An Experimental study of neutral and charged particle fluctuations in Pb Pb collisions at 158-A-GeV

Relativistic quantum field theory treats the vacuum as a medium, with bulk properties characterized by long-range order parameters. This has led to suggestions that regions of disoriented chiral condensate might be formed in high energy colli sion processes. In particular, the approximate chiral symmetry of QCD could lead to regions of vacuum which have chiral order parameters disoriented to directions which have non-zero isospin, i.e. disoriented chiral condensate. The prime accessible signature of DCC is the ratio of neutral to charged pions in a certain range of phase space which should exhibit non-statistical fluctuations. The WA98 experiment has been used to measure the charged and photon multiplicities in the central region of Pb+Pb collisions at the CERN SPS. PMD which has the ability to count photons is positioned at 21.5m from the target, covering the pseudorapidity range 2.9 < t) < 4.4. A lead converter of thickness 3 radiation length is used to produce pre-showers. 28 box modules were placed directly behind the converter. Each module consisted of square plastic(BC400) scintillator pads, arranged in 50 x 38 arrays. WLS fibre was inserted diagonally into each pad. To avoid stray light, fibres were covered with PVC sleeves. The other end of fibres were attached to image intensifier and CCD camera system where the signals are read out. Pads with energy deposition more than 3 MIP’s are labeled as due to incident gamma photon. The photon counting efficiency is found to be 68% to 73% for central and peripheral cases where as purity of the photon sample is 65% to 54% for the two cases. Silicon Pad Multiplicity Detector (SPMD) detects the charged particles in the pseudorapidity range 2.35 < 77 < 3.75. It is circular in shape and is placed at 32.8cm downstream the target. The whole detector is divided into four quadrants with 1012 pads each. Each quadrant has 22 bins in rapidity and 46 bins in azimuth. The efficiency of the detector is 99%. The main source of error is the uncertainty in the calculation of gain which contributes about 2% towards the systematic error. Another major source of error (« 2%) is due to the use of Landau average of energy in calculating NCh. Further, the error due to “line” effect is of the order of 1%. The combined effective error in the determination of Nch was found to be less than 4%. v We have looked for the fluctuations in pseudorapidity and azimuthal distributions of charged particles and photons. Various measures for fluctuations have been applied but the detailed investigation is carried out by the power spectrum technique to filter out the events having non - statistical fluctuations. The results from data have been compared with those of monte carlo simulated VENUS events processed through GEANT and with the set of mixed events generated from real experimental data. Some events have been filtered which have shown large non-statistical fluctuations though their probability is small. For DCC search, we have analysed the data set consisting of « 294 K events. A method has been developed which can scan an event in Ary - A phase space for the region where the neutral pion fraction / attains the maximum value. The top 15% of the events are analysed in small Ary — A bins. Results from data are compared with those of simulated VENUS events processed through GEANT. The results are also compared with the two sets of mixed events generated by different methods. The data has more number of events having photon excesses in Ary — A4> region as compared to mixed events (I and II) and V+G events. To test the authenticity of the filtered events, these events are passed through various checks. The domains are found to be distributed uniformly in azimuth. The fraction of events having anomalous / values increases with decrease in centrality and increases with decrease in patch size. Search is also made for the ring type DCC formation. No positive evidence has been obtained regarding this search. The results obtained for data are consistent with mixed and V+G events. The top 15-65% of the events are analysed globally in the overlap region of PMD and SPMD. We have found « (0.67+0.04)% events with E? = 50 - 100 GeV and (0.16+0.03)% events with E? — 100 - 150 GeV, having neutral pion fraction, / > 0.70. Some events have been filtered which have shown excess of photons either in small 7y — 4> phase space or in the entire available phase space. It is worth mentioning that these events closely resemble the “anticentauro events” seen by JACEE collaboration in cosmic ray interactions. This is for the first time that anticentauro type events have been observed in heavy ion collisions in an accelerator based experiments.