V. P. Konchakovski

Hadronic and partonic sources of direct photons in relativistic heavy-ion collisions

The direct photon spectra and flow (v2,xa0v3) in heavy-ion collisions at CERN Super Proton Synchrotron, BNL Relativistic Heavy Ion Collider, and CERN Large Hadron Collider energies are investigated within a relativistic transport approach incorporating both hadronic and partonic phases, the parton-hadron-string dynamics (PHSD). In the present work, four extensions are introduced compared to our previous calculations: (i) going beyond the soft-photon approximation (SPA) in the calculation of the bremsstrahlung processes meson+meson→meson+meson+γ, (ii) quantifying the suppression owing to the Landau-Pomeranchuk-Migdal (LPM) coherence effect, (iii) adding the additional channels V+N→N+γ and Δ→N+γ, and (iv) providing PHSD calculations for Pb+Pb collisions at sNN=2.76TeV. The first issue extends the applicability of the bremsstrahlung calculations to higher photon energies to understand the relevant sources in the region pT=0.5–1.5GeV, while the LPM correction turns out to be important for pT<0.4GeV in the partonic phase. The results suggest that a large elliptic flow v2 of the direct photons signals a significant contribution of photons produced in interactions of secondary mesons and baryons in the late (hadronic) stage of the heavy-ion collision. To further differentiate the origin of the direct photon azimuthal asymmetry (late hadron interactions vs electromagnetic fields in the initial stage), we provide predictions for the photon spectra, elliptic flow, and triangular flow v3(pT) of direct photons at different centralities to be tested by the experimental measurements at the LHC energies. Additionally, we illustrate the magnitude of the photon production in the partonic and hadronic phases as functions of time and local energy density. Finally, the “cocktail” method for an estimation of the background photon elliptic flow, which is widely used in the experimental works, is supported by the calculations within the PHSD transport approach.

Azimuthal anisotropies for Au+Au collisions in the parton-hadron transient energy range

The azimuthal anisotropies of the collective transverse flow of charged hadrons are investigated in a wide range of heavy-ion collision energies within the microscopic parton-hadron-string dynamics (PHSD) transport approach which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation of state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow

Photon elliptic flow in relativistic heavy-ion collisions: hadronic versus partonic sources

v_2

Rise of azimuthal anisotropies as a signature of the Quark-Gluon-Plasma in relativistic heavy-ion collisions

of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The PHSD scaling properties of various collective observables are confronted with experimental data as well as with hydrodynamic predictions. The analysis of higher-order harmonics

Fluctuations and correlations in nucleus–nucleus collisions within transport models

v_3

Theoretical analysis of a possible observation of the chiral magnetic effect in Au+Au collisions within the RHIC beam energy scan program

and

Baryon number and electric charge fluctuations in pb+pb collisions at SPS energies

v_4

Event-by-event background in estimates of the chiral magnetic effect

in the azimuthal angular distribution shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing collision energy. This demonstrates that the excitation functions of azimuthal anisotropies reflect the increasing role of quark-gluon degrees of freedom in the early phase of relativistic heavy-ion collisions. Furthermore, the specific variation of the ratio

Parton-hadron matter in- and out-off equilibrium

v_4/(v_2)^2

K/π, K/p and p/π ratio fluctuations within the HSD transport approach

with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying dynamics.