Single photons from Pb   Pb collisions at CERN SPS: quark gluon plasma versus hadronic gas

Abstract

In a hydrodynamic model, we have analyzed the direct photon data obtained by the WA98 collaboration in 158 A GeV Pb+Pb collisions at CERN SPS. The transverse expansion of the system was taken into account. Two scenarios, (i) formation of quark-gluon plasma and (ii) formation of hot hadronic gas, were considered. Both the scenarios describe the data equally well. However, hadronic gas scenario require very high initial temperature (∼ 300 MeV) and it is difficult to conceive existence of hadron gas at that high temperature.If the hadronic fluid has small radial velocity (0.2c-0.3c) initially, the data are well explained in the hadronic gas scenario with reasonable initial temperatures.PACS numbers(s): 12.38.Mh,13.85.Qk,24.85.+p, Recently WA98 collaboration has published their single photon emission data for 158 A GeV Pb+Pb collisions at CERN SPS [1]. Much interest was aroused after the publication of the WA80 preliminary results of the S+Au single photon data [2], as it was hoped that the they can be a conclusive probe of the much debated quark-gluon plasma (QGP), expected to be produced in relativistic heavy ion collisions. The preliminary data were analyzed by several authors. Xiong and Shuryak [3] analyzed the data assuming a mixed phase formation and found excess photons. Srivastava and Sinha [4] analyzed the data considering two possible scenarios after the collision, one with the phase transition to QGP, the other without it. It was claimed that the data were explained only in the phase transition scenario. We had also analyzed the preliminary versions of the WA80 direct photon data [5]. It was shown that formation of viscous hadron gas in the initial state, can explain the data. The revised version of the data [6] were also analyzed by several authors including us [7]. It was concluded that the data are not sensitive enough to discriminate between the two alternate pictures, e.g. formation of quark-gluon plasma and formation of hot hadronic gas.Several authors have analyzed the recent WA98 single photon data [8][9][10][11]. Peressounko et al [11] concluded that the data could be explained only with a small initial radial velocity. Also the data could not distinguish between a QGP and a hot hadronic gas scenario. Srivastava et al [8] could fit the data in the QGP scenari, without initial radial velocity. But the two loop photon rate used by them was not corrected for a factor of 4. They did not considered the hadronic scenario. They argued that in the pure hadronic scenario, initial temperature of the hadronic fluid will be large. Hadronic density will be ∼ 10 hadrons/f m 3 . It is unphysical to consider hadronic gas at such a high density. Initial Hadronic scenario was considered in [9]. It was concluded that only hadronic model with medium modification (hadrons were formed with zero mass) could explain the data.In the present paper we analyze the WA98 single photon data in the no phase transition (NPT) scenario. A hot hadronic gas is assumed to be formed in the initial state. It expands, cools and freezes o...

Hydrodynamic Models for Heavy Ion Collisions

“…Because the rates are proportional to exp(− p · u/T ), where p is the four momentum of the photon and u is the flow four-velocity, stronger transverse flow allows lower temperatures to produce equal yield at high p T . Peressounko & Pokrovsky (128) argued the necessity of such an initial flow, and Alam et al (127) and Chaudhuri (129) later studied its effects. It can be argued that gradients in initial particle production would lead to buildup of flow during thermalization, but it is very difficult to quantify how large flow velocities could build up this way.…”

“…In the other two calculations (126,129), the initial state was only required to have the same entropy as the final-state particles. It is thus unknown whether these calculations are consistent with the hadron data.…”

“…Direct photon production in √ s N N = 17.3 GeV Pb+Pb collisions at the CERN-SPS was measured by the WA98 collaboration (127). Several authors have compared this data with hydrodynamical calculations (128)(129)(130)(131)(132). All authors agreed that the photon spectrum could be explained if one assumes sufficiently hot (T > 200 MeV) initial state, but the required initial temperature varied largely from T ∼ 200 MeV (129) to T = 335 MeV (128).…”

“…The hadron spectra were reproduced in References (130,132), and, when no initial transverse flow is assumed, also in Reference (129). In the other two cal-culations (128,131), the initial state was only required to have the same entropy as the final-state particles. It is thus unknown whether these calculations are consistent with the hadron data.…”