RESONAANCES

Fermi says "nothing"...like sure sure?

2009-11-19T06:32:00.005+01:00

I wrote recently about a couple of theory groups who claim to have discovered intriguing signals in the gamma-ray data acquired by the Fermi satellite. The Fermi collaboration hastened to trash both these signals, visibly annoyed by pesky theorists meddling in their affairs. Therefore a status update is in order. Then I'll move to realizing the holy mission of yellow blogs, which is spreading wild rumors.

The first of the theorist's claims concerned the gamma-ray excess from the galactic center, allegedly consistent with a 30 GeV dark matter particle annihilating into b-quark pairs. The relevant data are displayed on this plot released recently by Fermi, which shows the gamma-ray spectrum in the seven-by-seven degrees patch around the galactic center. There indeed seems to be an excess in the 2-4 GeV region. However, given the size of the error bars and of the systematic uncertainties, not to mention how badly we understand the astrophysical processes in the galactic center, one can safely say that there is nothing to be excited about for the moment.

The status of the Fermi haze is far less clear. Here is the story so far. In a recent paper, Doug Finkbeiner and collaborators looked into the Fermi gamma-ray data and found an evidence for a population of very energetic electrons and positrons in the center of our galaxy. These electrons would emit gamma rays when colliding with starlight, in the process known as inverse Compton scattering. They would also emit microwave photons via synchrotron radiation, of which hints are present in the WMAP data. The high-energy electrons could plausibly be a sign of dark matter activity, and fit very well with the PAMELA positron excess, although one cannot exclude that they are produced by conventional astrophysical processes. But Fermi argues that there is no haze in their data. During the Fermi Symposium last week the collaboration was chanting anti-haze songs and tarred-and-feathered anyone humming Hazy shade of winter. Interestingly, it seems that each collaboration member has a slightly different reasons for doubts. Some say the haze is just heavy cosmic-ray elements faking gamma-ray photons. Some say the haze does exist but it can be easily explained by tuned-up galactic models without invoking an energetic population of electrons. Some say the haze is LOOP-1 - a nearby supernova remnant that happens to lie roughly in the direction of the galactic center. But none of the above explanations seems to be on a firm footing, and the jury is definitely out. In the worst case, the matter should be clarified by the Planck satellite (already up in the sky) who is going to make more accurate maps of photon emission at lower frequencies that will lead to a better understanding of astrophysical backgrounds.

And now wild rumors... which, let's make it clear, are likely due to daydreaming over-imagination of data-hungry theorists. The rumors concern Fermi's search for subhalos, which is one of the most promising methods of detecting dark matter in the sky. Subhalos are dwarf galaxies orbiting our Milky Way who are made almost entirely of dark matter. Two dozens of subhalos have been discovered so far (by observing small clumps of stars that they host) but simulations predict several hundreds of these objects. The darkest of the discovered subhalos has a mass-to-light ratio larger than a thousand, indicating large concentration of dark matter. Because of that, one expects dark matter particles to efficiently annihilate and emit gamma rays (typically, via final state radiation or inverse Compton scattering of the annihilation products). Although the resulting gamma-ray flux is expected to be smaller than that from the galactic center, the subhalos with its small visible matter content offer a much cleaner environment to search for a signal.

So, Fermi is searching for spatially extended object away from the galactic plane that steadily emit a lot of gamma rays but are not visible in other frequencies. The results based on 10-months data have been presented in this poster. Apparently, they found no less than four candidates at the 5-sigma level!!! However, according to the poster, these candidates do not fit the spectra of three random dark matter models. For this reason, the conclusion of the search is that no subhalos have been detected, even though it is not clear what astrophysical processes could produce the signal they have found.

Well, I bet an average theorist would need fifteen minutes to write down a dark matter model fitting whatever spectrum Fermi has measured. On the other hand, the collaboration must have better reasons, not revealed to us mortals, to ditch the candidates they have found. On yet another hand, the fact that Fermi is not revealing the positions and the measured spectra of these four candidates makes the matter very very intriguing. So, we need to wait for more data. Or for a snitch :-)

Higgs chased away from another hole

2009-11-07T06:04:00.005+01:00

The hunt for the Higgs continues. Tevatron is running at full steam hoping to catch a glimpse of the sucker before the LHC joins in the game. If the standard model is correct, the entire range of allowed Higgs masses will be covered within next 3-4 years. But there is one disturbing puzzle: indirect measurement indicate that we should have already found the Higgs! Indeed, precision measurements at LEP and Tevatron - mostly lepton asymmetries of Z decay and the value of the W boson mass - are best explained if the Higgs mass is some 80-90 GeV, whereas the direct limit from LEP implies that it must be heavier than 115 GeV.

There is one more reason, this time purely theoretical, to expect that the Higgs may be lighter than the naive LEP bound. If supersymmetry is relevant at the weak scale it is in general very uncomfortable with a heavy Higgs. Well, they keep telling you that the upper limit in the MSSM is 130 GeV. But that requires stretching the parameters of the model to the point of breaking, while the natural prediction is 90-100 GeV. Indeed, not finding the Higgs at LEP is probably the primary reason to disbelieve that supersymmetry is relevant at low energies.

Is it possible that Higgs is lighter than 115 GeV and LEP missed it? The answer is yes, because the LEP searches have left many loopholes. Sensitivity of LEP analyses deteriorates if the Higgs decays into a many-body final state, which is possible in some extensions of the standard model. One popular theory where this could happen is the NMSSM - the 2.0 version of the MSSM with an additional singlet. Roughly, the Higgs could first decay into the new singlet, who in turn decays into two tau leptons, which amounts to Higgs decaying into four tau leptons. This funny decay topology could escape LEP searches even if the Higgs is as light as 86 GeV! That is the case not because of deep physical reasons, but simply because LEP collaborations were too lazy to search for it (in comparison, Higgs decaying into four b-quarks, which was studied by LEP, is excluded for the Higgs mass up to 110 GeV).

But not anymore - this particular gaping hole has been recently sealed. A group of brave adventure-seekers ventured into CERN caverns, excavated the ancient LEP data and analyzed them lookig for the Higgs-to-4tau signal. The results were presented this week at the ALEPH meeting celebrating the 20th anniversary and 9th anniversary of its demise. Of course, there is nothing there, in case you had any doubts. The new limit for the Higgs-to-4tau channel excludes the Higgs mass smaller than 105-110 GeV. Yet the beautiful thing in that analysis is that going back to the LEP data is still possible, if only there is reason, and will, and cheap work force.
So, is the idea of the hidden light Higgs dead? It has definitely received a serious blow, but it can still survive in some perverse models where Higgs decays into four light jets, at least until someone ventures to kill that too. Anyway, never say dead; there is no experimental results that theorists could not find a way around ;-)

Hail to Freedom

2009-10-30T22:10:00.011+01:00

Experimental collaborations display vastly different attitudes toward sharing their data. In my previous post I described an extreme approach bordering on schizophrenia. On the other end of the spectrum is the Fermi collaboration (hail to Fermi). After one year of taking and analyzing data they posted on a public website the energy and direction of every gamma-ray photon they had detected. This is of course the standard procedure for all missions funded by NASA (hail to NASA). Now everybody, from a farmer in the Guangxi province to a professor in Harvard, has a chance to search for dark matter using real data.

The release of the Fermi data has already spawned two independent analyses by theorists. One is being widely discussed on blogs (here and here) and in popular magazines, whereas the other paper passed rather unnoticed. Both papers claim to have discovered an effect overlooked by the Fermi collaboration, and both hint to dark matter as the origin.

The first (chronologically, the second) of the two papers provides a new piece of evidence that the center of our galaxy hosts the so-called haze - a population of hard electrons (and/or positrons) whose spectrum is difficult to explain by conventional astrophysical processes. The haze was first observed by Jimi Hendrix ('Scuse me while I kiss the sky). Later, Doug Finkbeiner came across the haze when analyzing maps of cosmic microwave radiation provided by WMAP; in fact, that was also an independent analysis of publicly released data (hail to WMAP). The WMAP haze is supposedly produced by synchrotron radiation of the electrons. But the same electrons should also produce gamma rays when interacting with the interstellar light in the process known as the inverse Compton scattering (Inverse Compton was the younger brother of Arthur), the ICS in short. The claim is that Fermi has detected these ICS photons. You can even see it yourself if you stare long enough into the picture.

The second paper also takes a look at the gamma rays arriving from the the galactic center, but uncovers a completely different signature. There seems to be a bumpy feature around a few GeV that does not fit a simple power-law spectrum expected from the background. The paper says that a dark matter particle of mass around 30 GeV annihilating into b quark pairs can fit the bump. The required annihilation cross section is fairly low, of order $10^{-25} cm^3/s$, only a factor of 3 larger than that needed to explain the observed abundance of dark matter via a thermal relic. That would put this dark matter particle closer to a standard WIMP, as opposed to the recently popular dark matter particles designed to explain the PAMELA positron excess who need a much larger mass and cross section.

Sadly, collider physics has a long way to go before reaching the same level of openness. Although collider experiments are 100% financed by public funds, and although acquired data have no commercial value, the data remains a property of the collaboration without ever being publicly released, not even after the collaboration has dissolved into nothingness. The only logical reason to explain that is inertia - a quick and easy access to data and analysis tools has only quite recently become available to everybody. Another argument raised on that occasion is that only the collaboration who produced the data is able to understand and properly handle them. That is of course irrelevant. Surely, the collaboration can make any analysis ten times better and more reliably. However, some analyses are simply never done either due to lack of manpower or laziness, and others are marred by theoretical prejudices. The LEP experiment is a perfect example here. Several important searches have never been done because, at the time, there was no motivation from popular theories. In particular, it is not excluded that the Higgs boson exist with a mass accessible to LEP (that is less than 115 GeV), but it was missed because some possible decay channels have not been studied. It may well be that ground breaking discoveries are stored on the LEP tapes rotting on dusty shelves in CERN catacombs. That danger could be easily avoided if the LEP data were publicly available in an accessible form.

In the end, what do we have to lose? In the worst case scenario, the unrestricted access to data will just lead to more entries in my blog ;-)

Update: At the FERMI Symposium this week in Washington the collaboration trashed both of the above dark matter claims.

What's really behind DAMA

2009-10-27T05:34:00.009+01:00

More than once I wrote in this blog about crazy theoretical ideas to explain the DAMA modulation signal. There is a good excuse. In this decade, DAMA has been the main source of inspiration to extend dark matter model building beyond the simple WIMP paradigm, in particular inelastic dark matter was conceived that way. This in turn has prompted to tighten the net of experimental searches to include signals from non-WIMP dark matter particles. More importantly, blog readers always require sensation, scandal and blood (I know, I'm a reader myself), so that spectacular new physics explanations are always preferred. Nevertheless, prompted by a commenter, I thought it might be useful to balance a bit and describe a more trivial explanation of the DAMA signal that involves a systematic effect rather than dark matter particles.

Unlike most dark matter detection experiments, the DAMA instrument has no sophisticated background rejection (they only reject coincident hits in multiple crystals). That might be an asset, because they are a priori sensitive to a variety of dark matter particles, whether scattering elastically or inelastically, whether scattering on nucleons or electrons, and so on. But at the same time most of their hits comes from mundane and poorly controlled sources such as natural radioactivity, which makes them vulnerable unknown or underestimated backgrounds.

One important source of the background is a contamination of DAMA's sodium-iodine crystals with radioactive elements like Uranium 238, Iodine 129 and Potassium 40. The last one is the main culprit because some of its decay products have the same energy as the putative DAMA signal. Potassium, being in the same Mendeleev column as sodium, can easily sneak into the lattice of the crystal. The radioactive isotope 40K is present with roughly 0.01 percent abundance in natural potassium. Ten percent of the times 40K decays to an excited state of Argon 40, which is followed by a de-excitation photon at 1.4 MeV and emission of Auger electrons with energy 3.2 keV. This process is known to occur in the DAMA detector with a sizable rate; in fact DAMA itself measured that background by looking for coincidences of MeV photons and 3 keV scintillation signals, see the plot above. That same background is also responsible for the little peak at 3keV in the single hit spectrum measured by DAMA, see below (note that this is not the modulated spectrum on which DAMA claim is based!). The peak here is exactly due to the Auger radiation.

Now, look at the spectrum of the time dependent component of the signal where DAMA claims to have found evidence for dark matter. The peak of the annual modulation signal occurs precisely at 3 keV.The fact that the putative signal is on top of the known background is VERY suspicious.
One should admit that it is not entirely clear what could cause the modulation of the background,
although some subtle annual effect affecting the efficiency for detecting the Auger radiation is not implausible. So far, DAMA has not shown any convincing arguments that would exclude 40K as the origin of their modulation signal.

Actually, it is easy to check whether it's 40K or not. Just put one of the DAMA crystals inside the environment where the efficiency for detecting the decay products of 40K is nearly 100 percent. Like for example, in the Borexino balloon that is waiting next door in the Gran Sasso Laboratory. In fact, the Borexino collaboration has made this very proposal to DAMA. The answer was a resounding no.

There is another way Borexino could quickly refute or confirm the DAMA claim. Why not buying the sodium-iodine crystals directly from Saint Gobain - the company that provided the crystals for DAMA? Not so fast. In the contract, DAMA has secured exclusive eternal rights for the use of sodium-iodine crystals produced by Saint Gobain. At this point it comes as no surprise that DAMA threatens legal actions if the company attempts to breach their "intellectual" property.

There is more stories that make hair on your chest stand on end. One often hears the phrase "a very specific collaboration" when referring to DAMA, which is a roundabout way of saying "a bunch of assholes". Indeed DAMA has worked very hard to earn their bad reputation, and sometimes it's difficult to tell whether at the roots is only paranoia or also bad will. The problem, however, is that history of physics has a few examples of technologically or intellectually less sophisticated experiments beating better competitors - take Penzias and Wilson for example.
So we will never know for sure whether the DAMA signal is real or not until it is definitely refuted or confirmed by another experiment. Fortunately, it seems that the people from Borexino have not given up yet. Recently I heard a talk of Cristiano Galbiati who said that the Princeton group is planning to grow their own sodium-iodine crystals. That will take time, but an advantage is that they will be able to obtain better, more radio-pure crystals, and thus reduce the potassium 40 background by many orders of magnitude. So maybe in two years from now the dark matter will be cleared...

Early LHC Discoveries

2009-10-05T23:07:00.004+01:00

It seems that the LHC restart will not be significantly delayed beyond this November. The moment when first protons collide at 7 TeV energy will send particle theorists into an excited state. From day one, we will start harassing our CMS and ATLAS colleagues, begging for a hint of an excess in the data, or offering sex for a glimpse on invariant mass distributions. That will be the case in spite of the very small odds for seeing any new physics during the first months. Indeed, the results acquired so far by the Tevatron make it very unlikely that spectacular phenomena could show up in the early LHC. Although the LHC in the first year will operate at a 3 times larger energy, the Tevatron will have the advantage of 100 times larger integrated luminosity, not to mention the better understanding of their detectors.

Nevertheless, it's fun to play the following game of imagination: what kind of new physics could show up in the early LHC without having been already discovered at the Tevatron? For that, two general conditions have to be satisfied:

There has to be a resonance coupled to the light quarks (so that it can be produced at the LHC with a large enough cross section) whose mass is just above the Tevatron reach, say 700-1000 GeV (so that the cross section at the Tevatron, but not at the LHC, is kinematically suppressed).
The resonance has to decay to electrons or muons with a sizable branching fraction (so that the decay products can be seen in relatively clean and background-free channels).

It turns out, however, that even these two favorable conditions may not guarantee an early discovery. The example of the Z' gauge boson is a good illustration here. In the particle slang, Z' refers to a new vector boson, a sort of a heavy photon, associated to a new broke U(1) gauge symmetry beyond the standard model. There is no particular motivation for such a creature to exist at the TeV scale, but the potentially large production cross section and clean experimental signatures (typical Z' couples to both the quarks and the leptons) make it the common case study for an early LHC discovery. Unfortunately, the existing bounds from LEP and the Tevatron forbid a large number of Z' events in the early LHC, as shown in this neat paper.

The possible couplings of Z' to quarks and leptons can be theoretically constrained: imposing anomaly cancellation, flavor independence, and the absence of exotic fermions at the TeV scale implies that the charges of the new U(1) acts on the standard model fermions as a linear combination of the familiar hypercharge and the B-L global symmetry. Thus, one can describe the parameter space of these Z' models by just three parameters: two couplings gY and gB-L and the Z' mass. This simple parametrization allows us to quickly scan through all possibilities. An example slice of the parameter space for the Z' mass 700 GeV is shown on the picture to the right. The region allowed by the Tevatron searches is painted blue, while the region allowed by electroweak precision tests is pink (the coupling of Z' to the electrons induces effective four-fermion operators that have been constrained by the LEP-II experiment). As you can see, these two constraints imply that both couplings have to be smallish, of order 0.2 at the most, which is even less than the hypercharge coupling g' in the standard model. That in turn implies that the production cross section at the LHC will be suppressed. Indeed, the region where the discovery at the LHC with 7 TeV and 100 inverse picobarns is impossible, marked as yellow, almost fully overlaps with the allowed parameter space. Only a tiny region (red arrow) is left for that particular mass, but even that pathetic scrap is likely to be wiped once the Tevatron updates their Z' analyses.

The above example illustrates how difficult is to cook up a model suitable for an early discovery at the LHC. A part of the reason why Z' is not a good candidate is that it is produced by quark-antiquark collisions. That is a frequent occurrence in the proton-antiproton collider like the Tevatron, whereas at the LHC, who is a proton-proton collider, one has to pay the PDF price of finding an antiquark in the proton. An interesting way out that goes under the name of diquark resonance was proposed in another recent paper. If the new resonance carries the quantum numbers of two quarks (rather than quark-antiquark pair) then the LHC would have a tremendous advantage over the Tevatron, as the resonance could be produced in quark-quark collisions that are more frequent at the LHC. Because of that, a large number of diquark events may be produced at the LHC in spite of the Tevatron constraints. The remaining piece of model building is to ensure that the diquark resonance decays to leptons often enough.

Diquarks are not present in the most popular extensions of the standard model and therefore they might appear to be artificial constructs. However, they can be found in somewhat more exotic models like for example the MSSM with a broken R-parity. That model allows for couplings like $u^c d^c \tilde b^c$, where $u^c,d^c$ are right-handed up and down quarks, while $\tilde b^c$ is the scalar partner of the right-handed bottom quark called the (right) sbottom. Obviously, this coupling violates R-symmetry because it contains only one superparticle (in the standard MSSM, supersymmetric particles couple always in pairs). The sbottom could then be produced by collisions of up and down quarks, both of which are easy to find in protons.

Decays of the sbottom are very model dependent: the parameter space of supersymmetric theories is as good as infinite and can accommodate numerous possibilities. Typically, the sbottom will undergo a complex cascade decay that may or may not involve leptons. For example, if the lightest supersymmetric particle is the scalar partner of the electron, then the sbottom can decay into a bottom quark + a neutralino who decays into an electron + a selectron who finally decays into an electron and 3 quarks:
$\tilde b^c -> b \chi^1 -> b e \tilde e -> b e e j j j$
As a result, the LHC would observe two hard electrons plus a number of jets in the final state, something that should not be missed.

To wrap up, the first year at the LHC will likely end up being an "engineering run", where the standard model will be "discovered" to the important end of calibrating the detectors. However, if the new physics is exotic enough, and the stars are lucky enough, then there might be some real excitement store.

Resonating dark matter

2009-09-27T22:39:00.005+01:00

On this blog I regularly follow the progress in dark matter building. One reason is that next-to-nothing is happening on the collider front: Tevatron invariably confirms the standard model predictions up to a few pathetic 2 point null sigma bleeps now and then. In these grim times particle theorists sit entrenched inside their old models waiting for the imminent LHC assault. The dark matter industry, on the other hand, enjoys a flood of exciting experimental data, including a number of puzzling results that might be hints of new physics.

One of these puzzles - the anomalous modulation signal reported by DAMA - continues to inspire theorists. It is a challenge to reconcile DAMA with null results from other experiment, and any model attempting that has to go beyond the simple picture of elastic scattering of dark matter on nuclei. The most plausible proposal so far is the so-called inelastic dark matter. Last week a new idea entered the market under the name of resonant dark matter. Since this blog warmly embraces all sorts of resonances I couldn't miss the opportunity to share a few words about it.

In the resonant dark matter scenario the dark matter particle is a part of a larger multiplet that transforms under weak SU(2). This means that the dark matter particle (who as usual is electrically neutral) has partners of approximately the same mass that carry an electric charge. Quantum effects split the masses of charged and neutral particles making the charged guys a bit heavier (this is completely analogous to the $\pi_+ - \pi_0$ mass splitting in the standard model). Most naturally, that splitting would be of order 100 MeV; some theoretical hocus-pocus is needed to lower it down to 10 MeV (otherwise the splitting is to large compared to nuclear scales, and the idea cannot be implemented in practice), which is presumably the weakest point in this construction.

Now, when dark matter particles scatter on nuclei in a detector there is a possibility of forming a narrow bound state of the charged partner with an excited state of the nucleus, see the picture. That would imply that the scattering cross-section sharply peaks at a certain velocity corresponding to the resonance.The existence of the resonance is very sensitive to many nuclear parameters: mass, charge, atomic number and the energies of excitation levels. It is conceivable that the resonant enhancement occurs only for one target, say, iodine present in DAMA's sodium-iodine crystals, while it is absent for other targets like germanium, silicon, xenon etc. that are employed in other dark matter experiments. For example, the resonant velocity for these other elements might be outside the range of velocities of dark matter in our galaxy (the escape velocity is some 500 km/s so that there is an upper limit to scattering velocities).

So, that looks like a perfect hideaway for DAMA, as other experiment would have a hard time exclude the resonant dark matter hypothesis in a model independent way. Fortunately, there is another ongoing dark matter experiment involving iodine: the Korean KIMS based on cesium-iodine crystals. After one year of data taking the results from KIMS combined with those from DAMA put some constraints on the allowed values of the dark matter mass, the position of the resonance and its width, but they leave large chunks of allowed parameter space. More data from KIMS will surely shed more light on the resonant dark matter scenario.

Scherzo II

2009-09-22T23:37:00.006+01:00

I'm still in a holiday mood so forgive one more prank before moving to serious physics (as if this blog were ever serious ;-). Traveling to conferences this summer I noticed that humans had acquired a new useful skill. Have a look at this picture:
and another:
and there are many more similar ones stored on my disc ;-) Of course, the interesting point here is not sleeping during a talk - everybody does that. The remarkable point is perfect balancing of a laptop while sleeping. In the extreme case it went on for 45 min without awaking and without dropping the laptop to the floor. Amazing!

Using Darwin's theory of evolution one can predict that this new skill will quickly spread in the scientific community.Those who do not possess it will inevitably destroy their laptops while sleeping at conferences, thus losing years of work and dropping in the academic ranking. As a consequence, these handicapped ones will acquire less mating partners and thus less chances to pass their genes. Unless laptops become shock resistant in the meantime.

Scherzo

2009-09-21T17:25:00.005+01:00

Last week I was stranded at a conference far in Italy. There was no internet to speak of; instead, the venue was offering alternative commodities. Like for example a chapel:

It striked me as a really good idea. They say that science and religion cannot go hand in hand, but that's obviously not true. Take the first example in a row: the Higgs searches at Tevatron.
According to the plot on the right, Tevatron has roughly a 30 percent chance of finding a 3 sigma evidence for the Higgs in the most interesting region between 115 and 130 GeV. Since we speak of chances the matter is open for prayers. Therefore chapels should be available not only at conferences but also at every accelerator facility.

In that case one would expect the following pattern:

Tevatron would pray that they find the Higgs
The LHC would pray that Tevatron does not find the Higgs
Graduate students at the LHC would pray for any data at all before they turn fifty
Everybody would pray that the magnets do not explode
Except for Hawaii surfers and Holger Nielsen who would pray for the contrary

And, as usual, gods will cast dice to decide whose requests should be granted.

New ideas in dark matter

2009-08-28T21:33:00.002+01:00

I keep receiving complaints about my meager blogging output. Concerning the last few weeks I have a good excuse: I was studying Mont Blanc (that narrow resonance visible at the LHC on a clear day); and after that I was being tired; and after that I was being lazy; and in any case nothing happens during summer months.

But the coming autumn is going to be exciting. The approaching LHC start-up is one but not the unique reason. On a completely different frontier, huge progress is expected in the area of direct detection of dark matter. The XENON100 experiment in Gran Sasso is going to kick off next month, while its bitter enemy - the LUX experiment in the Homestake mine - is hoping to follow before the end of the year. Within one year experimental sensitivity to the WIMP-nucleon cross section should be improved by two orders of magnitude, biting deep into the parameter space region where numerous popular theories predict a signal.

In the best case scenario the very first months or even days can lead to a discovery. This is the prediction of the dark matter models designed to resolve the DAMA puzzle. Recall that the DAMA experiment in Gran Sasso claims to have detected dark matter by observing the annual modulation of the count rate in their sodium-iodine detector. Particle theorists struggle to reconcile the DAMA signal with the null results from a dozen of other, in principle more sensitive, experiments. That is not quite impossible because various experiments use different targets and different detection techniques, in particular, the masses of the target nuclei as well as the range of the observable recoil energies are specific to each experiment. The game is thus to arrange the properties of dark matter such that the nuclear recoils due to scattering of dark matter particles could have been observed only by DAMA.

The standard WIMP is not an option here: DAMA would require large cross section at the level that has been excluded by CDMS, XENON10 (the little brother of XENON100) and others. But theorists are not easily discouraged and they are trying to come up with alternative ideas. Recently the so-called inelastic dark matter has gained a lot of publicity. In that scenario, dark matter particles scatter inelastically off nuclei into a slightly (hundred of keV) heavier state. Thus, one needs to provide enough energy to produce the heavier state which implies a minimum velocity of the initial dark matter particle for the scattering to occur. The splitting can be tuned such that DAMA is able to see the signal while the others are not. That of course requires some amount of conspiracy. Fortunately, the inelastic dark matter theory predicts a thunderstorm of events in the upcoming runs of XENON100 and LUX.

Until recently inelastic dark matter was the only plausible explanation of DAMA. But this week there was a paper exploring a different idea. In this new scenario, the scattering of dark matter on nucleons is elastic, but the scattering amplitude depends in a non-trivial way on the momentum transfer, hence the name form factor dark matter. If the form factor is suppressed outside the window to which DAMA happens to be sensitive, the null results of other experiments can be explained.

Non-trivial form factors can be arranged by some dirty model building tricks. The paper presents an example of a scalar dark matter particle with dipole-type ($D_\mu X^\dagger D_\nu X F_{\mu\nu}$) interactions with some new hidden vector fields. The latter mix with the photon which provides a coupling to the ordinary matter. To explain the DAMA phenomenology one needs at least two vector fields with opposite couplings to the photon and comparable masses, which makes the whole construction a bit contrived. Again, the model predicts a characteristic recoil spectrum, and a large number of events in XENON100 and LUX.

What seems to be most valuable in these constructions is that they demonstrate that dark matter can have very different properties than the standard WIMP. That should encourage the experimenters to extend the scope of their searches; so far their search algorithms have been tailor-made for the standard WIMP case, and they could have easily missed something interesting. The fantastic experimental progress makes the dark matter models testable well before the LHC can offer us any interesting data. If you have a good idea in store now is the good time to come out.

10? 6.66? Eleventeen?

2009-08-05T00:29:00.006+01:00

Today (Wednesday) the CERN management is going to reach a decision that will affect the life of everybody on this planet. Namely, the operating energy of the LHC machine in the first year will be decided today. Senior readers may remember that the LHC used to be a 14 TeV collider. However that energy cannot be achieved in near future due to poor quality of the magnets provided by industry. Reaching the nominal energy will require a long process of magnet training, and the prospect for upgrade seem unlikely within the next 3 years. For this reason, 10 TeV was the energy planned for the last year false start, as well as for the restart scheduled for mid-November.

However, the rumor is that even this smaller energy will not be achieved next year due to the well known problems with bad splices. The hundreds of individual magnets around the LHC ring are connected using a process called soldering - an advanced cutting-edge technique whose many aspects are clouded in mystery. There are in fact two separate problems with soldering that have been detected at the LHC. One is a poor quality of interconnections between the superconducting magnets. That leads to excessive resistance (like nanoohms) and, in consequence, the current flowing through the interconnection generates heat that triggers a quench of the superconductor. The other problem are faulty interconnections between copper bus bars who are supposed to carry the current when the superconductor quenches. It is suspected that the solder in the bus bars was sometimes accidentally melted during subsequent soldering of the superconducting cable connections. In fact, it was a combination of the two above mentioned problems that triggered the fireworks of September 19.

Bad splices are known to be present in the LHC ring, and those residing in cold sectors cannot be repaired without a considerable slip to the schedule. So the alternative is to either postpone the LHC restart or run at slightly lower energies (the latter implies slightly smaller currents running through magnets and thus a slightly smaller risk of another catastrophe). During the last few months numerous simulations and experiments have been performed to determine the maximum safe current.

After a careful study of the plot above, listening to the experts, and weighing all pros and cons, the director general is going to roll a pair of dice, and the sum of dots will determine the LHC energy for the coming restart. As for the rumors, I have heard any rational number between 4 and 10 TeV. So, now is the last moment place your bets. Theoretical analysis of the 2-dice experiment suggests that 7 is the most likely outcome :-).

Once we know the operating energy, we will have a better idea what kind of results to expect in the first year. It is already certain that for a while the LHC cannot compete with the Tevatron in the area of Higgs searches. In fact, almost all reasonable new physics signatures require at least one inverse femtobarn of integrated luminosity, much more than the 100 inverse picobarns expected in the first year. This leaves boring standard model signatures, including slightly less boring top quark physics (but even in the case of the top quark competing with the Tevatron results may be tough if the center-of-mass energy is lower than 10 TeV). However, some spectacular (and unlikely) signatures like a new 1 TeV Z' gauge boson or light superparticles may be within reach if the center of mass energy is not much less than 10 TeV. But realistically, we have to keep patient until at least 2012.

And the winner is...

Seven!

FERMI is seeing something?

2009-07-24T09:23:00.015+01:00

It has become a tradition that release of new astrophysical data proceeds in the atmosphere of scandal, sex, and intrigues. Less than two weeks ago in this blog I was whining that the FERMI collaboration is guarding their secrets too effectively. Not any more. Not even guns and barbed wire fences could keep theorists off, once they have smelled real data.

Once again the story is related to the searches of indirect signals of dark matter in cosmic rays. In the previous episodes, the PAMELA satellite reported an excess of cosmic ray positrons between 10 and 100 GeV, and FERMI announced that the spectrum of electrons and positrons is harder (falls off more slowly with energy) than predicted by conventional cosmic ray propagation models. Although there exist plausible explanations in terms of mundane astrophysics, the excess positrons and electrons can also be understood as products of annihilation or decay of dark matter in our galaxy. If that is the case, there is one robust consequence. The electrons and positrons produced by dark matter throughout the galaxy should interact with the photons of the cosmic microwave background and starlight in the process known as the inverse Compton scattering, ICS in short. A high energy electron scattering off a photon transfers most of its energy to the photon. Thus, dark matter models explaining the PAMELA and FERMI results also predict an excess of gamma ray photons from the galactic center at energies 100 GeV and more. That's why the astroparticle community has been eagerly awaiting the release of FERMI measurements of gamma rays from the galactic center.

A week ago FERMI announced some new results at the TeV Particle Astrophysics conference held at SLAC. The new data included measurements of the gamma ray spectrum from the galactic center. The results from the one-by-one degree square around the galactic center, while providing new constraints on dark matter models, do not show any exciting features, see the upper plot. However, the data from a larger portion of the sky referred to as the inner galaxy do show an excess, or a hardening of the spectrum, starting at 100 GeV, see the plot on the left. The hardening occurs exactly where the dark matter models predict it! The FERMI collaboration did not want to post the latter result because it is still contaminated with poorly understood backgrounds. But somehow, mysteriously, the plot made it into the summary talk given by Persis Drell and the slides were posted at the conference page. These slides have now been removed; too late alas too late. Today there is a new paper on arXiv that interprets the new FERMI data in terms of the PAMELA/FERMI motivated models dark matter. The plot below reproduced from that paper shows the FERMI data together with expected backgrounds and predictions from dark matter models.

So is FERMI seeing dark matter? Most likely not. Members of the FERMI collaboration suspect that the feature in the gamma ray spectrum around 100 GeV is due to an unexpected background from other cosmic ray particles. Further analysis should clarify the situation. What is definitely true is that we're living in interesting times...

Bullets Fly

2009-07-19T00:32:00.009+01:00

I'm sure that everybody has heard of the Bullet cluster aka 1E 0657-56:

This picture from August 2006 made the headlines because it offers a new way to see the presence of dark matter in the universe (the key of course is to paint it blue). The depth of the gravitational potential deduced from gravitational lensing is marked blue, while the matter that shines ordinary photons is marked red. The picture is interpreted as showing two clusters of galaxies that have recently undergone a head-on collision. The dark matter components (and also most of the ordinary stars in the galaxies) just passed through each other with little or no interaction, while the interstellar gas made of familiar protons and electrons collided and got left behind. The observation of the Bullet cluster gave another blow to dark matter alternatives like MOND-type modified gravity theories: in the latter context it is hard to explain why the gravitational potential is not spatially correlated with the ordinary matter distribution.

What might be a little less known is that the Bullet cluster is not the only one. In August 2007 Abell 520 aka Train Wreck was revealed:

This one is much more messy. In fact, in this case the dark matter interpretation is less straightforward. The reason is that the galaxies seems to have been removed from the densest core of dark matter, and it is not clear what mechanism could have caused it. Then in August 2008 we had a pleasure to meet MACS J0025.4-1222 aka Baby Bullet:
which is another pretty clear evidence in favor of dark matter. Apparently, galactic collisions happen every year in August, so next month we should be presented with another picture of this kind :-)

The most important thing about these observations is that they look cool in pictures. But they also carry some practical consequence for particle theorists who sweat to construct models of dark matter. From the fact that the dark matter components pass through each other so easily one can derive a constraint on the self-interaction cross section of dark matter. The paper of Randall et al (not that Randall) based on the analysis of the Bullet cluster quotes the limit

$\sigma/M \leq 3 \cdot 10^3/GeV^3$.

That's an order of magnitude better than the so-called Spergel-Steinhard bound that can be deduced from the dynamics of our galaxy. While this bound is irrelevant for a standard 100 GeV WIMP, it might be a useful constraint for recently popular theories of dark matter where the dark sector consists of strongly interacting particles bound by some new unknown GeV scale forces.

That's Another One for the Fire

2009-07-10T20:37:00.003+01:00

It's a lazy summer season: everybody's on the beach and nothing's much going on. To stay in business I have to feed you with some microwaved news. Last week the FERMI collaboration uploaded a pile of papers on arXiv, one of which caught my attention. FERMI is a space gamma-ray observatory, but first of all he is a ruthless terminator with a mission to eliminate other astrophysical experiments. A while ago in May the widely publicized measurement of the cosmic-ray electron+positron spectrum pierced the ATIC balloon that had been pumped for several months. Earlier this year FERMI made another kill: it shot down EGRET, its direct predecessor in cosmic gamma-ray observations. That result has been presented at conferences for a few months, but only last week it made it to arXiv (there's also a longer PRL paper announced).

EGRET was a sattelite gamma-ray observatory that in particular studied the diffuse gamma ray emission in the 30 MeV-100 GeV range. Diffuse means spread over the sky rather than originating from point sources. The main source of diffuse radiation is scattering of the cosmic rays on the milk of the Milky Way. Dark matter annihilation into standard model particles can also contribute to the diffuse flux. The EGRET results showed an excess of gamma rays above 1 GeV, which was quickly hailed as the harbinger of dark matter and supersymmetry. But FERMI's measurement now demonstrates that there's nothing exciting going on below 10GeV: the experimental curve nicely follows the theoretical prediction of the standard propagation model. No exotic physics in sight. ATIC, EGRET...who's next?

FERMI's goal is to measure the diffuse gamma-ray spectrum up to some 300 GeV. The high energy data around are even more interesting for theorists as many popular models of dark matter - especially those that explain the PAMELA positron excess - predict a large signal peaking around a few hundred GeV. More results are expected in August since on August 12th the collaboration is supposed to make all their photon data public. If you hear cries and squealing later this summer that's the dark matter models being slaughtered. Or maybe FERMI sees an excess in which case all hell will break loose? The rumor is...the weird thing is that there are no rumors. Last time, quite accurate descriptions of the electron spectrum circulated in the community months before publication of the FERMI data, and theory papers from outside of the collaboration were out days after FERMI's publication (or even violating causality in one case). Probably because of that experience the collaboration has now entrenched in its camp with barbed wire fences, dogs, booby traps to keep off the theorists. Oh come on, dont be so serious, we also wanna know ;-)

Angles and Demons

2009-06-25T18:48:00.009+01:00

Since writing down the standard model back in the summer of love the only progress in particle theory has been the discovery that neutrinos have masses. This fact makes the leptons similar in spirit to the quarks in the sense that transitions between different flavors are possible. In both cases the flavor eigenstates, that is the states to which the W bosons couple, are not the same as the mass eigenstates but linear combinations thereof. This fact opens the door to a fascinating endeavor of measuring the angles in the unitary matrix that relates the flavor and the mass eigenstates.

The angles in the quark sector have been measured from every angle. The final conclusion is that what they taught at school was right: the sum of the angles in a (unitarity) triangle is equal to 180 degrees. In the lepton sector, experiment is lagging behind: so far we know only two of the angles. The one known as the atmospheric or $\theta_{23}$ angle (responsible in particular for the transitions of atmospheric muon neutrinos into tau ones) is close to 45 degrees. The solar or $\theta_{12}$ angle (responsible for the vanishing of electron neutrinos produced in the Sun) turned out to be a bit smaller, about 30 degrees. For the last angle, at the moment we have only an upper bound from the CHOOZ reactor experiment: $\theta_{13} < 11$ degrees at 90 percent C.L.

Now what have demons to do with it? I recently came across a talk from the MINOS collaboration a few month ago in Fermilab. MINOS, when on leave from Hades, studies the muon neutrino beam sent over the distance of 735 kilometers from Fermilab to a far detector located in the Soudan mine in Minnesota. One reason to bother (just imagine what it takes to dig such a long tunnel to send the neutrino beam over several states) is a precise measurement of the angle $\theta_{23}$ which controls the fraction of muon neutrinos that disappear on the way. But there may be more fun than that. Most of the muon neutrinos that vanish turn into tau neutrinos who escape detection. However, if $\theta_{13}$ is non-zero then a small fraction of the muon neutrinos should turn into electron neutrinos, and those receive a warm welcome in Soudan. Thus, MINOS is one in a long queue of experiments trying to pinpoint $\theta_{13}$.

In February this year MÌNOS announced their first results concerning the electron neutrino appearance. They see 35 electron event, roughly 1.5 sigma above the expected background of $27$ events. Not too significant, but already tantalizing. Moreover, if the MINOS data are combined with all available neutrino data the hint for a non-zero $\theta_{13}$ is strengthened to 2 sigma. The central value for $\theta_{13}$ inferred from the overall fit is 8 degrees (plus minus 4) - just below the CHOOZ limit.

If the current hints converge to a full-fledged measurement of $\theta_{13}$ in the 5-10 degrees ballpark then there are some far reaching consequences. First of all, measuring the $\theta_{13}$ angle paves the way to measuring yet another angle (isn't particle physics exciting?), more precisely the CP violating phase in the neutrino mixing matrix. Secondly, it would appear that the mixing angles in the lepton sector are pretty random numbers with no structure, in stark contrast to the quark sector where the CKM matrix displays a very hierarchical structure. In other words, neutrinos would prove to be anarchic. That would mean that anarchy is at rule, at least in the lepton sector, for the first time since Barcelona'36.

Boston Tea Party

2009-06-12T03:42:00.006+01:00

Last weekend I made a trip to Boston and had a privilege to see bits of the SUSY'09 conference. Backreaction and Quantum Diaries have already run their stories but in my opinion they did not fully capture the grandeur of the event. 9 days in a row, including foreplay. Over 400 participants, not counting squatters. 42 plenary speakers, most of whom witnessed the glorious days when supersymmetry was conceived. Seven parallel parallel sessions to cover every aspect of supersymmetry that has not yet been covered thoroughly enough. Royal coffee break menu fully adequate to the royal conference fee. And so on and on since 16 years and into the future.

Meanwhile, there is no single hint from experiment that supersymmetry is realized in nature... but that should not upset anyone. As my fellow blogger skillfully put it, supersymmetry is the "shining beacon", the "raison d’etre" and for this reason "the conundrum is how it will be discovered, not if". That's why every year we come together to enjoy old familiar faces and old familiar talks. The point is, while waiting for the inevitable, to maintain that kind of spirit that David Lodge praised in his books.

On the picture below, the photographer about to make a photograph of the SUSY'09 participants.

Life After FERMI

2009-06-09T04:07:00.006+01:00

I mean, after FERMI's first electron data that shed new light on the currently hottest topic in astroparticle physics - the origin of the cosmic-ray positron excess measured by PAMELA. The PAMELA anomaly trumpeted last summer, combined with the data from ATIC (who claims a spectacular excess of electrons at few hundred GeV), prompted zillions of publications that speculate of its dark matter origin. A few weeks ago FERMI revealed their first measurement of the electron+positron cosmic ray spectrum up to 1 TeV. Here is my summary of what that implies for the models of dark matter.

Let's begin with a handful of facts:

Thanks to FERMI and HESS we now have a pretty accurate picture of the cosmic-ray electron+positron spectrum. Up to 1 TeV, the spectrum is well approximated by power law, $\sim E^{-3}$, while above TeV it becomes softer (larger power).
ATIC screwed: there is no clear feature in the electron spectrum. In principle, one should describe the present situation as "two experiments giving inconsistent results", given that ATIC's brand new data continue displaying a distinct bump with even smaller errorbars. But, somehow, the public opinion prefers a fancy high-tech satellite over a dirty leaking balloon somewhere in the cold Antarctica. More seriously, FERMI simply beats ATIC with a hundred times more statistics.
The electron spectrum near TeV is above the background predicted by conventional cosmic-ray propagation models (which prefer a larger power, $\sim E^{-3.3}$) in which the electrons are produced by scattering of cosmic ray protons. But that alone is hardly an anomaly, and the spectrum can be easily fitted by cranking up the propagation model.
However, combining FERMI with the PAMELA positron data strongly indicates the presence of a new primary component of electrons/positrons (as opposed to the secondary production by protons) .
The primary component could be injected by nearby pulsars, or by another so far unidentified astrophysical process, or by dark matter annihilating or decaying in our galaxy. We have to wait for the jury to reach the verdict.
But, light (less than 1 TeV) dark matter particle as a source of the PAMELA and FERMI excess is now strongly disfavored. The reason is that a light particle would lead to sharp spectral features at energies comparable to its mass, whereas FERMI sees none of that. If dark matter is the cause of the excess, it has to be relatively heavy, 1 TeV or more.

In fact, the new FERMI data did not really lead to a slaughter of the dark matter models or its authors. New papers keep appearing in which the excess is fitted with axions, neutralinos, winos, KK particles, F-theory or three little pigs. But Occam is waving his razor menacingly, and we are reaching the point where boring astrophysics becomes the simplest explanation of all available data. The dark matter models, although still viable, have to be intelligently designed to yield observable signals in the positron and electron channels, but none in antiprotons, gamma rays or neutrinos. Incidentally, the electron channel is the one where the astrophysical background is most difficult to control...

The cool thing is that by the end of this summer we may further disfavor dark matter or return it to grace. The new crucial piece of information will be FERMI's measurement of the diffuse gamma ray spectrum that will extend its previous measurement to larger energies. If the electrons and positrons observed by PAMELA and FERMI originate from dark matter it means that they are produced all over the galaxy. Once produced, the electrons lose their energy by scattering on starlight and on the CMB or by synchrotron radiation in the galactic magnetic fields, which leads to a diffuse flux of photons at energies of a few hundred GeV. Given the number of electrons needed to explain PAMELA and FERMI, the diffuse signal should be detectable by 1-year FERMI data. The important thing is that boring astrophysics cannot easily fake that signal. On the other hand, the absence of features in diffuse gamma would be a huge setback for the dark matter interpretation. The experimental data are expected on August 12, so just a little patience please...

For more details and plots see the slides of Alessandro Strumia's talk at Planck'09; check also for the connection between dark matter and dialectic materialism.

News and Gossips from the LHC

2009-05-27T08:04:00.008+01:00

Since two weeks Planck stands first of all for a satellite CMB observatory, but it's also the name for an annual series of conferences on physics beyond the standard model. This year's edition is taking place in the furnace of Padova. Since Tommaso is around, he will surely describe everything in great detail, including color of the tie of each speaker, while I should later write a summary of the interesting ideas discussed here in case there is any. But for now I'd like to share a handful of interesting facts about the progress of the LHC that I learned from a supercool talk delivered here by Jörg Wenninger. I guess most of what's below is not new and should be familiar to those closely following the LHC saga.

One interesting fact I was not aware of: a quench (a phase transition from superconductivity to normal conductivity) of an LHC magnet can be induced by just a few milijoules of energy. That energy may be provided by a bunch of strayed protons from the beam . To avoid quenching, LHC cannot lose more than a millionth part of its beam. For comparison, the Tevatron loses about one thousandth of its beam during acceleration. In that respect, Jörg was very convincing that the LHC would ever work ;-) But then, miracles do happen, sometimes.

Another interesting part of the talk was the explanation why the LHC will run at 10 TeV in the center of mass, instead of the nominal 14 TeV. The story goes as follows. Before installing, the LHC magnets have to be "trained", that is to say, to undergo a series of quenches to let their coils settle down at stable positions. After being installed in the tunnel they are supposed to come back to their test performance with no or few quenches. It turns out that the magnets provided by one of the three manufacturing companies need an extraordinary number of quenches to settle down. Although the company in question was not pointed at, everybody knows that the name is Ansaldo. In the case of that company, the number of quenches required for stable operation at 7 TeV per beam is currently unknown, it is probably somewhere between a hundred and a thousand. At the moment it is not clear if the LHC will ever reach 14 TeV; 12-13 TeV might be a more realistic goal.

The talk gave also a detailed account of the incident of September 19 known as the 9/11 of particle physics. Although the evidence has evaporated, one can quite reliably outline the sequence of events. An abnormally large resistance in one of the magnets acted as a heat source that quenched the superconducting cable at one interconnection. In case of a quench the current should start flowing or a few minutes through the copper bus-bar that encloses the cable until the energy stored in the magnet is removed. However, due to bad soldering of an interconnection the current could not flow normally and an electric arc was created. This melted copper, punctured the helium enclosure which led to spilling of 6 tons of helium into the tunnel. The logo of the company that made the faulty magnet is always erased in the pictures, although everybody knows that the name is Ansaldo.

So what's next? The repairs of the damaged sector are almost finished. The current plan is to head for collisions this year (with a caveat "depends how one defines collisions"). Beam commissioning is scheduled for September/October and the first collisions could happen in November. The schedule is very tight and, moreover, the quality control of has revealed problems like bad soldering or reduced electrical contact in a number of places, including sectors that are already cold. The rumor is that some of the LHC magnets in reality turned out to be electric kettles.

The slides here.

Bon Voyage, Planck

2009-05-20T04:44:00.010+01:00

The rumors of new imminent delays at the LHC imply that this year we need to look for action elsewhere. Fortunately, experimental astroparticle physics is truly enjoyable these days. FERMI and PAMELA are probing high energy cosmic rays, and there is still a plenty of hope that telltale signals of dark matter will be uncovered. That celebrity couple may soon be outshined by another competitor at shorter wavelengths - the Planck cosmic microwave background observatory. Contrary to my fears, the Ariane rocket which carried Planck into space was not shot down by an Imperial cruiser. Planck is now on its way to the L2 point and in one and half year or so we will bath in a plenty of new precise cosmological data.

Planck is the third in a row, after COBE and WMAP, to chase after small anisotropies of the CMB. At first sight the mission comes close to Lord Kelvin's nightmare: Planck will measure what its predecessors have measured, but more precisely, with better resolution, and in more colors. From the propaganda plot on the right one can see that one practical virtue of Planck is the access to higher multipoles of the CMB temperature anisotropy. Probing more acoustic peaks and the damping tail will allow us to precisely determine the cosmological parameters and put the currently ruling Lambda-CDM cosmological model to a thorough test.

Doesn't sound too exciting? Of course, there is always a good chance that something unexpected will emerge from the data. However, I'm going to argue that even confirming the boring cosmological standard model may provide us with extremely interesting pieces of information. In particular, there is one important question to which Planck, with a little bit of luck, may provide an answer: what is the scale of inflation?

The past missions have collected some shreds and pieces of information about inflation. First of all, we know that the highly primitive model of inflation - a single scalar field slowly rolling down its potential - perfectly describes all available data. That is to say, the power spectrum of the primordial density fluctuations that ultimately produced the CMB temperature fluctuations can be explained by quantum fluctuations of that scalar field. Furthermore, we know something about the potential that provides for vacuum energy driving the accelerated expansion during inflation. In particular, the overall scale of the potential can be inferred from the amplitude of the temperature fluctuations observed by COBE and WMAP. This yields
$(V/\epsilon)^{1/4} \sim 3 \cdot 10^{16}$ GeV
where $\epsilon = (V'/V)^2/2M_{Pl}^2$ is one of the so-called slow-roll parameters. The slow-roll parameters must be small during inflation, of order 0.01 or less, which sets the upper bound on the scale of inflation. But, in principle, there's no lower limit on $\epsilon$, and at this point we cannot make a definitive statement about the magnitude of V.

Planck has a good chance to ultimately pinpoint the scale of inflation. The hopes are based on Planck's fantastic ability to measure the CMB polarization. Thomson scattering at the last scattering surface results in linear polarization of the CMB photons. The polarization can be decomposed into the E-mode (gradient) and the B-mode (curl), each of which is then decomposed into multipoles, much as the temperature fluctuations. The lower multipoles of the E-mode have already been detected by WMAP; the B-mode is more tricky and it is waiting for Planck.

The importance of the B-mode follows from the fact that, at the linear level, it is not produced by scalar density perturbations, but only by tensor perturbations, that is by the primordial gravity waves. The amount of tensor perturbations is directly related to the scale of the inflationary potential. The larger V, the higher is the ratio of tensor to scalar primoridial perturbations. As an example, Planck's sensitivity to the primordial B-mode for the tensor-to-scalar ratio = .1 is plotted on the right. If the tensor-to-scalar ratio is high enough for Planck to detect the primordial B-mode, then we will have the first evidence of the existence of a very high-energy scale in particle physics. (who said neutrinos? it's not certain if they're really Majorana, and besides who cares about neutrinos anyway).

But of course the tensor-to-scalar ratio can be too small for Planck to measure. In the worst case scenario Planck may share the tragic fate of LEP: a successful experiment without much success. Let's cross our fingers.

More info in Planck Bluebook.

All eyes on Denver

2009-05-01T20:11:00.013+01:00

Tomorrow (Saturday) morning, the FERMI/GLAST collaboration is going to announce their first results at the APS meeting in Denver. FERMI/GLAST is a satellite cosmic gamma-ray observatory, but it also has capabilities to measure the electron+positron spectrum. The latter is eagerly awaited by the particle physics community. Last year, the measurements of the cosmic ray positron fraction by PAMELA and of the combined electron+positron flux by ATIC have sparked some 150 theory papers and one paparazzi affair. Recall that PAMELA sees an excess of positrons over the background (whatever the background means) in the 10-100 GeV range, while ATIC claims there is a clear bump in the spectrum at around 700 GeV. One tantalizing interpretation of these data is that the excess positrons originate from annihilation or decay of TeV scale dark matter particles.

If you can't wait till tomorrow have a look at the plot extracted from a theory paper of two weeks ago. The solid black line on that plot by sheer accident reproduces pretty well the FERMI/GLAST data to come. The sexy ATIC bump is gone and is replaced with milder features: a shallow deep around 100 GeV followed by a mild rise toward 800 GeV, and then a steep decline consistent with the earlier HESS measurements. These new results neither exclude (there's still an excess) nor significantly support the dark matter cause (there's no smoking gun features). Next week arXiv will be flooded with papers refitting the earlier theoretical models to the new data.

Update: FERMI/GLAST has revealed the new measurement of the e+e- cosmic ray spectrum but the plot is not available yet - it will be published coming Monday. According to those who saw Denver's talk, the spectrum is indeed similar to the one plotted above, although the low energy (20-80 GeV) data points lie slightly below the background curve and the dip is even less pronounced. Also, FERMI's data stop at 1 TeV so the high-energy decline cannot be clearly seen. So at this point everything is clear: it's either dark matter or a pulsar or an alien civilization or maybe the galactic propagation model needs refining ;-). More insight should come from FERMI's measurement of the diffuse photon spectrum which is expected by late summer.

Update 2: and here is the original plot from Fermi's today paper on arXiv:
Also HESS got its foot in the door and just published new results for the electron+positron flux above 340 GeV, consistent with those of FERMI and inconsistent with ATIC.

Higgs Was At LEP

2009-05-01T05:58:00.007+01:00

Everybody knows that the LEP experiment set a stringent limit on the Higgs boson mass - it has be larger than 114.4 GeV. The common expectation is that Higgs is just around the corner, and will be hunted down and roasted alive at the LHC or at the Tevatron. But this is not the only conceivable scenario that future may unfold: the world can be Higgsless, or the Higgs may be too wide or too invisible or too whatever to be detected at a hadron collider. There is yet another possibility that is definitely a bit crazy but is nevertheless not completely excluded. Namely, it is possible that the Higgs is lighter than 115 GeV and therefore kinematically available at LEP but ... we missed it.

How could Higgs have been missed? The point is that the 114.4 GeV limit strictly applies to a particle that walks, talks and couples just like the Standard Model Higgs boson. If we meddle with the Higgs couplings then, with a bit of skill, we can make Higgs effectively invisible to LEP. One obvious way to achieve that is to suppress the production rate. At LEP, Higgs would be dominantly produced by the process petnamed Higgsstrahlung where the e+e- collision first produces a Z boson which then radiates a Higgs boson. The LEP limits can be relaxed by suppressing the Higgs-Z-Z vertex by a factor of 3-4, see the black line on the plot below. However, this is not a theoretically plausible direction, as the electroweak precision observables suggest the existence of a light Higgs particle whose coupling to W and Z bosons is not suppressed. Besides, on the more philosophical side, a particle with a reduced coupling to Z should not be called Higgs (but rather, a scalar particle that slightly mixes with the Higgs). So let's leave the Higgs-Z-Z vertex alone. In that case we can still try to hide the Higgs from LEP by meddling with the Higgs decays.

The LEP collaboration was not that stupid and they also searched for the Higgs decaying in a non-standard way. You could think that Higgs could be hidden by making it invisible, that is to say, it could decay to some light, almost non-interacting particles that leave the detector undetected. This does not work: the signature involving a Z boson plus missing energy (carried out by the invisible stuff) is not easy to miss in a lepton collider, and in consequence the limit on the invisible is 114 GeV, almost as strong as that on the standard Higgs. Thus, paradoxically, to make Higgs invisible one must make it decay into something visible. LEP has concluded the following:

Higgs decaying to a pair of jets of any flavor (rather than dominantly into b-jets as the standard Higgs) has to be heavier than 113 GeV
Fermiophobic Higgs decaying dominantly to off-shell WW and ZZ has to be heavier than 110 GeV
Higgs decaying dominantly into two photons has to be heavier than 117 GeV

All in all, Higgs lighter than 110 GeV decaying into a two-body final state is excluded. But the situation is far less clear if the final state contains more particles. For example, the Higgs can undergo a cascade decay: it first decays into a pair of light scalars or pseudoscalars which subsequently decay into a pair of quarks or leptons each. In that case we deal with a four-body final state, for example with four b-quarks or four tau-leptons (typically, the pseudoscalars decays into the heaviest quark or lepton that is kinematically available). This is of course impossible in the Standard Model, while in the MSSM it occurs only in an obscure corner of the parameter space. But in several popular extensions of the Standard Model, for example in the NMSSM (MSSM adorned by a singlet superfield) or in little Higgs theories such cascade decays appear often and willingly.

The possibility of avoiding the LEP bounds via the cascade decays was first pointed out by Radovan Dermisek and Jack Gunion in the context of NMMSM. In that model, there are new pseudoscalar states in the Higgs sector which can naturally be light and to which the true Higgs (the one that couples to Z with the largest strength) can decay. These pseudoscalars then decay into a pair of b quarks each, or into tau quarks if the pseudoscalar is lighter than twice the b-quark mass. The former possibility was excluded by a subsequent LEP analysis - the limit on the Higgs decaying into four b-jets is now 110 GeV - but the four-tau or the four-light-jet final states allow for a much lighter Higgs particle. See the exclusion limits for the case of four-tau cascade decay - the allowed region on this plot is almost non-existing but there is no limit above the Higgs mass of 85 GeV. The reason why that analysis stopped at 85 GeV is not physical but psychological: in the MSSM there is no parameter space that would allow to consider Higgs heavier than 85 GeV. This is a clinical case of the damage that happens when experimenters take theorists and their theories too seriously (following this logic, if the MSSM did not allow for a light Higgs one could completely skip the LEP experiment).

Hiding the Higgs is a nice prank in itself, but there are also some theoretical and phenomenological motivations for playing this game. Firstly, the electroweak precision observables are best fitted by a fairly light Higgs mass with the central value of order 80 GeV,
and the light Higgs of 90-100 GeV would alleviate the tension. Secondly, LEP saw a 2.3 sigma excess of Higgs-like bbar events around the mass of 100 GeV. That cannot be interpreted as the standard Higgs (the number of events would have been five times much higher), but can be perfectly explained by the Higgs decaying most of the times into four light quarks or leptons and one fifth of the times into the b quarks. Recall that in the final year of LEP a smaller excess created much larger theoretical activity.

Of course, a light elusive Higgs is a nightmare for the LHC. Fortunately, theories that motivate such a scenario typically predict a lot of new phenomena at the TeV scale to provide enough fun for the LHC experiment. Just that some people will have to wait a bit longer for their Nobel prize.

Here is the review of the non-standard Higgs decays.

Inelastic

2009-04-23T03:10:00.008+01:00

As for today, only the DAMA experiment in Gran Sasso claims to have detected dark matter particles. The claim is based on observing the annual modulation of the number of scattering events in DAMA's sodium-iodine detector. Such an effect could arise due the motion of the Earth around the Sun that implies the annual variation of the Earth's velocity with respect to the sea of dark matter pervading our galaxy.

The experimental community is divided about DAMA. One half considers them ignorants who have no idea what they're doing, whereas the other half thinks that they deliberately rigged their results. Theorists, on the other hand, are by construction more open-minded (or maybe just bored) and they sometimes entertain the possibility that the DAMA signal might actually be dark matter. The challenge is then to explain why other, in principle more sensitive detection techniques have yielded null results. There has been several, less or more contrived proposals to reconcile DAMA with the stringent limits from other direct detection experiments like CDMS, XENON, CRESST, ZEPLIN and KIMS. The DAMA signal can be explained by the standard WIMP dark mater scattering on the sodium atoms if the dark matter particle has a fairly small mass of order 5 GeV (although there is some controversy about this interpretation). This post is about another scenario called inelastic dark matter, iDM in short. It was originally proposed quite some time ago, but recently it is becoming more and more fashionable.

A typical WIMP particle scatters elastically on the target nucleons, that is to say, it retains its identity in the process. In the iDM scenario, on the other hand, the cross section for elastic scattering is assumed to be suppressed. Instead, the dark matter particle scatters inelastically into a slightly heavier partner. If the mass splitting between the two dark matter particles is of order 100 keV - the typical kinetic energy in the dark matter sea - the DAMA signal can be, with a bit of luck, reconciled with the bounds from other experiments.

The way it works is the following. All direct detection experiments attempt to measure the recoil energy of a nucleon that has been hit by a passing dark matter particle. In the iDM scenario, the minimal velocity of the incoming dark matter particle needed to produce the recoil $E_R$ is given by the formula
$v_{min} = \frac{\delta+ m_N E_R/\mu_N }{\sqrt{2 m_N E_R}}$,
where $\mu_N$ is the reduced mass of the dark matter + nucleon system and $\delta$ is the mass splitting between the two dark matter states. As long as the splitting term dominates, heavier targets require lower velocity to give them a kick. DAMA's target contains pretty heavy iodine (A=127) (as compared to CDMS germanium with A=73). The sea of dark matter is expected to have the Maxwellian distribution of velocities that rapidly fall above the peak velocity which is of order $v \sim 0.001$, so that even a small change of the minimal velocity may significantly affect the number of events. Also for that reason, the oscillation signal studied by DAMA is enhanced, because the small summer/winter variation of the dark matter velocity distribution (in the Earth reference frame) may lead to a large variation of the signal. All in all, there remains some allowed parameter space, as can be seen in the example plot borrowed from this paper. For a fixed dark matter mass, the DAMA region in the mass splitting - cross section plane is marked in magenta, while black lines are the current bounds, the most stringent coming from CDMS (solid) and CRESST (dashed).

There is also a purely sociological reason why the bounds from other experiments get relaxed: iDM has not really been searched for...The nature of iDM leads to a very peculiar nucleon recoil spectrum. Whereas for the standard WIMP the number of events grows exponentially at low recoil energies, the recoil spectrum in the iDM scenario is suppressed at low energies and displays a "resonant" shape. Most experiments derive their bounds assuming the standard recoil spectrum and they do not optimize their search strategies to probe non-standard scenarios. For this reason, the idea of iDM is relevant for dark matter searches irrespectively of DAMA. It is a phenomenologically distinct possibility that should be taken into account, and one may easily miss the Nobel prize by restricting to the standard WIMP paradigm.

From the theoretical point of view, models of iDM are not difficult to write down. One simple possibility is the dark matter particle being a Dirac fermion with a large mass of order 100 GeV spiced up by a small 100 keV Majorana mass. The later leads to the required splitting between the two Majorana mass eigenstates. Furthermore, if the Dirac fermion has vector interactions the vector couples non-diagonally in the eigenstate basis, and the elastic scattering is suppressed with respect to the inelastic one. Another simple realization of iDM is a complex scalar whose two real components are split by a small "holomorphic" mass term. There is no obstacles to embed iDM into mainstream theories beyond the Standard Model. For example, in the MSSM, the Standard Model neutrino is partnered by a sneutrino who is a complex scalar, and the mass splitting could originate from a small lepton-violating term $(L H)^2$ in the superpotential.

So, just keep our fingers crossed while waiting for the new results from CRESST, XENON-100, LUX, KIMS and many others.

See also this post on Dirac Sea.

Inconstant

2009-04-10T02:16:00.006+01:00

The funniest April Fools prank was definitely the one about time variation of $\pi$. That idea is of course absurd because the Bible unambiguously sets the value of $\pi$ to be equal three. But the physical constants like the QCD scale or the Fermi constant are not mentioned in the Bible which suggests that they might not be constants. Recently, Harald Fritzsch put on ArXiv a neat status report of various theoretical and experimental pursuits of varying fundamental constants.

For almost a century the idea of varying fundamental constants has been attracting most brilliant minds and complete crackpots alike. At the theoretical level the mechanism is easy to imagine: the physical constants can be set by a vacuum expectation value of a scalar field that evolves on cosmological timescales. In high-energy theory we already have one evolving scalar field for inflation and sometimes another one for quintessence, so that introducing yet another one for varying constants is not that difficult to swallow.

At the beginning of this century the idea has received renewed attention due to some experimental claims that the electromagnetic constant $\alpha$ may vary in time. A group of astrophysicists studying absorption spectra of very distant quasars concluded that 10 billion years ago $\alpha$ was smaller than today by $\Delta \alpha/\alpha \sim 10^{-5}$, corresponding to a time variation of order $10^{-15}$ per year. This claim is very controversial because of various assumptions involved in the determination $\alpha$ and, most of all, because other groups did not confirm this result. A more recent claim that the proton-to-electron mass ratio was different 10 billion years ago also remains highly controversial.

Yet another reason why the above claims are taken with a huge grain of salt is that the so-called Oklo bounds imply a slower variation of $\alpha$. 2 billion years ago, when the Earth was young and beautiful, the uranium-235 isotope was five times more abundant than today. Thanks to that fact and some other lucky coincidences, near the river Oklo in today's Gabon nature could create a fully organic nuclear reactor which operated for 100 million years. The uranium fission produced many rare isotopes, and the particular ratio of Samarium-149 to Samarium-147 can be used to constrain variation of the fundamental constants. The point is that the cross-section for the neutron capture on Samarium 149 is accidentally enhanced by a presence of resonance just 0.1 eV above the threshold. From the fact that the position of this resonance could not migrate by more than 0.1 eV one can set the bound $\Delta \alpha/\alpha \sim 10^{-7}$ (assuming that only the electromagnetic constant is varied) corresponding to a time variation $10^{-16}$ per year. If $\alpha$ was changing faster than that (as suggested by some astrophysical results) it had to stabilize at least two billion years ago.

In the neat future there is hope for more progress from precision measurements in a controlled laboratory environment. Experiments in quantum optics have recently reached a similar sensitivity to varying constants as the astrophysical observations. In particular, Theodor Haensch's group in Munich is running an experiment that studies time variation of the frequency of the 1s-2s transition in atom hydrogen (review here). The measurements from different years are related to the hyperfine transitions of Cesium-133 and to another precision measurement of quadrupole transitions in Mercury, which allows them to constrain the variation of both the electromagnetic constant and the QCD scale. The results published several years ago constrain the variation of both at the level of few times $10^{-15}$ per year.

Actually, Harald Fritzsch is spreading wild rumors that the most recent results from Munich imply the time variation of the QCD scale at the level of $3 \cdot 10^{-15}$ per year. Well, I'd rather bet that at the end of the day the constants will once more turn out to be constants. But who knows...in the end the Hubble constant has changed since the nucleosynthesis by some 17 orders of magnitude.

Dark Matter more like Baryons

2009-04-02T04:31:00.002+01:00

April Fools is over; I'm staying dead serious for the rest of the year. The most serious things in the months before the first LHC results are dark matter searches high and low. Here is another idea what they might find.

The most popular scenario for dark matter assumes that it consists of weakly interacting massive particles (WIMPs) who were once in thermal equilibrium. In the early hot and dense universe such a particle can efficiently annihilate into familiar particles like photon or electrons, and in this way dark matter is kept in equilibrium with the the rest of the cosmic plasma. The equilibrium ceases to hold when the temperature T of the universe falls below the dark matter particle mass M. In that regime, the number of dark matter particle very quickly decreases - as an exponential $e^{-M/T}$ - and at some point dark matter freezes out: there isn't enough dark matter particles around that they could find each other and annihilate. The surviving particles float around in the universe playing hide and seek with astronomers and physicists alike.

The WIMP scenario is nice and robust but it sheds little light on the surprising fact that the present abundanceof dark matter $\Omega_{DM}$ is very close to that of the ordinary matter who is today dominated by the baryon (proton and neutrons) abundance $\Omega_{B}$. After WMAP data we are confident that the ratio $\Omega_{DM}/\Omega_B$ is roughly five. Of course, one can always cook up the parameters of the WIMP model such that this constraint is satisfied, but nevertheless the proximity of $\Omega_B$ and $\Omega_{DM}$ is intriguing. It may suggests that baryons and dark matter have a common origin. But baryons are definitely NOT a cold relic!

In fact, we don't know for sure what is the origin of baryons in our universe but we have a bunch of ideas that go under the name of baryogenesis. The general idea is that the very early universe contains an equal number of baryons and antibaryons, but at some point in its evolution the fundamental interactions in the plasma produce a tiny $10^{-10}$ asymmetry between matter and antimatter. Once the temperature falls below the baryon mass most of the baryons and anti-baryons annihilate with each other and turn into the sea of photons, leaving only the small unpaired $10^{-10}$ fraction of baryons. These are the protons and neutrons that make galaxies and stars today.

Is it conceivable that dark matter originates in a similar fashion? That is to say, the early universe contains dark matter and anti dark matter particles which almost completely annihilate away leaving only the small asymmetric fraction? Can the dark asymmetry and the baryon asymmetry have the common origin? The answer to these questions is yes, and the first practical realization I'm aware of is due to David B. Kaplan in early nineties. In that model, the dark matter particle carries a charge under an additional U(1) global symmetry who, much as the U(1) baryon symmetry, has a mixed anomaly with the electroweak SU(2) gauge symmetry. Because of the anomalies, non-perturbative electroweak interactions that are effective in the early universe violate both the baryon number and the dark matter number. Then, if some conditions are satisfied, the electroweak phase transition generates the baryon and the dark asymmetries roughly of the same order. At the end of the day on obtains the relation $\Omega_{DM}/\Omega_B \sim m_{DM}/m_{proton}$ and the experimentally measured ratio is recovered if the dark matter particle's mass is around 5 GeV.

Kaplan's original model is long gone for several reasons, but the idea is still floating in the backchannels of model building. The most recent approach in the context of supersymmetry was made by David E. Kaplan et al (David E. Kaplan is a more recent version of David B. Kaplan with more features). In that model, there's no new quantum number invented especially for the dark matter particle; instead, it carries the lepton (or baryon in another version) quantum number. Furthermore, the model does not rely on electroweak baryogenesis but rather it assumes that the the B - L asymmetry is generated at high energies (for example by leptogenesis). That asymmetry is later redistributed between baryons and dark matter by higher-dimensional interactions. When these interactions fall out of equilibrium, dark matter asymmetry is frozen in and one again ends up with $\Omega_{DM}/\Omega_B \sim m_{DM}/ m_{proton}$. All that remains is to find 5-15 GeV dark matter in the sky, or in colliders or by direct detection...

No Higgs particle after all?

2009-04-01T05:17:00.008+01:00

While the LHC, after initial difficulties, is on the straight path for first collisions the particle physics community has been given yet another bitter pill to swallow. Yesterday at CERN Peter Higgs gave a seminar whose message was really shocking. No more no less, Higgs demonstrated that the famous particle that carries his name cannot exist! He pointed out a mistake in his original '64 paper which did not take into account the topological anomalies of the symmetry group of the Standard Model. He also presented a concise and elegant proof that the existence of the Higgs particle would lead to an instability of the vacuum.

In reply to angry voices from the audience, Higgs said:

Yes i got it wrong back then. I mean, i got it right at first, but then the referee confused me and I added the particle to get the paper published quickly.

Why did Higgs wait more than 40 years to correct his mistake? He explained:

You know, I was flattered - not everybody has his own particle. I thought that sooner or later someone else would point out the mistake anyway. But now there's so much talking of that particle at the LHC that I had to come out to prevent greater disappointment.

Cern theorist John Ellis commented:

This seems unbelievable, but the math is there on the blackboard... I'm afraid Higgs is right this time. The whole story demonstrates how important is to independently verify scientific results rather than to follow fashion. We have learned our lesson.

But is it not too late? If the Higgs particle is not out there, does this make the 5 billion worth LHC accelarator a useless toy? CERN director general Rolf Heuer carefully chooses his words:

One can never predict the course of scientific developments . Important discoveries may arise as side effects of the LHC program, as it happened before with the World Wide Web.

But even in these grim circumstances there are some who see the glass half-full. As someone from the audience pointed out:

At least the Tevatron won't have it either...

Happy April Fools everyone :-)

Dr Jekyll and Mr Higgs

2009-02-28T16:54:00.014+01:00

There has been recently a lot of excitement around the world concerning the race for the Higgs. Fermilab now claims that they have 50-50 chance to get a 3 sigma Higgs signal before the LHC, thus providing a modern reenactment of Aesop's The tortoise and the hare. As for me, the prospective discovery itself, although eagerly awaited, is not actually that thrilling. There are compelling theoretical arguments that Higgs does exist (he was recently spotted in Edinburgh opera). Moreover, experiment directly constrains the Higgs mass to be larger than 115 GeV and, indirectly, smaller than some 150 GeV. This leaves a narrow ballpark making the situation somewhat similar to the top-quark discovery. The more exciting question is *which* Higgs will we find. Yes, the God particle has numerous incarnations that answer different prayers of theorists. Here is a brief summary of the most popular Higgs avatars.

Standard Higgs

A perfect guy, to the point of being boring. He does everything he is supposed to do, and perfectly matches all experimental results so far (apart from the small tension with electroweak precision tests). We know everything about him except for the mass. It is believed, however, that left alone and unprotected he would acquire a large mass. This purely theoretical argument prompts most of what follows.

Susy Higgs

The marriage of Susy and Higgs has lasted for more than 30 years. Susy provides stabilization to Higgs, keeping its mass small enough. Sadly enough, bad tongues and the LEP experiment have left deep scars on this relationship. The problem is that the minimal supersymmetric model ties the Higgs boson mass to the Z boson mass. The failure to discover Higgs LEP implies that the parameters of the minimal model must be finely-tuned in order to accommodate the higher mass, thus spoiling the naturalness of the whole construction.

Composite Higgs

Higgs does not have to be that elementary - it is natural to imagine that Higgs is a bound state like many other particles we have observed. For example, it could be a meson made of new quarks glued together by new strong interactions. The problem with this idea is that a simple back-of-a-napkin estimate suggests that the Higgs mass should not be much different from the scale of the new strong interactions. Since we have seen nothing like that up to a few hundreds of GeV, the mass of the composite Higgs would have to be larger, contrary to what electroweak precision tests seem to tell us. Or there must be some more structure that keeps the mass light enough...

Pseudo-Goldstone Higgs

Susy does not have exclusive rights on controlling quantum corrections to the Higgs mass. Particle's masses can also be protected by spontaneously broken global symmetries. A similar mechanism operates in real life and was awarded a Nobel prize last year: thanks to that mechanism the QCD pions remain lighter than the QCD scale. The Higgs boson could also arise as a pseudo-Goldstone boson when a new strong dynamics spontaneously breaks its own global symmetries. But at the end of the day this simple idea does not work as well as it is supposed to. First, the name is unattractive and difficult to pronounciate (worse still, around Chicago it becomes a pseudo-Nambu-Goldstone-boson-Higgs monster). Besides, the new strong interactions meddle with electroweak precision observables, and at the end of the day the fine-tuning is only slightly better than in minimal supersymmetry.

Little Higgs

Little Higgs is a variation on the theme of the pseudo-Goldstone Higgs. The new strong interactions are pushed to higher scales, around 10 TeV, while an additional structure - the so-called collective symmetry breaking - protects that scale separation. While this idea can be made completely realistic and the fine-tuning of parameters can be acceptably small, fully realistic constructions are situated somewhere between late baroque and early racoco.

Fat Higgs

Not that he's very pretty, but he has a cool name. This one combines the ideas of composite Higgs and supersymmetry. A strongly coupled Susy gauge theory sits in the conformal window all the way down till the TeV scale. At that point, due to the fact that some of the flavors have TeV scale masses, the theory drops out of the window and confines. The challenge is make all the numbers work and get rid of all the excess bagagge that comes along.

Invisible Higgs

Could it be that Higgs was at LEP but we missed it? Actually, in models with additional singlet fields it is common that Higgs decays into exotic particles that escape from the detector without being seen. Such a cheap trick would not fool LEP, however, and invisible Higgs is just as well constrained as the standard one. Nevertheless, one can devise more complicated models where Higgs is partly invisible and hides from LEP analyses even though his mass is below 115 GeV.

Unhiggs

Every kid has to go through a negation phase at some point. It may be that Higgs is neither a god nor a particle after all. Instead, it could be a fuzzy continuum of excitations and still perfectly fulfill its role.

Higgsless

Finally one should mention that Higgs might not exist. This athehigsm has some scientific support. Electroweak symmetry can be broken by a condensate in a strongly interacting theory, much as it happens to chiral symmetries in QCD. In that case Higgs is expandable, and his role is played by new resonances whose spin is one rather than zero. That is not as bad as it seems since these new resonances must have masses within the LHC reach to make the picture consistent. Higgsless theories are disfavored by electroweak precision and flavor tests, but the ultimate answer will be given by the LHC. Unless reality is Unhiggsless.

WHO IS GOING TO WIN THE RACE? WILL IT BE HIGGS-THE-PERFECT-BORING-GUY? OR HIGGS' LOVE FOR SUSY WILL OVERCOME THE OBSTACLES? OR MAYBE SOMEONE ELSE WILL MEDDLE IN THE RACE? STAY TUNED FOR THE NEXT EPISODES. To definitely nail down the nature of Higgs we'll probably need to wait for future linear colliders, but some partial answers should be provided in two years from now, if all goes well.