Exploring the partonic phase at finite chemical potential within an extended off-shell transport approach

Abstract

We extend the parton-hadron-string dynamics (PHSD) transport approach in the partonic sector by explicitly calculating the total and differential partonic scattering cross sections as a function of temperature T and baryon chemical potential µB on the basis of the effective propagators and couplings from the dynamical quasiparticle model (DQPM) that is matched to reproduce the equation of state of the partonic system above the deconfinement temperature Tc from lattice QCD. We calculate the collisional widths for the partonic degrees of freedom at finite T and µB in the timelike sector and conclude that the quasiparticle limit holds sufficiently well. Furthermore, the ratio of shear viscosity η over entropy density s, i.e., η/s, is evaluated using the collisional widths and compared to lattice QCD calculations for µB = 0 as well. We find that the novel ratio η/s does not differ very much from that calculated within the original DQPM on the basis of the Kubo formalism. Furthermore, there is only a very modest change of η/s with the baryon chemical µB as a function of the scaled temperature T /Tc(µB). This also holds for a variety of hadronic observables from central A + A collisions in the energy range 5 GeV ≤ √ sNN ≤ 200 GeV when implementing the differential cross sections into the PHSD approach. We only observe small differences in the antibaryon sector (p,Λ +Σ 0 ) at √ sNN = 17.3 GeV and 200 GeV with practically no sensitivity of rapidity and pT distributions to the µB dependence of the partonic cross sections. Small variations in the strangeness sector are obtained in all collisional systems studied (A + A and C + Au); however, it will be very hard to extract a robust signal experimentally. Since we find only small traces of a µB dependence in heavy-ion observables -although the effective partonic masses and widths as well as their partonic cross sections clearly depend on µB -this implies that one needs a sizable partonic density and large space-time QGP volume to explore the dynamics in the partonic phase. These conditions are only fulfilled at high bombarding energies where µB is, however, rather low. On the other hand, when decreasing the bombarding energy and thus increasing µB, the hadronic phase becomes dominant and accordingly it will be difficult to extract signals from the partonic dynamics based on "bulk" observables.

Exploring the Partonic Phase at Finite Chemical Potential in and out-of Equilibrium

“…We remind also that in Refs. [17,19,20] we find that the ratio η/s shows a very weak dependence on µ B and has a similar behavior as a function of temperature for all µ B ≤ 400 MeV.…”

“…It contains the information about the properties of our universe 'bulk' observables in HIC for different energies-from AGS to RHIC, and systems-strongly asymmetric C+Au and symmetric Au+Au/Pb+Pb collisions. We have found only a small influence of µ B dependences of parton properties (masses and widths) and their interaction cross sections in bulk observables [17].…”

“…In this work, we extend our study from Ref. [17] to the collective flow (v 1 , v 2 ) coefficients and their sensitivity to the µ B dependences of partonic cross sections. In addition, we explore the relations between the in and out-of equilibrium QGP by means of transport coefficients and collective flows.…”

“…The µ B dependence of ζ/s has been investigated within the DQPM in Refs. [17,19,20], where it has been shown that it is rather weak for µ B ≤ 400 MeV, similar to η/s. As follows from hydrodynamical calculations, the results for the flow harmonic v n is sensitive to the transport coefficients [10,11,34].…”

“…[25] as well as in the latest study with the DQPM model in Refs. [17,19]. Another way to calculate transport coefficients (explored also in [17,19]) is to use the relaxation-time approximation (RTA) [26][27][28][29].…”