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CERN Hadron Horizons

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CERN Hadron Horizons Comparison of proton-antiproton (top) and proton-proton (bottom) total cross-sections at different collision energies (lower horizontal axis). The points on the right with large errors are the new results from the SPS proton-antiproton collider, showing that the total cross-section countinues to rise, and extrapolates well with what is seen at lower energies. The round data points, centre, come from the Intersecting Storage Rings. CERN Hadron horizons An article in the May issue (page 136) described some of the new hadron scattering results which indicate that at high energies there might be an universal underlying behaviour, inde­ pendent of the quark composition of the colliding particles. Just over ten years ago, contro­ versy raged about the general trend of hadron behaviour at higher ener­ gies. The dust only settled when the first results appeared from the CERN Intersecting Storage Rings. Today there is less controversy, but the topic has been given a new lease of life by the ability to compare proton-proton and proton-antipro­ ton behaviour under similar con­ ditions at the ISR, and by the new energy range opened up at the SPS proton-antiproton collider. Total and elastic cross-sections For elastic scattering and total related to the effective 'size' of th are important measurements of in- cross-section measurements, these colliding particles. ter-hadron affinity. The total cross- included a CERN / Naples / Pisa / For both proton-proton and pre section indicates how reactive the Stony Brook group measuring total ton-antiproton scattering, there a\ particular particles are, while the cross-sections by the observed pears to be a slight increase in th elastic cross-section describes how reaction rate, a Louvain / Northwes­ exponential falloff at smaller momei the scattered particles simply tern team using the time-honoured turn transfer. At lower energies, th 'bounce' off each other, without 'Roman Pot' technique of movable falloff is also sharper in the protoi changing their relative composition. detectors to probe the extreme for­ antiproton case, however the diffe A systematic comparison of these ward scattering cones, and an Ames ence disappears as the collision e two important parameters of hadron / Bologna / CERN / Dortmund / Hei­ ergy is increased. behaviour provides significant clues delberg / Warsaw team working at Overall, the observed behavio about the underlying interaction me­ the Split Field Magnet. seems to be well described by tl chanisms. Comparison of particle- These results, when taken togeth­ conventional model of 'Regge' e particle (proton-proton) and particle- er, now give a good comparison of changes, without recourse to ne antiparticle (proton-antiproton) re­ proton-proton and proton-antipro­ ideas, such as 'odderons'. sults is also revealing. ton scattering over the available ki­ When the ISR came into operate Over the years, the behaviour of nematic range. just over ten years ago, the observ the proton-proton elastic and total Broadly speaking, the elastic and rise of the proton-proton total cros cross-sections has been measured total cross-sections in the two cases section was something of a surpm with precision at the ISR. New stu­ appear to converge as the collision Until then, it had been widely t dies were mounted to profit from the energy is increased. The shape of the lieved that particle interaction rat availability from 1981 of proton-anti­ elastic scattering spectrum is con­ were levelling off at previously avi proton collisions (see June 1981 is­ ventionally described by a decreas­ able energies in preparation for sue, page 196). ing exponential, the exponent being asymptotic limit. But the ISR show 182 CERN Courier, June 1S Preliminary proton-antiproton elastic scattering spectrum as measured by the UA4 experiment at the CERN SPS proton-antiproton collider. The observed exponential decrease is sharper for small momentum transfers (between 0 and 0.2 GeV2), and then becomes less steep. At about 0.8 GeV2, there appears to be a 'shoulder', reminiscent of what is seen at lower energies in the Intersecting Storage Rings. 'shoulder' in the elastic scattering spectrum. This looks like the vestige of one of the early discoveries at the ISR — a dip in the proton-proton scattering spectra, strongly reminis­ cent of optical diffraction. However, even under ISR con­ ditions its position moves with ener­ gy and the dip itself becomes less pronounced. This is underlined by the SPS proton-antiproton results, which show that the shoulder has that asymptopia was still a long way off. With the advent of the SPS proton- antiproton collider, another new en­ ergy range opened up. Proton-anti­ proton cross-section information comes from the mighty UA1 exper­ iment (which measures as many things as it can) and from the special UA4 study (Amsterdam / CERN / Genoa / Naples / Pisa, see Septem­ ber 1982 issue, page 271). Results show that the proton-anti­ proton total cross-section continues to rise, reaching about 65-70 milli- barns, broadly in line with extrapola­ tions from lower (ISR) energies. This seems to indicate that the cross-sec­ tion grows as fast as allowed by the general principles of the theory. Both experiments now report a re­ latively sharp exponential decrease (exponent about 17) for small mo­ mentum transfers (between 0 and 0.2 GeV2). At larger momentum transfers the exponent falls to about 13.5. It is not yet evident whether there is an abrupt change between these two regions of behaviour, or whether there is an identifiable trans­ ition region. At larger momentum transfers (about 0.8 GeV2) there is a definite Comparison of proton-proton and oroton-antiproton elastic scattering at the Intersecting Storage Rings by an Ames / Bologna / CERN / Dortmund / Heidelberg / Warsaw collaboration. CERN Courier, June 1983 183 Units of the system developed at CERN which greatly increases the efficiency and speed of scanning and measuring nuclear emulsions for particle interactions. The introduction of a computer-aided digitized microscope (CADIM) has helped relaunch the emulsion technique, particularly in the study of very short lived particles. (Photo CERN 233.4.83) shifted considerably from its ISR position. Several theoretical models at­ tempt to describe these results. As well as an effective size, the proton (and antiproton) has an 'opacity'. Under laboratory conditions, the proton is not completely opaque ('black') to an incoming beam, but is 'grey' and penetrable. One outstand­ ing question is whether at infinite energy the proton would become opaque, whether it would remain grey, or whether it would become transparent. Further studies at the collider will help provide us with the answer. A thing of beauty After many fundamental contribu­ tions in the early days of particle phy­ sics, the nuclear emulsion technique had some ten years of almost total neglect in physics at accelerators. The reason was not so much its bas­ All these developments are trate on events where the number o ic abilities as the fact that searching brought together in so-called 'hy­ tracks seen a few millimetres down for events in the emulsion and their brid' experiments (such as the stream exceeds the number of track subsequent measurement was a WA71 and WA75 studies at CERN) emerging from the vertex in th process of mind-bending tedium. A to study the production and the life­ emulsion, indicating the possible de revival came with the need to study times of particles containing the cay of a charmed particle, which h charm and beauty particles whose beauty quark. WA71 uses the re­ self may result from the prompt de lifetimes, though short, are still ob­ vamped Omega spectrometer, and cay of a beauty particle. servable by detectors able to distin­ is a CERN / Genova / Milan / Mos­ The emulsions are scanned unde guish events at the micron level. This cow / Paris / Rome / Santander / a computer-aided digitized micrc is exactly the special feature of nu­ Valencia collaboration with G. Diam- scope (CADIM) and can be moved b clear emulsions — they can resolve brini-Palazzi as spokesman. A intervals of less than 1 jam in thre events in space to distances of a few 350 GeV negative pion beam gener­ dimensions via refined stepping mc microns or less. ated by protons from the SPS hits tors. Coordinate digitizers linked 1 Happily this ability can now be ex­ nuclear emulsion (two pellicles each the microscope can record the pos ploited without the pain of bygone 0.5 mm thick stacked perpendicular tion of any point in the field of vie1 years. Other detectors in association to the beam). Silicon detectors, a when accurate measurements ai with the emulsion can select and re­ time projection chamber and multi- required. A much magnified image < construct the event and indicate wire proportional chambers follow the slice of emulsion in focus can t where to look in the emulsion. More downstream of the emulsion in the visualized on a large TV screen. modern mechanisms in association magnetic field of the Omega magnet. Analysis proceeds by the compi with microelectronics can also allow A beam hodoscope, a ring imaging er reconstruction of the event fro rapid scanning and precise measure­ Cherenkov counter and a gamma de­ the downstream information indicc ment of a whole event recorded in tector complete the apparatus. ing where to look in the emulsion ar the emulsion. The beauty search will concen­ also giving a visual display of the pc 184 CERN Courier, June 19 .
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