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Bigger and More Absorbent Bottom, comparison of proton-proton and proton-antiproton total cross-sections (a measure of the effective 'size' of the colliding particles) with increasing energy. The point on the right comes from the UA4 experiment at the CERN Collider. Top, comparison of elastic to total cross-sections. Together these results show that at Collider energies the particles become both larger and more absorbent. be accessed indirectly at electron- positron machines. First results to emerge from the experiment (announced at this sum­ 0,24 mer's conferences) concern the so- called chi states, where the char- monium quarks, as well as having 0.22 angular momentum due to their aligned spins, also have mutual orbi­ tal angular momentum. This gives a 0.20 triplet of possible levels (chi 0, 1 and 2). 0.18 The chi 1's mass is found to be 3511 MeV, with a partial width for the proton-antiproton channel of just 0.16 62 eV, while the chi 2 is found at 3557 MeV, with a proton-antiproton width of 200 eV. The intrinsic accu­ PPPPc 65 A Ukh0V racy of this experiment is reflected in v • ™?iFNAL |refJ . 10 o ISR (1979) *' 11 its small errors, the possible vari­ • - ISR (R210) " 12 ation of the mass values being esti­ o • ISR (R211) » 13 m Th'is experiment (UAM mated as below 1 MeV. More data 60 from these and other charmonium states is being analysed. 55 Bigger and more absorbent 50F \ PP Elastic scattering, when particles 'bounce' off each other without 45h \ ! changing their form, gives important insights into particle behaviour. With 40h the world's highest available colli­ sion energies, measurements of pro­ ton-antiproton elastic scattering are 35 L 50 100 500 1000 high on the list of priorities at the /s (GeV) CERN Collider. Initial results from both the big UA1 experiment (which measures beta mode, with magnetic fields lower collision energies at the CERN just about everything) and the special squeezing the colliding beams to Intersecting Storage Rings, but the UA4 study (Amsterdam / CERN / boost the collision rate.) Collider values for the exponential Genoa / Naples / Pisa — see Sep­ At the Collider, the elastic scatter­ slopes are about 2.5 units higher — tember 1982 issue, page 271) gave ing spectrum falls off exponentially the elastic scattering spectrum hints of a number of interesting ef­ with momentum transfer, with this shrinks. fects (see June 1983 issue, page exponential 'slope' changing at The total proton-antiproton cross- 183). To pin these down, the UA4 about 0.14 GeV2. For smaller mo­ section (a measure of the effective experiment took more data last year mentum transfers, the exponential 'size' of the colliding particles) at­ in a dedicated 'high beta' run of the slope is about 15, flattening to 13 at tains 62 mb at the Collider energy of Collider. (For hunting rare particles, higher values. This 'break' is qualita­ 540 GeV. Compared with the behav­ the Collider is normally run in the low tively similar to the behaviour seen at iour seen at lower energies, this 336 CERN Courier, October 1984 Schematic diagram of the apparatus used by an Aarhus/CERN/Strasbourg experiment studying the angular distribution of channelling radiation. shows that the proton size increases almost as fast as allowed by general principles (the Froissart bound). Especially intriguing is the result that the ratio of elastic to total cross- sections increases from around 0.175 at ISR energies to 0.215 at the Collider. Not only does the proton become larger, it also becomes more absorbent. DETECTORS New semiconductors... In the relentless search for the Best Detector, new materials, as well as new techniques, are being continual­ ly exploited. Semiconductors, such as silicon, are sensitive to charged particles, releasing many times more electrons per particle than a gas. Silicon and germanium strip detectors were de­ veloped some twenty years ago and extensively used by nuclear spec- troscopists, but only recently has the technique become widely adopted for particle physics. Now large posi­ tion-sensitive silicon strip detectors are being developed for many new experiments. Although industry has a big invest­ ment in silicon and germanium, they are not the only semiconductors available, and research groups around the world are looking at other semiconductors in the continual search for better detecting mate­ rials. The list of materials being worked on is both long and exotic. A prom­ ising material is cadmium telluride, which has been investigated for sev­ eral years at the French Centre de Recherches Nucléaires, Strasbourg, The channelling radiation was monitored by an array of cadmium telluride semiconducting detectors. CERN Courier, October 1984 337 .
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