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 $\mu_B$ 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 $T_c$ from lattice QCD. The ratio of shear viscosity $\eta$ over entropy density $s$, i.e. $\eta/s$, is evaluated using the collisional widths and compared to lQCD calculations for $\mu_B$ = 0 as well. We find only a very modest change of $\eta/s$ with the baryon chemical $\mu_B$. This also holds for a variety of hadronic observables from central A+A and C+Au collisions in the energy range 5 GeV $\leq \sqrt{s_{NN}} \leq$ 200 GeV when implementing the differential cross sections into the PHSD approach. We only observe small differences in the strangeness and antibaryon sector with practically no sensitivity of rapidity and $p_T$ distributions to the $\mu_B$ dependence of the partonic cross sections. Since we find only small traces of a $\mu_B$-dependence in heavy-ion observables - although the effective partonic masses and widths as well as their partonic cross sections clearly depend on $\mu_B$ - 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 $\mu_B$ is, however, rather low. On the other hand, when decreasing the bombarding energy and thus increasing $\mu_B$, the hadronic phase becomes dominant and accordingly, it will be difficult to extract signals from the partonic dynamics based on "bulk" observables.
Highlights
The phase transition from partonic degrees of freedom to interacting hadrons is a central topic of modern high-energy physics
The following elastic and inelastic interactions are included in parton–hadron–string dynamics (PHSD) qq ↔ qq, q q ↔ q q, gg ↔ gg, gg ↔ g, qq ↔ g, qg ↔ qg, gq ↔ gq exploiting “detailed-balance” with cross sections calculated from the leading Feynman diagrams using the effective propagators and couplings g2(T/Tc) from the dynamical quasiparticle model (DQPM) (Moreau et al 2019)
We mention that, when implementing the differential cross sections and parton masses into the PHSD5.0 approach, one has to specify the” Lagrange parameters” T and μB in each computational cell in space–time. This has been conducted by using the lattice equation of state, which is practically identical to the lattice quantum chromodynamics (QCD) equation of state, and a diagonalization of the energy–momentum tensor from PHSD as described in Moreau et al (2019)
Summary
The phase transition from partonic degrees of freedom (quarks and gluons) to interacting hadrons is a central topic of modern high-energy physics. As relativistic heavy-ion collisions start with impinging nuclei in their ground states, a proper nonequilibrium description of the entire dynamics through possibly different phases up to the final asymptotic hadronic states—eventually showing some degree of equilibration—is mandatory. To this aim, the parton–hadron–string dynamics (PHSD) transport approach (Bratkovskaya et al 2011; Linnyk et al 2013) has been formulated about a decade ago and was found to well describe observables from p + A and A + A collisions from SPS to LHC energies, including electromagnetic probes such as photons and dileptons (Linnyk et al 2016). In order to explore the partonic systems at higher μB, the PHSD approach is extended to incorporate partonic quasiparticles and their differential cross sections that depend on T and μB explicitly
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