A measurement of direct photon production in {sup 208}Pb+ {sup 208}Pb 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{sub T}<4 GeV/c . A significant direct photon signal, compared to statistical and systematical errors, is seen at p{sub 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.
Transverse energy, charged particle pseudorapidity distributions and photon transverse momentum spectra have been studied as a function of the number of participants (\(N_{part}\)) and the number of ...binary nucleon-nucleon collisions (\(N_{coll}\)) in 158\(\cdot A\) GeV Pb+Pb collisions over a wide impact parameter range. A scaling of the transverse energy pseudorapidity density at midrapidity as \(\sim N_{part}^{1.08 \pm 0.06}\) and \(\sim N_{coll}^{0.83 \pm 0.05}\) is observed. For the charged particle pseudorapidity density at midrapidity we find a scaling as \(\sim N_{part}^{1.07 \pm 0.04}\) and \(\sim N_{coll}^{0.82 \pm 0.03}\). This faster than linear scaling with \(N_{part}\) indicates a violation of the naive Wounded Nucleon Model.
Three-particle correlations have been measured for identified pi(-) 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.
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.
Anisotropic transverse flow has been studied with a magnetic spectrometer at midrapidity and the Plastic Ball detector at target rapidity in the WA98 experiment. Our preliminary results show the ...existence of directed and elliptic flow in semi-central Pb+Pb collisions. The magnitude of the directed flow is found to be significantly smaller than those observed at the AGS energies.
Disoriented Chiral Condensates (DCC) have been predicted to form in high energy heavy ion collisions where the approximate chiral symmetry of QCD has been restored. This leads to large imbalances in ...the production of charged to neutral pions. Sophisticated analysis methods are being developed to disentangle DCC events out of the large background of events with conventionally produced particles. We present a short review of current analysis methods and future prospects.
An event-by-event analysis of the azimuthal angular correlation with respect to the reaction plane has been carried out for
K
+ and
π
+ 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
π
+ 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.
We have studied the azimuthal angle distributions of identified charged particles near mid-rapidity region in semi-central Pb+Pb collisions at SPS energies. Our preliminary results show that π
+ ...mesons and protons seem to be emitted in the reaction plane, while out-of-plane for
K
+ mesons. This phenomenon might be explained by the existence of another possible effect such as in-medium potential as well as the collective motion.
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