Post a Comment On: Backreaction

When a beam of electrons is brought to collision with a beam of positrons in a particle accelerator, different things can happen: The electron (e-) and its antiparticle, the positron (e+), can just bounce off each other, or they can annihilate, and the energy then set free can, if high enough, materialise in new particles. For example, a muon (μ-, a massive cousin of the electron) and an antimuon (μ+) can be created, or one or more hadrons, such as pions.

To measure the probability of these different possible events, one can imagine the particles rushing onto each other as small disks - the larger the area of the disk, the higher the probability that two particles will hit each other and something will happen. The area of these imaginary disks is called the cross-section σ.

This figure shows the cross-sections in electron-positron collisions for the different possibilities mentioned before (from bottom to top: e+e- scattering, μ+μ- production, and hadron production) as a function of the centre-of-mass energy of the colliding electron-positron pair:

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Source: Burton Richter's Nobel Lecture From the Psi to Charm – The Experiments of 1975 and 1976, The Nobel Foundation 1976.

Energy, on the horizontal axis, is measured in billion electron volt (Giga electron volt, GeV) - this is quite a lot for electrons: the rest mass of an electron is only 0.00051 GeV. The cross-section on the vertical axis is measured in a unit of area with the funny name nanobarn (nb) - that's one billionth barn, or 10-37 m2. Note that the cross-section is shown in a logarithmic scale to allow the representation of large changes.

There is a marked bump - technically called a resonance - in the cross-section at an energy of 3.095 GeV. The resonance is visible in all three signals, but most prominent in the hadron production, which shows a sharp increase by a factor 100 with increasing energy. Obviously, something special is happening at this energy: The bump, which came as quite a big surprise in the experiment that took the data, marks the creation of a new particle.

The group of Burton Richter at the Stanford Linear Accelerator Center SLAC, who measured the data shown in the figure, called it the Ψ particle. At about the same time in November 1974, the team of Samuel Ting at the Brookhaven National Laboratory BNL found the same signal. At BNL, they dubbed the particle the J.

Today, it is known as the J/Ψ - it's a meson with a mass of 3.097 GeV, and it is made up of a charm quark and an anticharm quark - a fourth quark flavour theoretically conjectured already before 1974, but not yet detected then in experiment. Richter and Ting shared the Nobel Prize in Physics 1976 for their discovery of the bump in the cross-section shown in the figure above. And the J/Ψ has remained a particle which has been studied intensively ever since- for example to improve our understanding of the forces between quarks, or as a probe particle to contribute to the mapping of the phase diagram of nuclear matter.


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Computer reconstruction of the decay of a Ψ', an excited state of the J/Ψ, as measured in the Mark I detector at SLAC in 1974 - by a beautiful coincidence, it's just the Greek letter Psi. (Source: SLAC slide747).



The groups of Ting and Richter reported their discoveries in the same issue of the Physical Review Letters in December 1974: J. J. Aubert et al.: Experimental Observation of a Heavy Particle J, Phys. Rev. Lett. 33 (1974) 1404, and J.-E. Augustin et al.: Discovery of a Narrow Resonance in e+e- Annihilation, Phys. Rev. Lett. 33 (1974) 1406 (subscription required).

SLAC has dedicated a web site to the "November Revolution in Physics", when "two separate experiments at SLAC and at Brookhaven independently discovered the first of a new set of particle states, the J/Psi particle", giving more background and with a few nice historical photos.

For the discovery plot of the BNL group and the role of the J/Ψ as a representative of the fourth generation of quarks, see Tommaso's recent post Top quark: a short history - part I.


This post is part of our 2007 advent calendar A Plottl A Day.