Single photons from relativistic collisions of lead nuclei at energies available at the CERN Super Proton Synchrotron (SPS): A reanalysis

Abstract

We present a reanalysis of single photon production from relativistic collision of lead nuclei at CERN SPS measured by the WA98 experiment. The refinements include use of iso-spin and shadowing corrected NLO pQCD treatment for prompt photon production using an optimized scale for factorization, renormalization, and fragmentation and use of hydrodynamics suited for non-central collisions along with a well tested equation of state admitting a quark-hadron phase transition. A quantitative explanation of the data requires a large initial temperature (at a small formation time of about 0.2 fm/c) and a moderate increase in the prompt yield which could perhaps be attributed to the Cronin effect in nuclei. The data can also be explained using a moderate initial temperature (at a formation time of about 1 fm/c) with a very large K-factor multiplying the prompt yield. We show that different initial times give rise to different values for the elliptic flow parameter v2 for thermal photons. We also show that a measurement of v2 for thermal photons could also distinguish between the scenarios with or without a phase transition.

Electromagnetic Radiations from Heavy Ion Collision

“…The thermal photons with initial temperature = 200 MeV along with the prompt contributions explain the WA98 data well ( Figure 6), with the inclusion of nonzero chemical potentials for all hadronic species considered [25,26,69,71,72]. In some of the previous works [73][74][75][76][77][78] the effect of chemical freezeout is ignored. As a result either a higher value of or a substantial reduction of hadronic masses in the medium was required [73].…”

“…The quantity, out / side gives the duration of particle emission [134,135,140,141] for various domains of . The difference between 2 side ( ) and 2 out ( ) at nonzero in is then only due to the explicit dependence in (78), that is, the term V ⟨ 2 ⟩. This implies that the explicit dependence dominates if the emission duration is sufficiently large or if the position-momentum correlations in the source are sufficiently weak:…”

“…In this case, the difference between these two HBT radius parameters gives direct access to the average emission duration ⟨ 2 ⟩ of the source and allows to partially disentangle the spatial and temporal information contained in (78). Figure 35 shows the out / side and the difference √ 2 out − 2 side as a function of ⟨ ⟩ for Au+Au collisions at √ = 200 GeV.…”

Identified particle production, azimuthal anisotropy, and interferometry measurements inAu+Aucollisions atsNN<

“…The µ B values shown were estimated at chemical freeze-out. The initial temperatures T initial achieved at top RHIC and SPS energies are obtained from models [63] that explain the direct photon measurements from the PHENIX experiment at RHIC [64] and from the WA98 experiment at SPS [65]. From these models, which assume that thermalization is achieved in the collisions within a time between 0.1 and 1.2 fm/c, the T initial extracted is greater than 300 MeV at RHIC and greater than 200 MeV at SPS.…”

Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC

“…This result is hard to be explained by many models. Several models qualitatively predicted the positive flow of the photons assuming the photons are boosted with hydrodynamic expansion of the system, but the amount is significantly lower than the measurement [13]. There is one model that gives relatively large flow by including hadron-gas interaction [14].…”