Pion and Photon Production in Heavy Ion Collisions
Pion and Photon Production in Heavy Ion Collisions

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Published: 2008

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Measurement of neutral pions and direct photons are closely connected experimentally, on the other hand they probe quite different aspects of relativistic heavy ion collisions. In this short review of the [pi]° results from the PHENIX experiment at RHIC our focus is on the [phi]-integrated nuclear modification factor, its energy and system size dependence, and the impact of these results on parton energy loss models. We also discuss the current status of high p{sub T} and thermal direct photon measurements both in p+p and Au+Au collisions. Recognizing the advantages of measuring not only the 'signal', but also all the 'references' needed for proper interpretation in the same experiments (with same or similar systematics) we argue that RHIC should regularly include d+A and even d+d collisions into its system size and energy scan.

Neutral Pion Production in Nucleus-nucleus Collisions at 158 and 200 GeV/nucleon
Neutral Pion Production in Nucleus-nucleus Collisions at 158 and 200 GeV/nucleon

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Published: 1998

Total Pages: 11

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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.

Photon and Neutral Pi 0 Production in (197)Au+ (197)Au Collisions at Sqrt S(NN)
Photon and Neutral Pi 0 Production in (197)Au+ (197)Au Collisions at Sqrt S(NN)

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Published: 2003

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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.