Direct-photon spectrum and elliptic flow produced from Pb+Pb collisions at sNN=2.76 TeV at the CERN Large Hadron Collider within an integrated hydrokinetic model

Abstract

The photon transverse momentum spectrum and its anisotropy from Pb+Pb collisions at the CERN Large Hadron Collider energy √ s NN = 2.76 TeV are investigated within the integrated hydrokinetic model (iHKM). Photon production is accumulated from the different processes at the various stages of relativistic heavy ion collisions: from the primary hard photons of very early stage of parton collisions to the thermal photons from equilibrated quark-gluon and hadron gas stages. Along the way a hadronic medium evolution is treated in two distinct, in a sense opposite, approaches: chemically equilibrated and chemically frozen system expansion. Studying the centrality dependence of the results obtained allows us to conclude that a relatively strong transverse momentum anisotropy of thermal radiation is suppressed by prompt photon emission which is an isotropic. We find out that this effect is getting stronger as centrality increases because of the simultaneous increase in the relative contribution of prompt photons in the soft part of the spectra. The substantial results obtained in iHKM with nonzero viscosity (η/s = 0.08) for photon spectra and v 2 coefficients are mostly within the error bars of experimental data, but there is some systematic underestimation of both observables for the near central events. We claim that a situation could be significantly improved if an additional photon radiation that accompanies the presence of a deconfined environment is included. Since a matter of a space-time layer where hadronization takes place is actively involved in anisotropic transverse flow, both positive contributions to the spectra and v 2 are considerable, albeit such an argument needs further research and elaboration.

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

“…In the work [45] an approach was developed allowing to account for the extra portion of soft photons, which presumably should be emitted from the system during the process of Quark-Gluon matter confinement (in our study within iHKM it corresponds to an extra γ emission at the particlization phase of the system's evolution). Such additional electromagnetic radiation could take place, e.g., via the mechanism of "magnetic bremsstrahlung-like" (synchrotron) radiation, coming from the interaction of quarks, which escape from QGP, with the confining collective color field [40].…”

“…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:…”

“…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.…”

Inclusive spectra and Bose-Einstein correlations in small thermal quantum systems

“…The particle yields and their ratios, hadron and photon spectra, anisotropic flows v n , quantum statistical correlation functions that bring information about the chaoticity parameter and interferometry radii, and other observables in A þ A collisions are quite successfully described at the top RHIC energy and all the available LHC energies on the basis of relativistic viscous hydrodynamics-in particular, within the integrated hydrokinetic model (iHKM); see Refs. [9][10][11][12][13][14][15]. The reason for the success of standard hydrodynamic and kinetic methods is that at the active stage of spectra formation in A þ A collisions, the thermal/ effective particle wavelengths are much smaller than the sizes of the system-more precisely, than the corresponding homogeneity lengths [16,17].…”