News from Gargamelle
Having obtained evidence for new hadronic constituents [partons] by electromagnetic and strongly interacting probes, the question arises whether they also manifest themselves in weak interactions. Preliminary measurements of the total cross-sections for antineutrinos and neutrinos in the heavy-liquid bubble chamber Gargamelle show both cross-sections rising linearly with energy in the range from 1 to 9 GeV, with the ratio between them of one third. These results agree with models where the partons are quarks, and imply that nucleons consist of three quarks and some quark–antiquark pairs.
However, free quarks have not been found. Why do nucleons exhibit a granular structure but it is not possible to knock out one of these grains? In experiments at the ISR, where quark masses up to 22 GeV could be observed, no particles with the fractional charges predicted for quarks have been seen. Maybe their masses are beyond presently available energies. Or maybe they are just mathematically useful concepts in describing particle behaviour without correspondence in physical reality.
• Compiled from texts on pp3–5.
At the synchro-cyclotron
During the past year, the physics programme of the 600 MeV synchro-cyclotron (SC) has included 18 experiments (plus those at the isotope separator, ISOLDE) involving scientists from about 30 research centres. The experiments have mainly concerned nuclear physics, but several are studying aspects of particle physics and there is also a modest programme of radiobiology.
At ISOLDE, fed by the SC, short-lived “exotic” nuclei are studied. In the SC experimental areas there are related experiments, such as an Orsay investigation of fragmentation, which provides astrophysics with data on stellar and interstellar interactions, and a Cologne study using targets of lunar-rock composition to estimate the effect of cosmic rays on the surface of the Moon.
The energy of the SC was selected so that it would be capable of copious production of pions, and this has been used in a long sequence of experiments where pions have been fired at nuclei. As well as protons and pions, muons are also available. One of the muon experiments produces muonic atoms, where the muon becomes a satellite of the atom in the same way as an electron. When the muon is in the higher atomic energy levels, it is sensitive to any asymmetry in movement of protons in the nuclear volume and it has been possible to measure the quadrupole moment associated with the rotation of ellipsoidal, or cigar-shaped, nuclei.
The peak internal beam current is about 1.5 μA. The main aims of the improvement programme are to increase this by a factor of 10 and raise the extraction efficiency well above its present value of around 6%. This will be achieved by replacing several important machine components, including a “hooded-arc” ion source and new central electrodes with an r.f. system capable of providing voltages as high as 30 kV, a frequency modulation from 30 to 16.7 MHz, and an increase the SC pulse repetition rate from 55 to over 500 Hz.
The starting date of the shutdown to carry out these improvements is not yet fixed since the rotary condenser, the mechanical tuning element of the r.f. system, has not been fully tested. The Advisory Panel of physics users strongly recommends bringing the condenser to CERN for completion after a preliminary test at the manufacturers. If this does not reveal any major technical difficulties, the shutdown, expected to last for just under a year, could start before the middle of the year.
• Compiled from texts on pp34–36.
The Gargamelle chamber is exhibited in the Microcosm garden at CERN. In 1973/1974, Gargamelle provided existential evidence for Z bosons, neutral partners of the hypothesized charged weak-interaction bosons, W±, and detailed testing of the electroweak Standard Model became top priority, especially at CERN. In 1983, W and Z bosons were created in the proton–antiproton collider, in 1989 experiments at LEP restricted the number of matter “generations” to three, and in 2012 Higgs bosons, the unverified components of the model, were observed at the LHC.
The SC improvement programme was started in July 1973 and completed in January 1975. This versatile and venerable machine, CERN’s first accelerator, commissioned in 1957, delivered particle beams until its closure in 1990. It now offers a unique look into CERN’s history as part of the visits programme.