Photon spectra and anisotropic flow in heavy ion collisions at the top RHIC energy within the integrated hydrokinetic model with photon hadronization emission

Abstract

The integrated HydroKinetic Model (iHKM) is applied to analyse the results of direct photon spectra as well as elliptic and triangular flow measurements in 200A GeV Au+Au collisions at RHIC for different centrality bins. Experiments detect the strong centrality dependence of photon elliptic and triangular flow as increasing v n (p T )-coefficients towards peripheral collisions. The photon production in the model is accumulated from the different sources along with the process of relativistic heavy ion collision developing. Those include the primary hard photons from the parton collisions at the very early stage of the process, the photons generated at the pre-thermal phase of dense matter evolution, then thermal photons at partially equilibrated hydrodynamic quark-gluon stage, together with radiation displaying a confinement and, finally, from the hadron gas phase. Along the way a hadronic medium evolution is treated in two distinct, in a sense opposite, approaches: chemically equilibrium and chemically non-equilibrium, namely, chemically frozen expansion. We find the description of direct photon spectra, elliptic and triangular flow are significantly improved, similar to that found in iHKM for the LHC energies, if an additional portion of photon radiation associated with the confinement processes, the "hadronization photons", is included into consideration.

Direct Photon Production in High-Energy Heavy Ion Collisions within the Integrated Hydrokinetic Model

“…The pre-thermal and thermal photon contributions are treated in our consideration based on the matter evolution description, obtained within the iHKM model. The model simulates the full process of the evolution of the system formed in a high-energy nuclear collision, which can be divided into several stages, each described using a relevant approach (see our previous papers, e.g., [1,2,46] for a detailed model description).…”

“…Note, that the quantity D c (representing the emission temporal width in the obtained Expression (6) for momentum spectra) is assumed to depend on collision centrality c, reflecting the fact that the created fireball expands and hadronizes faster for non-central collisions due to higher gradients in transverse direction, caused by smaller transverse size of the system in this case. Let us further assume, following the work [44] and our articles [45,46], that the photon emission function, approximated in our consideration by the product of the two functions F • D c , can be presented in the following thermal-like form:…”

“…The quantity γ hadr in the above formula characterizes the QGP confinement process itself (its basic properties), while the parameter d c ∝ D characterizes its temporal width and thus can be possibly different for different centrality classes. However, in our analysis, carried out in the References [45,46], we combined the product of these two parameters, γ hadr and d c , into a single parameter β ≡ d c γ hadr and used the same its value to describe the hadronization emission of photons for all the considered centrality classes at given collision energy. The value β = 0.04 was used in the case of Au+Au collisions at the top RHIC energy [46] and the value β = 0.02 was used even earlier for 2.76A TeV Pb+Pb collisions at the LHC [45].…”

“…A new possible contribution to the photon spectra connected with the hadronization process was also proposed aiming to improve the observables description and, if possible, to resolve the direct photon puzzle. In this review paper we summarize and re-examine the results obtained in the original works [45,46].…”

“…Recently, the analysis of direct-photon production in heavy-ion collisions at the mentioned RHIC and LHC energies, was also performed within the integrated hydrokinetic model [45,46]. The calculations of thermal photon emission within iHKM were carried out in the two different approaches to the description of the matter evolution at the posthydrodynamic stage-the chemically equilibrated and the chemically frozen ones.…”