Neutral Pion Production in Nucleus-Nucleus Collisions at 158 and 200 GeV/Nucleon
Two years ago, at the 12th Winter Workshop on Nuclear Dynamics held here in Snowbird, I presented WA80 limits on direct photon production in 200-GeV 32S+Au collisions.1 It was found that the results were consistent (within 1 σ) with the absence of an excess of photons over those that can be accounted for by the two-photon decay branches of π0 and η mesons and by the small photon contributions from other radiative decays. We are in the process of finalizing our direct-photon production results from collisions of lead nuclei at 158 GeV/nucleon. I will briefly discuss the status of the analysis and give some preliminary results at the end of this talk. However, most of this presentation is concerned with a very different aspect of our 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 us with information concerning the system at freeze out. Transverse momentum spectra at low and intermediate PT relate to ther-modynamic and hydrodynamic descriptions of the hot, dense systems. In addition, the high-pT region reflects the hard-scattering regime and may help us understand initial-state particle production by forming a bridge to proton-proton and proton-nucleus results. It follows that it is essential that the π0 measurements cover a large pT range.
KeywordsQuark Matter Neutral Pion Direct Photon Local Slope Peripheral Collision
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