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CENTAURO EVENTS and a SEARCH for THEM in Ual DATA at E 155 Tev LAB --'---'-= 203 CENTAURO EVENTS AND A SEARCH FOR THEM IN UAl DATA AT E 155 TeV LAB --'---'-= D.P. Dallman EP Division, CERN and Institut fur Hochenergiephysik der �AW, Vienna Abstract The phenomenon of Centauro events reported from cosmic ray experiments is discussed. A search for such events in data from the UAl experiment at the CERN SPS pp collider has so far yielded a negative result. The upper limit on the cross-section is about 1 pb. 204 1. Introduction The Centauro phenomenon is an example of a confrontation between two different fields of experimental physics, namely those of cosmic ray physics and particle accelerator physics. The different techniques involved (and some­ times even the different jargon) make direct comparisons rather difficult. In the case 'of accelerator experiments one has the advantage of controllable initial conditions, simpler interactions and a high event rate. In contrast, even the primary cosmic ray interaction is complex, involving at least one multi-nucleon nucleus, and, since the incident flux falls off rapidly with increasing energy l) , for sufficiently high energies the chance of directly observing this primary interaction in the detector is small. Where cosmic rays will always win over accelerator experiments in the forseeable future is in the maximum energy which can be attained, for the primary cosmic ray spectrum extends to at least 108TeV. In comparison, the present energy of the CERN SPS pp collider (540 GeV in the centre-of-mass system) is equivalent to 155 TeV for an interaction on a stationary target. Even if the proposed rise to 900 GeV centre-of-mass energy becomes feasible 2) , this will still give only E LAB = 430 TeV. 2. Centauro events in cosmic ray experiments The cosmic ray experiments which are relevant for the Centauro phenomenon are three high-altitude emulsion chamber experiments 3) , at Chacaltaya in Bolivia (Brazil-Japan collaboration) , Pamir in the Soviet Union (Poland-S.U. collaboration) and Mt. Fuji in Japan (Japanese collaboration) . The detectors used in these experiments are basically similar to one another . For example, in the Chacaltaya experiment, there are upper and lower chambers, 10 cm, 7 cm thick respectively, composed of layers of lead, X-ray film and nuclear emulsion and separated by about 1.5 metres . The events are first located by a visual scan of the X-ray film which allows the detection of all electromagnetic showers above 2 TeV in energy. This is then followed by detailed microscopic measurements in the emulsion. Immediately below the upper chamber is a layer of petroleum pitch (23 cm thick) which is intended to be the interaction region. Several hundred events occurring in this layer (known as C-jets since the target is most often a carbon nucleus) with visible energies in excess of 100 TeV have been observed so far. These events have many properties in common with the most common interactions seen at the SPS pp 205 collider, for example as regards the s-dependence of the total charged particle multiplicity and the increase of the average transverse momentum of charged particles with energy 4) , The success of such comparisons gives one some confidence in extending them to other types of event. The five examples of Centauro events so far found have all occurred in the atmosphere above the 3 detector rather than in the pitch target layer ,S), Their particular characteristics are a high multiplicity of charged hadrons with a high average transverse momentum, together with an almost total absence of electromagnetic energy. In these emulsion chamber experiments only electromagnetic energy is observed and only then when the shower energy exceeds the 2 TeV threshold mentioned above. To obtain the total electromagnetic energy it is necessary to correct for this threshold . In addition, to obtain the total hadronic energy it is necessary to correct for hadrons which do not interact in the apparatus and, for those which do interact, for the fraction of their energy which does not appear as electromagnetic energy. Denoting by Ky the fraction of the energy of a hadron which appears as electromagnetic energy, one has values of Ky which range from 0.2 (for baryons or antibaryons), through 0.24 (the average for a mixture of hadrons near the core of cosmic ray showers to about &) ) 0.33, an extrapolation to the case of charged pious only . Furthermore, due to 2 the TeV threshold, Ky increases radially outwards from the core axis. To obtain multiplicities, the observed showers are "decascaded", that is pairs of showers whose relative transverse energy is below a certain chosen value are combined . In addition it is necessary to correct for secondary interactions occurring in the atmosphere above the apparatus (A-jets) . Monte Carlo studies have been carried out to investigate the possibility that the Centauro events do not represent a new phenomenon but are a fluctuation of normal A-j et events7) . Three of the five events can be accounted for in this way but the remaining two events still stand out conspicuously. One of these is the original Centauro event which, since it occurred only about SO metres above the detector, is free from some of the large corrections needed to obtain the hadronic and electromagnetic multiplicities in the primary interaction. Assuming Ky� 0.2, the total energies of the Centauro events lie in the range 1500-1800 TeV. Such' a grouping could be explained as a combination of a threshold effect together with the steeply - -falling primary energy spectrum. 206 3. Search for Centauros in UAl data Although the observed Centauro events seem to have energies which are some­ what higher than the current SPS collider energy, the collider does have nearly 10 times the centre-of-mass energy of any previous machine, so we have thought it worthwhile to make a search for such events in our data 8) . The data sample used was 48000 low bias events from the December 1981 run, obtained by a trigger requirement of at least one charged particle in the angular region 12-56 mrad with respect to the beam direction at both the p and p ends . The analysis made use of the number of charged tracks reconstructed by the central detector and associated to the pp interaction vertex, and the energy depositions in the electromagnetic and hadron calorimeters over the angular ranges 30°-150° (gondola region) and 5°-30° (bouchon regions) with respect to the incident beam directions . Details of the UAl detector and its performance are given in reference 9, For the purpose of this search, so as to correspond as nearly as possible to the emulsion chamber experiments , the electromagnetic energy was defined to be the energy in sampling 1 of the electromagnetic calorimeters (first 4 radiation lengths) and the hadronic energy was taken to be that in samplings 3 and 4 of the electromagnetic calorimeters together with the energy in the hadron calorimeters (energy beyond 12 radiation lengths) . Figure 1 shows scatter plots of electromagnetic versus hadronic energies according to the above definitions . No events are seen which deviate significantly from the general trend, and a Monte Carlo simulation of the gondola region using measured properties of these low-bias events (transverse momentum spectrum for charged particles lO) , charged particle multiplicity distibution ll) , etc.) seems to agree well with the real data (fig.ld) . Figure 2 shows , for the same events, the average transverse momentum of the charged tracks as a function of the charged particle mult1pl1c1ty . There are no events in the region of high multiplicity which depart significantly from the rest. We now turned the situation around and asked how tDe observed Centauro events would look in our detector . To be able to answer this it is necessary to transform the event into its centre-of-mass system, which depends both on the total incident energy (that is, on the value of and on the mechanism responsible for the events . For Ky ) Centauro I , the best documented event, we considered two hypotheses for the production mechanism: collision of an incident particle with an air nucleon (hyp.l) and the decay of a massive object (hyp.2). Combining these with two extreme choices for , (a) and 0.3 (b) , this gives four possibilities which are shown together witKy h our0.2 data in figure 3. This figure is the same as figure 1, except that we have removed most of the events by requiring a charged 207 mu ltiplicity of above 10 and an average transverse momentum for charged tracks of more than 1 GeV/c, these cuts being chosen to bias ourselves towards events of the Centauro type. No concentrations of events are seen in the relevant regions of the plot. We therefore quote an upper limit of l pbfor the Centauro production in pp interactions at TeV . E = LAB 155 Theoretical speculations 4 . There have been many theoretical ideas put forward to explain some or all lZ) of the features observed in the Centauro events . These involve such phenomena as quark-gluon plasma, unconfined quarks and some even more exotic possibilities . Until the experimental situation has become clearer it is perhaps best to treat them as what they are, namely interesting speculations. 5. Conclusions and outlook In addition to the UAl result, the UA5 experiment has also published a null result on Centauro production at the SPS collider There are se�eral ways in 13� which one can interpret the negative results of these searches : a) the present colli<ler energy is insufficient since the threshold for Centauro production is not reached; b) there is an exotic component in the primary cosmic ray spectrum , that is, Centauros exist as a distinct phenomenon but are not produced in pp interactions; c) Centauros are a fluctuation of otherwise normal cosmic ray interactions .
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