A systematic study on direct photon production from central heavy-ion collisions

Abstract

A theoretical derivation of photon bremsstrahlung, induced by the interactions of an energetic quark in a hot and dense quark-gluon plasma, is given in the framework of the reaction operator approach. For the physically relevant case of hard jet production, followed by few in-medium interactions , we find that the Landau-Pomeranchuk-Migdal suppression of the bremsstrahlung photon intensity is much stronger than in the previously discussed limit of on-shell quarks and a large number of soft scatterings. This result is incorporated in the first systematic study of direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au heavy ion collisions at the Relativistic Heavy Ion Collider at center of mass energies √ s = 62.4 GeV and 200 GeV. We find that the contribution of the photons created via final-state interactions is limited to 35% for 2 GeV< pT < 5 GeV and at high transverse momenta the modification of the direct photon cross section is dominated by initial-state cold nuclear matter effects.

Hybrid model calculations of direct photons in high-energy nuclear collisions

“…In our calculations, we use N f = 3, and therefore i q 2 i = 2 /3. The temperature dependence of α S is taken from [59] as…”

Measurement of Direct Photons in pp and Pb–Pb Collisions with ALICE

“…They may be modified by isospin effects and nuclear shadowing. Fragmentation photons are affected by jet quenching; saturation effects may lead to further modifications for both sources [2,3].…”

“…In the QGP phase, photons should be produced by the scattering of hard partons traversing the medium with thermalized partons, as well as by the scattering of thermalized partons. In the hot hadronic gas, produced after hadronization, direct photons should emerge from the scattering of the thermalized hadrons within the gas [2,3,4]. The thermalized nature of the production medium (QGP and hadron gas) should be reflected in the p T distribution of the produced thermal photons.…”

Thermal photons in QGP and non-ideal effects

“…Time evolution of the temperature can be calculated using hydrodynamics with appropriate initial conditions. Thus the spectra depend upon the equation of state (EoS) of the medium and they may be useful in finding a signature of the quark-gluon plasma [13][14][15][16]. Recently thermal photons are proposed as a tool to measure the shear viscosity of the strongly interacting matter produced in the collisions [17,18].…”