Around the Laboratories

Two views of a 540 GeV total energy proton-antiproton collision, as recorded in one of the big streamer chambers of the UA5 experiment at the CERN SPS.

CERN Antiprotons again

At the end of October, the CERN machines once more supplied anti- protons, and experiments at both the big SPS proton synchrotron and the Intersecting Storage Rings were able to record data from proton-antipro­ ton collisions. This time the big streamer chambers of the UA5 ex­ periment (Bonn / Brussels / Cam­ bridge / CERN / Stockholm collabora­ tion) came into action at the SPS (see June 1980 issue, page 148) and re­ corded some 2000 good examples of proton-antiproton collisions at 540 GeV total energy. Also in the SPS, the big UA1 experiment took more data to supplement that gath­ ered in its first run in August. How­ ever this time the U A1 central detec­ tor came into action for the first time, supplying detailed information on the interaction vertices. Initial physics in­ formation from these experiments readiness for increased antiproton Below left, a representation of the UA 1 could soon emerge. luminosities. apparatus used to study the energy flow

9 in 540 GeV proton-antiproton collisions at In the SPS, a bunch of some 10 Also during this run, the 'Roman the CERN SPS, showing the signals recorded 270 GeV antiprotons circulated pots' — drift chambers mounted in in the calorimeters. The central against two proton bunches, each movable sections of beam pipe — of electromagnetic calorimeters are enclosed by the large cubic instrumented return yoke containing about 5x1010 particles. the UA4 experiment took their first of the magnet. The forward detectors Thus both UA1 and UA5, in different samples of forward proton-antipro­ extend on either side in the SPS tunnel. regions of the ring, were able to stu­ ton elastic scattering at the SPS. This dy collisions in parallel. The SPS technique was first used in the early Below right, the signals picked up in the middle cell of the cylindrical central detector beams were twice stored overnight. days of the CERN ISR to measure which surrounds the beam pipe. These are In the near future it is hoped to test total reaction rates in the newly- the first tracks seen in the detector, with the new SPS low beta magnets in available energy range. The UA4 a small fraction of readout installed for the October run.

446 CERN Courier, December 1981 Layout of the ARGUS detector scheduled for the revamped DORIS-II electron-positron ring at DESY.

group (Amsterdam / CERN / Genoa / Naples / Pisa) soon hopes to provide a value for the total proton-antipro­ ton reaction rate at 540 GeV. While all this action was going on at the SPS, a two milliamp antiproton beam was quietly circulating in the ISR. The highly successful run was terminated on 2 November after the antiprotons had been kept for more than thirteen days! With a proton beam of 12 A, the proton-antiproton luminosity attained 9 x 1026 cm-2 s~1. As a result of the run, a new lower limit for the antiproton lifetime was established.

Baryons from quarks

According to our present picture of the deep inelastic interactions of high energy leptons, the incoming lepton penetrates into the interior of the struck nucleon and interacts with one or other of its constituent quarks. Both the struck quark (cur­ rent fragmentation) and the specta­ lower energies with electron beams, DESY tor quarks (target fragmentation) and could turn out to be related to the then produce sprays of hadronic unexpectedly high baryon yields New detector matter. In principle, the hadrons from seen in high energy electron-posi­ for DORIS-II the struck and spectator quarks tron annihilations at DESY's PETRA should be distinguishable, although ring. ARGUS is one of the two big detec­ this distinction only becomes really According to simple quark count­ tors which will be installed next clear at higher energies. Because ing rules, equal numbers of baryons spring at the revamped DORIS-II only one quark is assumed to be and antibaryons should be seen, electron-positron storage ring at struck per interaction, the current however the EMC data show an ex­ DESY (see November issue, p. 397). fragmentation would be expected to cess of protons over antiprotons. The second interaction region will be dominated by mesons (containing This is puzzling, but could be due to house the Crystal Ball detector from a quark and an antiquark), while ba­ the difficulty of separating the pro­ SPEAR. The improved running con­ ryons (three quarks) would be pro­ ducts of current and target fragmen­ ditions, in particular the higher lumi­ duced in the target fragmentation. tation, even at SPS energies. It could nosity expected at total energies up The European Muon Collaboration also be due to one of the mechan­ to 11.2 GeV, make DORIS-II an ex­ (EMC) experiment in the North Exper­ isms of current fragmentation. cellent tool for studying upsilon par­ imental Area of CERN's SPS 400 The EMC experiment is now em­ ticles and beauty mesons. However GeV proton synchrotron has studied ploying a streamer chamber as ver­ DORIS-II also covers the entire charm the interactions of 120 and 280 GeV tex detector together with more production region, down to the J/psi muons with protons and finds not sophisticated particle identification at 3.1 GeV. only mesons, but also protons and immediately downstream. The new ARGUS is a universal detector with antiprotons in the hadronic debris of apparatus will be described in detail magnetic momentum analysis of current fragmentation. This baryon in our January/February 1982 is­ charged particles, shower counters production had not been seen at sue. and muon identification covering

CERN Courier, December 1981 447