**5. Conclusions**

In this work, we have studied the influence of the baryon chemical potential *μB* on the properties of the QGP in equilibrium as well as the QGP created in heavy-ion collisions also far from equilibrium.

For the description of the QGP, we employed the extended effective Dynamical QuasiParticle Model (DQPM) that is matched to reproduce the lQCD crossover equation-of-state versus temperature *T* and at finite baryon chemical potential *μ<sup>B</sup>*. We compared the DQPM results for transport coefficients such as shear viscosity *η* and bulk viscosity *ζ* with available lQCD data and the non-conformal holographic model at *μB* = 0 and with results from a Bayesian analysis of experimental heavy-ion data. We find that the ratios *η*/*s* and *ζ*/*s* from the DQPM agree very well with the lQCD results from Ref. [36] and show a similar behavior as the ratio obtained from a Bayesian fit [35]. As found in [17,21], the transport coefficients show a mild dependence on *μ<sup>B</sup>*.

Following [17], we based our study of the non-equilibrium QGP—as created in heavy-ion collisions—on the extended Parton–Hadron–String Dynamics (PHSD) transport approach in which i) the masses and widths of quarks and gluons depend on *T* and *μB* explicitly; ii) the partonic interaction cross sections are obtained by calculations of the leading order Feynman diagrams from the DQPM and explicitly depend on the invariant energy √*<sup>s</sup>*, temperature *T* and baryon chemical potential *μ<sup>B</sup>*. This extension is realized in the full version of PHSD5.0 [17].

In order to investigate the traces of the *μB* dependence of the QGP in observables, the results of PHSD5.0 with *μB* dependences have been compared to the results of PHSD5.0 for *μB* = 0 as well as with PHSD4.0 where the masses/width of quarks and gluons as well as their interaction cross sections depend only on *T* following Ref. [20]. We have presented the PHSD results for different observables: (i) rapidity and *pT* distributions of identified hadrons for asymmetric Cu+Au collisions at energies of 30 AGeV (future NICA energy) as well as for the top RHIC energy of √*sNN* = 200 GeV; (ii) directed flow *v*1 of identified hadrons for *Au* + *Au* at invariant energy √*sNN* = 27 GeV; (iii) elliptic flow *v*2 of identified hadrons for *Au* + *Au* at invariant energies √*sNN* = 27 and 200 GeV. We find only small differences between PHSD4.0 and PHSD5.0 results on the hadronic observables considered here at high as well as at intermediate energies. This is related to the fact that at high energies, where the matter is dominated by the QGP, one probes a very small baryon chemical potential in central collisions at midrapidity, while, with decreasing energy, where *μB* becomes larger, the fraction of the QGP drops rapidly, such that in total the final observables are dominated by the hadrons which participated in hadronic rescattering and thus the information about their QGP origin is washed out. We have shown that the *μB* dependence of QGP interactions is more pronounced in observables for strange hadrons - kaons and especially anti-strange hyperons, as well as for antiprotons. This gives an experimental hint for the searching of *μB* traces of the QGP for experiments at the future NICA accelerator, even if it will be a very challenging experimental task.

**Author Contributions:** Conceptualization, E.B.; methodology, E.B., O.S. and P.M.; software, O.S., P.M., L.O., V.V., V.K. and T.S.; validation, O.S., P.M., L.O., V.V., V.K. and T.S.; formal analysis, O.S., P.M., L.O., V.V., V.K. and T.S.; investigation, O.S., P.M., V.V., L.O., V.K. and T.S.; resources, E.B.; data curation, E.B.; writing—original draft preparation, E.B. and O.S.; writing—review and editing, E.B. and O.S.; visualization, O.S., L.O. and P.M.; supervision, E.B.; project administration, E.B.; funding acquisition, E.B. and L.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Deutsche Forschungsgemeinschaft(DFG, German Research Foundation) through the gran<sup>t</sup> CRC-TR 211 'Strong-interaction matter under extreme conditions' - Project number 315477589 - TRR 211. O.S. acknowledges support from HGS-HIRe for FAIR; L.O. and E.B. thank the COST Action THOR, CA15213. L.O. has been financially supported in part by the Alexander von Humboldt Foundation.

**Acknowledgments:** The authors acknowledge inspiring discussions with Jörg Aichelin, Wolfgang Cassing, Vadim Kolesnikov, Ilya Selyuzhenkov and Arkadiy Taranenko. We thank Maximillian Attems for providing us the results from Ref. [39] in data form. The computational resources have been provided by the LOEWE-Centerfor Scientific Computing.

**Conflicts of Interest:** The authors declare no conflict of interest.
