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.
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.
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.
Neutral pion production in central 158A GeV sup 208Pb+ sup 208 Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The pisup 0 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. copyright ital 1999 ital The American Physical Society
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