The new particles
1974 has been one of the most fascinating years ever experienced in high-energy physics. Anyone in touch will be well aware of the ferment created by the recent news from Brookhaven and Stanford, followed by Frascati and DESY, of the existence of new particles. Why the excitement? A brief answer is that the particles have been found in a mass region where they were completely unexpected with stability properties which, at this stage, are inexplicable.
Since spring, a MIT/Brookhaven team, led by Sam Ting, has been looking at collisions between protons which yield an electron–positron pair. They use a slow ejected 28.5 GeV proton beam from the Brookhaven 33 GeV synchrotron to bombard beryllium. The probability that collisions will yield such a pair is very low and the detection system has to be capable of picking out an event from a million or more.
From about August, the system was totting up an unusually large number of events with a combined electron–positron energy of 3.1 GeV. They were on to something important – a resonance, an unstable particle which breaks up too quickly to be seen, its mass identified by the combined energy of more stable particles emerging from its decay. By the end of October, they had collected about 500 events but were soon prodded into print by dramatic news from the other coast of America.
In June, a Berkeley/Stanford team at the Stanford Linear Accelerator Center electron–positron storage ring SPEAR had seen some “funny” readings at collision energies between 3.1 and 3.2 GeV. While meditating during the summer transformation of SPEAR I into SPEAR II, the suspicion grew that a resonance could lie at these energies. Following the upgrade, the team went into action and on the weekend of 9–10 November the hunt began, changing the beam energies in 0.5 MeV steps. By 11.00 a.m. Sunday morning the new particle had been unequivocally found. A jump in cross-section from 20 to 200 nanobarns soared to 2000 nanobarns as the data were refined. It was nothing short of shattering. Burt Richter described it as “the most exciting and frantic week-end in particle physics I have ever been through”.
Within hours of the SPEAR measurements, telephone wires across the Atlantic were humming as enquiries and rumours were exchanged. As soon as it became clear what had happened, European Laboratories looked to see how they could contribute to the excitement. Obvious candidates to be in on the act quickly were the electron–positron storage rings at Frascati and DESY.
From 13 November, three experimental teams on the ADONE storage ring at Frascati began to search in the same energy region and on 15 November the new particle was seen by all three. At DESY, the DORIS storage ring was brought into action with the PLUTO and DASP detection systems. During the weekend of 23–24 November, a clear signal at about 3.1 GeV energy was seen in both systems.
For the past year, something has been expected in the hadron–lepton relationship. Are the new particles behind this and if so, how? Do they carry a new quantum number? Theorists have recently invoked two new properties that could influence which interactions can take place – colour and charm. Colour is suggested as a 3-valued property of quarks, the constituents of hadrons, to make sense of the statistics used to calculate the consequences of their existence. Charm is a property suggested to explain some observations concerning neutral current interactions.
Still reeling from the 1973 discovery of neutral currents, 1974 began with the SPEAR hadron production mystery, continued with new high-energy information from Fermilab and the CERN ISR, including the high lepton production rate, and finished with the discovery of new particles. All against a background of feverish theoretical activity trying to keep pace with what the accelerators and storage rings have been uncovering.
• Compiled from texts on pp415–419.
As noted in last month’s Courier (p50), charm was the property attributed to the 1974 particles. The J/ψ – J at BNL, ψ at SLAC, earning the 1976 Nobel Prize in Physics for Ting and Richter – was declared to be a charmed quark–antiquark meson, completing a second family of matter particles. Though not known at the time, one and only one family remained to be discovered. The bottom quark, with a mass around 4 GeV, was found at Fermilab in 1977 (CERN Courier June 2017 p18), compelling physicists to search for its partner. But the top didn’t materialise until 1995, when Fermilab’s Tevatron energies were sufficient to create this astonishingly heavy quark. Weighing in around 173 GeV, its mass resembles that of a gold nucleus containing 197 protons and neutrons.