Analysis of dilepton production inAu+Aucollisions atsNN=200GeV within the parto

Abstract

We address dilepton production in Au+Au collisions at √ sNN = 200 GeV by employing the parton-hadron-string dynamics (PHSD) off-shell transport approach. Within the PHSD one goes beyond the quasiparticle approximation by solving generalized transport equations on the basis of the off-shell Kadanoff-Baym equations for the Green's functions in the phase-space representation. The approach consistently describes the full evolution of a relativistic heavy-ion collision from the initial hard scatterings and string formation through the dynamical deconfinement phase transition to the quark-gluon plasma (QGP) as well as hadronization and to the subsequent interactions in the hadronic phase. With partons described in the PHSD by the dynamical quasiparticle model (DQPM) -matched to reproduce lattice QCD results in thermodynamic equilibrium -we calculate, in particular, the dilepton radiation from partonic interactions through the reactions qq → γ * , qq → γ * + g and qg → γ * q (qg → γ * q ) in the early stage of relativistic heavy-ion collisions. By comparing our results to the data from the PHENIX Collaboration, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant. Furthermore, explicit predictions are presented for dileptons within the acceptance of the STAR detector system and compared to the preliminary data.

The STAR Dilepton Physics Program

“…While there remains a quantitative discrepancy between the PHENIX and STAR measurements at √ s NN = 200 GeV [17,18], we observe a comparable agreement between the CERES measurements in the Pb+Au at √ s NN =17.2 GeV [7,19]. Various models have shown to be in good agreement with LMR spectrum measured by STAR in central Au+Au collisions at 200 GeV [21,22,23]. The BES measurements provide an excellent opportunity to further verify robust and consistent model descriptions down to SPS energies.…”

Impact of the initial size of spatial fluctuations on the collective flow in Pb–Pb collisions at $\sqrt{{{s}_{{\rm NN}}}}$ = 2.76 TeV

“…We find that this approach works also reasonably for Pb-Pb collisions at √ s N N = 2.76 TeV with respect to charged particle spectra as well as collective flow coefficients v 2 , v 3 , v 4 and v 5 for different centralities with a quality comparable to that achieved at RHIC energies before [21,49,50,51,52,54]. Our finding implies that the 'soft' physics in Pb-Pb collisions at the LHC and Au-Au interactions at the top RHIC energies -despite a factor of ∼ 14 in √ s N N -is very similar and in line with the dynamical quasiparticle model (DQPM) that defines the parton properties for PHSD in equilibrium.…”

“…[48]; model results and their comparison with experimental observables for heavy-ion collisions from the lower super-proton-synchrotron (SPS) to relativistic-heavy-ion-collider (RHIC) energies can be found in Refs. [21,49,50,51] including electromagnetic probes such as e + e − or µ + µ − pairs [52,53] or real photons [54].…”

“…Note that except for the upgrade in the PYTHIA version no additional parameters or changes have been introduced in the PHSD that had been employed before in Refs. [21,49,50,51,52,54] from lower SPS up to top RHIC energies. In this respect the approximate reproduction of the midrapidity p T spectra for central collisions over 7 orders of magnitude in Fig.…”

Dilepton production from RHIC to the LHC

“…STAR measures a smaller enhancement, shown in the left panel of Fig. 4, that does not depend as strongly on centrality and is well described by theoretical models of a broadened ρ spectral function [14,15]. Besides a slightly different mass region, in which the enhancement is defined, one should point out two further differences in the calculation of the "enhancement above hadron cocktail" factor: the charm cross section used by STAR (dσ NN cc /dy = 171 ± 26 μb) is a factor 1.4 larger than the one used by PHENIX (dσ NN cc /dy = 123 ± 12 ± 37 μb).…”