S. Fokin

Centrality Dependence of Neutral Pion Production in 158A GeV

The production of neutral pions in 158AGeV Pb+Pb collisions has been studied in the WA98 experiment at the CERN SPS. Transverse momentum spectra are studied for the range 0.3 GeV/c<mT-m0<4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons which is most naturally explained by assuming an equilibrated system.

^{208}

Abstract Minimum Bias production cross sections of η mesons have been measured in 200 AGeV/cS+Au and S+S collisions at the CERN SPS by reconstructing the η → γγ decay. The measurements have been made over the rapidity range 2.1 ≤ y ≤ 2.9 using the leadglass spectrometer of WA80. Within the statistical and systematical uncertainties the spectral shapes of π0 and η mesons yields are identical when their invariant differential cross section is plotted as a function of the transverse mass. The relative normalization of the η to π0 transverse mass spectra is found to be 0.53±0.07 for S+Au and 0.43±0.15 for S+S reactions. Extrapolation to full phase space leads to an integrated cross section ratio of η to π0 mesons of 0.147±0.017(stat.)±0.015(syst.), and 0.120±0.034(stat.)±0.022(syst.) for S+Au and S+S collisions, respectively.

Pb+

Abstract The multiplicity of inclusive photons has been measured on an event-by-event basis for 158· A GeV Pb induced reactions on Ni, Nb, and Pb targets. The systematics of the pseudorapidity densities at midrapidity ( ρ max ) and the width of the pseudorapidity distributions have been studied for varying centralities for these collisions. A power law fit to the photon yield as a function of the number of participating nucleons gives a value of 1.12±0.03 for the exponent. The mean transverse momentum, 〈 p T 〉, of photons determined from the ratio of the measured electromagnetic transverse energy and photon multiplicity, remains almost constant with increasing ρ max . Results are compared with model predictions.

^{208}

Abstract Azimuthal correlations of photons produced at mid-rapidity in 200 A GeV S + Au collisions have been studied using a preshower photon multiplicity detector in the WA93 experiment. The Fourier expansion method has been employd to estimate the event plane via the anisotropy of the event as a function of centrality. The event plane correlation technique has been used to determine the true event anisotropy, beyond the anisotropy which arises due to finite multiplicity. The VENUS event generator with rescattering and proper simulation of the detector response can explain only a portion of the observed anisotropy. The residual anisotropy is found to be of the order of 5% for semi-central collisions. This suggests that directed collective flow of the produced particles is present at SPS energies.

Pb Collisions

First results on the measurement of localized fluctuations in the multiplicity of charged particles and photons produced in central 158 A GeV Pb+Pb collisions are presented. The charged versus neutral correlations in common phase space regions of varying azimuthal size are analyzed by two different methods. The measured results are compared to those from simulations and to those from different types of mixed events. The comparison indicates the presence of non-statistical fluctuations in both charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charge-neutral fluctuations are observed.

Production of η mesons in 200 AGeV/cS+S and S+Au reactions

An event-by-event analysis of the azimuthal angular correlation with respect to the reaction plane has been carried out for K+ and pi(+) emission near mid-rapidity in 158 A . GeV Pb + Pb collisions. In semi-central collisions, K+ mesons are found to be preferentially emitted out of the reaction plane, while pi(+) mesons are emitted in the reaction plane. The results suggest that the kaon emission is influenced by in-medium potential effects in addition to collective flow effects

Systematics of inclusive photon production in 158-A-GeV Pb induced reactions on Ni, Nb, and Pb targets

The azimuthal distributions of photons and charged particles with respect to the event plane are investigated as a function of centrality in Pb-208 + Pb-208 collisions at 158 (.) A GeV/c in the WA98 experiment at the CERN SPS. The anisotropy of the azimuthal distributions is characterized using a Fourier analysis. For both the photon and charged particle distributions the first two Fourier coefficients are observed to decrease with increasing centrality. The observed anisotropies of the photon distributions compare well with the expectations from the charged particle measurements for all centralities.Abstract.The azimuthal distributions of photons and charged particles with respect to the event plane are investigated as a function of centrality in 208Pb + 208Pb collisions at 158

Azimuthal anisotropy in S + Au reactions at 200 A GeV

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Localized charged-neutral fluctuations in 158A GeVPb plus Pb collisions

GeV/c in the WA98 experiment at the CERN SPS. The anisotropy of the azimuthal distributions is characterized using a Fourier analysis. For both the photon and charged particle distributions the first two Fourier coefficients are observed to decrease with increasing centrality. The observed anisotropies of the photon distributions compare well with the expectations from the charged particle measurements for all centralities.

Elliptic emission of K+ and π+ in 158 A·GeV Pb + Pb collisions

The mean transverse momentum of photons has been determined on an event by event basis in S + Au collisions at 200 A.GeV from the ratio of the measured electromagnetic transverse energy (E-T(em)) to the photon multiplicity (N-gamma). The average value obtained is similar to that determined for the same system using spectroscopic techniques. The centrality dependence of the measured values are in agreement with the predictions of the VENUS event generator