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The Ghost of Baryonium

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The Ghost of Baryonium The spectrum from an experiment which used the CERN SPS hyperon beam, showing a clear peak at 2.46 GeV. This is interpreted as a baryon carrying both charm and strangeness. The signal contains 82 events, against 147 of background. The ghost of baryonium For many years, the fortunes of so- called 'baryonium' states in nucleon- antinucleon annihilation reactions changed with the continual ebb and flow of experimental statistics. At first there were some promising sig• nals, but gradually the sightings be• came less frequent and it looked as though baryonium was destined for the scrapheap of interesting, but ir• relevant, physics ideas. The initial motivation for the search for baryonium came, oddly enough, from the lack of structure in the nucleon-nucleon channel, where the deuteron is the only commonly encountered bound state. If this is interpreted as being due to a basi• cally repulsive force between two nucleons, then an attractive force might be expected in the 'crossed' nucleon-antinucleon channel. Candidate baryonium states were out in the West Experimental Area of would be produced in the decay of a reported from a wide range of exper• the SPS proton synchrotron by a charmed strange baryon. These iments, but one by one they failed to Bristol / Geneva / Heidelberg / Lau• quantum numbers could not be pro• reappear in searches using larger sanne / London / Rutherford team. duced from the decay of three light data samples. In the trade, the joke The hyperons (135 GeV negatively (up, down, strange) quarks. was that baryonium was being re• charged sigmas) were selected from The initial study concentrated on placed by buryonium I the secondary SPS beam by a DISC the decays producing a lambda and a Most of the initial candidate ba• Cherenkov counter (2 x 104 sigmas negative kaon together with two po• ryonium signals came in the form of per 1.5 s pulse containing one and a sitive pions. Other final states are still explicit nucleon-antinucleon reson• half million negative particles) and hit under study. After careful selection ances. One notable exception was a beryllium target. The reaction pro• and tests to eliminate effects which provided by a Basle / Karlsruhe / ducts were measured in a magnetic could produce spurious signals, the Stockholm collaboration at the CERN spectrometer equipped with wire four-particle mass spectrum shows 28 GeV PS proton synchrotron counters and drift chambers, togeth• a sharp peak at 2.46 GeV contai• studying the interactions of antipro- er with threshold Cherenkov count• ning 82 events. This is interpreted as tons brought to rest in a target. This ers for identification of protons, the positively charged, charmed experiment looked for (and found) kaons and pions. (This SPS hyperon strange baryon, A. Companion parti• indications of the gamma rays which beam has now been dismantled.) cles are now being sought. would be expected if quasi-free pro• The search concentrated on the In the data analysis of this experi• tons and antiprotons fall into tightly production of a lambda baryon (de• ment, the contents of some 200 bound baryonium states. The gam• tected by its decay products) and a magnetic tapes were transferred ma spectrum looked quite promis• negative kaon, together with other from CERN to the computers at the ing, with evidence for three separate particles. The selected final states Rutherford Appleton Laboratory in and well defined proton-antiproton carry the charge (positive) and the UK using the STELLA satellite bound states. strangeness (—2) combination which high speed data transmission link. To confirm these initial findings. CERN Courier, March 1983 55 Results from the Basel / Karlsruhe / Stockholm / Strasbourg / Thessaloniki collaboration at the CERN in their searches for signs of 'baryonium' in the gamma rays coming from proton-antiproton annihilations at rest. The peaks indicated by arrows correspond (with increasing gamma energy) to masses of 1771, 1694, 1638 and 1210 MeV. The large peak at 130 MeV gamma energy is expected and produced by a slow negative pion being brought to rest in the target. The later runs used an improved detector system. previous line becomes less distinct, The collaboration has already pro• and other lines begin to show up as duced some interesting results on well. fixed target antiproton, interactions The yields are reduced compared and sees a rich structure setting in at with the earlier study, but the exper• a much lower energy than for the imenters have more confidence in proton-proton case (see July/Au• the new experiment. If not in perfect gust 1981 issue, page 246). health, baryonium is at least still al• Differential Cherenkov counters ive ! These results are both interest• (CEDARS) provided the necessary ing and encouraging in view of the particle identification, with additional experiments starting at CERN's new instrumentation measuring the beam LEAR low energy antiproton ring this particles and the beam intensity. year, which is ideally suited to study Because of the rare reactions be• these phenomena. ing studied, intensity was important. The secondary SPS beam typically Large angle scattering contained 5 x 107 particles per burst. The two-arm downstream spec• In hadron-hadron scattering, two dif• trometer covered centre-of-mass ferent types of general behaviour can scattering angles from 45° to be distinguished. In gentle 'peripher• 100°. al' interactions, where the interact• In order to minimize apparatus ing particles are only slightly de• dead time, the trigger was organized flected, the force is understood to be into three levels, each successively transmitted by 'Regge' exchanges. more sophisticated and consequent• However in more violent collisions ly with a longer decision time. Ho- producing wide angle deflections, doscope data was fed into six pro• the collaboration, including now also the interaction is assumed to be grammable fast matrix coincidence Strasbourg and Thessaloniki, em• caused by the forces between the units for a first selection. Additional barked on a new experiment at the constituent quarks inside the parti• matrix logic carried out more accu• PS, using an improved detector to cles. rate tests before passing the data to pick up the emitted gamma rays with In principle it should be possible to a specially designed hardwired mi• higher sensitivity and to intercept the study hadron-hadron elastic scatter• croprocessor. products of the proton-antiproton ing over a wide range of momentum A total of some 38 million triggers annihilations. transfer (scattering angle) and see were analysed, covering 20 GeV ne• Two major runs were carried out, the transition from one type of be• gative and positively charged one with 40 million and the other haviour to another. Hadron-hadron beams, and with 30 GeV negative with 110 million stopped antipro- elastic scattering, over the years, beam. Most of the selected events tons. Major components of the ex• has been extensively studied in the correspond to scattering of protons periment were modified between peripheral region. Apart from a few and of positive and negative pions runs, which therefore can be con• results at high energy, data on large with the fixed target. A few kaon sidered almost as independent ex• angle scattering for the wide range of events were intercepted. Under periments, providing two sets of different hadron-hadron collisions these conditions, no antiproton ev• gamma spectra. has been scanty. ents were seen. Both spectra were subject to high An experiment by an Annecy / The data were first compared with backgrounds, mainly due to gammas CERN / Copenhagen / Genoa / Oslo / the 'constituent interchange model', coming from the decay of neutral University College London team us• which assumes that the interactions pions. This background signal had to ing a high intensity unseparated ha- are built up from constituent quark- be carefully removed to isolate any dron beam from the CERN SPS 400 quark scatterings. The angular de• sharp gamma emission lines hidden GeV proton synchrotron has made a pendence of this model is much behind. Two of the lines seen in the survey of this wide angle scattering weaker than the observed behav• initial experiment are confirmed, one and finds the theories wanting. iour, particularly at the higher energy. 56 CERN Courier, March 1983 .
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