On Monday, December 2, 2024, it was exactly 50 years to the day since a single issue of Physical Review Letters (Volume 33, Number 23) published three articles
- Experimental Observation of a Heavy Particle J
- Discovery of a Narrow Resonance in e+e- Annihilation
- Preliminary Result of Frascati on the Nature of a New 3.1-GeV Particle Produced in e+e- Annihilation,
announcing a discovery that fundamentally impacted the further development of our understanding of the laws of the microworld. The first reached the editors on November 12, 1974, the second a day later, and the third on November 18. I will get back to this time sequence later.
What was discovered was the experimental proof of the existence of an unstable particle with spin 1 and a mass of 3.1 GeV, more than three times the mass of the proton, now called J/psi. This unusual name reflects the fact that the particle was discovered simultaneously in two different experiments.
The first paper describes how the particle was discovered in proton collisions with beryllium nuclei at the AGS accelerator at Brookhaven in the USA. A team led by Samuel Ting identified the particle in their decays to an electron-positron pair. In the second paper, a team led by Burton Richter identified the particle in electron-positron collisions at the SPEAR accelerator at SLAC in California. In this case, the particle appeared as a resonance in the energy dependence of the total cross section. Ting liked the letter J and Richter liked psi, which reminded him of what collisions looked in the detector.
The discovery of this particle came as a surprise, but it soon became clear that its properties closely matched those of the bound state of the fourth quark, which came to be called "charm", and its antiquark. While the latter was one of the options to extend the then well-established quark model of hadrons, no one expected it to be discovered in these particular processes. For the discovery of the J/psi particle, Samuel Ting and Burton Richter shared the Nobel Prize in Physics in 1976.
On the occasion of its 50th anniversary, I described the path to the formulation of the quark model and then to quantum chromodynamics in two articles:
Jak byly objeveny kvarky (How Quarks Were Discovered), Notes on the 50th Anniversary of the Formulation of the Quark Model, Part 1, Čs. čas. fyz. 64 (2014) 185,
Jak byly objeveny kvarky (How Quarks Were Discovered), Notes on the 50th Anniversary of the Formulation of the Quark Model, Part 2, Čs. čas. fyz. 64 (2014) 240.
In the articles, information can be found about what led Gell-Mann and Zweig to hypothesize the existence of quarks and how fundamentally their paths to and interpretations of the concept differed.
The fundamental significance of the discovery of the J/psi particle and its interpretation as a bound state of the fourth quark and its antiquark was that it provided convincing evidence that symmetry exists between quarks and leptons. By November 1974, four leptons (electron and electron neutrino and muon and muon neutrino) and three quarks (u, d, s) were known, each of which existed in three mutations called "colours". The fourth, "charm" quark, referred to as the c quark, is the partner of the s quark in the quark model, just as the u quark is the partner of the d quark.
Quarks (u, d) and an electron and its neutrino form one so-called generation of fundamental fermions, and quarks (c, s) and a muon and its neutrino a second generation. All these particles have the same spin ½. The path to finding experimental evidence for symmetry between quarks and leptons was not easy and took 10 years.
Symmetry between quarks and leptons also exists in today's Standard Model, which has been added a third generation of quarks (t, b) and leptons (tauon and tauon neutrino). Symmetry between quarks and leptons has solved several theoretical problems and is a starting point in some efforts to extend the Standard Model to include new forces and particles.
And now to the third paper, which arrived at Physical Review Letters six days later. There was considerable rivalry between Ting's and Richter's groups, even though they were investigating completely different processes, and there is evidence that they watched each other. There is also unconfirmed information about which of these groups saw the evidence confirming the new particle first, but the reality is that the two papers arrived at the journal within a few hours.
The group working at the Frascati Laboratory in Italy on the ADONE accelerator, which was very similar to the SPEAR accelerator at SLAC, was downright unlucky. ADONE was designed for a maximum collision energy of 3 GeV. Since the J/psi particle has a mass of 3.1 GeV and is very narrow, the measurement region did not allow for it to be recorded and the Frascati group could not detect it. But once the group heard about its discovery, they slightly increased the energy of the collisions and immediately detected the J/psi particle as well.