Search Results (9 total)

Localized fluctuations in the multiplicity of charged particles and photons produced in central 158A GeV/c Pb+Pb collisions are studied. The charged versus neutral correlations in common η-φ phase space regions of varying azimuthal size are analyzed by two different methods. The analysis provides a model-independent demonstration of nonstatistical fluctuations in both charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charge-neutral fluctuations are observed, contrary to expectations for the production of a disoriented chiral condensate. The result is not explained by the widely used VENUS model.

A measurement of direct photon production in ²⁰⁸Pb+²⁰⁸Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5<p_T<4 GeV/c. A significant direct photon signal, compared to statistical and systematical errors, is seen at p_T>1.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.

Three-particle correlations have been measured for identified π^- from central 158A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found as expected for a mainly chaotic and symmetric source.

Neutral pion production in central 158A GeV ²⁰⁸Pb+ ²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The π⁰ transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out.

The production of neutral pions in 158A GeV ²⁰⁸Pb+²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron (SPS). Transverse momentum spectra are studied for the range 0.3≤m_T-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. This is most naturally explained by assuming an equilibrated system.

The photon multiplicity has been measured for the first time in S+Au collisions at 200A GeV over a wide pseudorapidity range (2.8<~η<~5.2) employing a fine granularity preshower detector. The pseudorapidity density of photons increases with centrality, reaching ∼200 at the highest centrality studied. The results are compared with measurements of the charged particle multiplicity and with predictions of the VENUS event generator.

Inclusive photons of low transverse momenta have been measured in 200 GeV/nucleon ³²S+Au collisions at the CERN SPS. Data were taken in the WA93 experiment using a small acceptance BGO detector with longitudinal segmentation. The results are compared to WA80 measurements for the same system and results from hadron decay calculations. An excess of soft photons over the expectations from neutral meson decays is observed.

A search for the production of direct photons in S+Au collisions at 200A GeV has been carried out in the CERN-WA80 experiment. For central collisions the measured photon excess at each p_T, averaged over the range 0.5≤p_T≤2.5 GeV/c, corresponded to 5.0% of the total inclusive photon yield with a statistical error of σ_stat  =  0.8% and a systematic error of σ_syst  =  5.8%. Upper limits on the invariant yield for direct photon production at the 90% C.L. are presented. Possible implications for the dynamics of high-energy heavy-ion collisions are discussed.