We report the first inclusive photon measurements about mid-rapidity (

Two years ago, at the 12th Winter Workshop on Nuclear Dynamics held here in Snowbird, the authors presented WA80 limits on direct photon production in 200-GeV 32S + Au collisions. It was found that the results were consistent (within 1[sigma]) with the absence of an excess of photons over those that can be accounted for by the two-photon decay branches of [pi]° and [eta] mesons and by the small photon contributions from other radiative decays. They are in the process of finalizing the direct-photon production results from collisions of lead nuclei at 158 GeV/nucleon. The author briefly discusses the status of the analysis and gives some preliminary results at the end of this talk. However, most of this presentation is concerned with a very different aspect of the photon measurements: distributions of neutral pions. In contrast to direct photons which probe initial collision conditions, hadrons, such as neutral pions, interact strongly and decouple late in the reaction evolution and, thus, provide information concerning the system at freeze out. Transverse momentum spectra at low and intermediate p{sub T} relate to thermodynamic and hydrodynamic descriptions of the hot, dense systems. In addition, the high-p{sub T} region reflects the hard-scattering regime and may help one understand initial-state particle production by forming a bridge to proton-proton and proton-nucleus results. It follows that it is essential that the [pi]° measurements cover a large p{sub T} range.

Inclusive transverse momentum spectra of photons and[pi][sup 0]s at mid-rapidity are studied as a function of collision centrality for[sup 197]Au+[sup 197]Au collisions at[radical]s[sub NN]= 130 GeV. Photon pair conversions have been reconstructed from charged tracks measured by the main Time Project Chamber of the STAR experiment at the RHIC heavy ion facility. The transverse momentum resolution of photons with this method is estimated to be[Delta]p[sub t]/p[sub t]= 2% at 0.125 GeV/c and 5% at 2.5 GeV/c. Photon spectra were measured up to a transverse momentum of 2.4 GeV/c between[+-] 0.5 units of rapidity. The dominant photon production mechanism, the[pi][sup 0][yields][gamma][gamma] decay, was measured between 0.25-2.5 GeV/c and[+-] 1 units of rapidity. Spectra are reported for the top 11%, 11-34% and 34-85% centrality classes. It was observed that in mid-central and central collisions the relative contribution of the[pi][sup 0][yields][gamma][gamma] decay to the inclusive photon spectrum decreases above a transverse momentum of 1.65 GeV/c. In central collisions the magnitude of the decrease from p[sub t]= 1.65 GeV/c to 2.4 GeV/c is 20%. It is unlikely that contributions from other[pi][sup 0] decay channels and other particle decays fully explain this decrease. The centrality dependence on the shapes of the[pi][sup 0] spectra was analyzed with Boltmann and Bose-Einstein functions. In the transverse momentum windows of the spectra, the extracted temperatures are near 0.295 GeV. These temperatures are substantially higher than those extracted from[pi][sup[+-]] spectra in a lower range of transverse momentum. This is an indication that the shapes of the pion spectra deviate from purely exponential shapes.

The discovery of asymptotic freedom in the theory of the strong interaction has initiated the high-energy heavy-ion collisions program. It is expected such collisions to produce a deconfined phase of quarks and gluons. The prediction of the phase transition to occur in the vicinity of non-pQCD regime increase the challenges at the theoretical and experimental levels. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory was constructed to explore the QGP-hadronic matter phase transition.

Direct photon productions in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au at center of mass energies √s = 62.4 GeV and 200GeV at RHIC are investigated systematically by taking into account jet quenching effect, medium-induced photon bremsstrahlung and jet-photon conversion in the hot QGP as well as known cold nuclear matter effects such as the isospin effect, the Cronin effect, shadowing effect, EMC effect and cold nuclear matter energy loss. It is shown that at high p{sub T} the nuclear modification factor for direct photon R{sub AA}(p{sub T}) is suppressed and dominated by cold nuclear matter effects, and there is no large enhancement due to medium-induced photon bremsstrahlung and jet-photon conversion in the hot QGP. Comparison of numerical simulations with experimental data rules out large Cronin enhancement and incoherent photon emission in medium, though large error bars in currently experimental data can not provide tight constraints on other nuclear matter effects.