Shear viscosity in microscopic calculations of A+A collisions at energies available at the Nuclotron-based Ion Collider fAcility (NICAстатьяИсследовательская статья
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Аннотация:Time evolution of shear viscosity $\eta$, entropy density $s$, and theirratio $\eta / s$ in the central area of central gold-gold collisions atNICA energy range is studied within the UrQMD transport model. Theextracted values of energy density, net baryon density and netstrangeness density are used as input to (i) statistical model of idealhadron gas to define temperature, baryo-chemical potential andstrangeness chemical potential, and to (ii) UrQMD box with periodicboundary conditions to study the relaxation process of highly excitedmatter. During the relaxation stage, the shear viscosity is determinedin the framework of Green-Kubo approach. The procedure is performed foreach of 20 time slices, corresponding to conditions in the central areaof the fireball at times from 1~fm/$c$ to 20~fm/$c$. For all testedenergies the ratio $\eta / s$ reaches minimum, $\ds \left( \eta/s\right)_{min} \approx 0.3$ at $t \approx 5$~fm/$c$. Then it increases upto the late stages of the system evolution. This rise is accompanied bythe drop of both, temperature and strangeness chemical potential, andincrease of baryo-chemical potential.