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Register a new userSpecific shear viscosity in hot rotating systems of paired fermions
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The specific shear viscosity $bareta$ of a classically rotating system of nucleons that interact via a monopole pairing interaction is calculated including the effects of thermal fluctuations and coupling to pair vibrations within the selfconsistent quasiparticle random-phase approximation. It is found that $bareta$ increases with angular momentum $M$ at a given temperature $T$. In medium and heavy systems, $bareta$ decreases with increasing $T$ at $Tgeq$ 2 MeV and this feature is not affected much by angular momentum. But in lighter systems (with the mass number $Aleq$ 20), $bareta$ increases with $T$ at a value of $M$ close to the maximal value $M_{max}$, which is defined as the limiting angular momentum for each system. The values of $bareta$ obtained within the schematic model as well as for systems with realistic single-particle energies are always larger than the universal lower-bound conjecture $hbar/(4pi k_B)$ up to $T$=5 MeV.
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The shear viscosity of hot nuclear matter is investigated by using the mean free path method within the framework of IQMD model. Finite size nuclear sources at different density and temperature are initialized based on the Fermi-Dirac distribution. The results show that shear viscosity to entropy density ratio decreases with the increase of temperature and tends toward a constant value for $rhosimrho_0$, which is consistent with the previous studies on nuclear matter formed during heavy-ion collisions. At $rhosimfrac{1}{2}rho_0$, a minimum of $eta/s$ is seen at around $T=10$ MeV and a maximum of the multiplicity of intermediate mass fragment ($M_{text{IMF}}$) is also observed at the same temperature which is an indication of the liquid-gas phase transition.
This work reports on investigations of the effects on the evolution of viscous hydrodynamics and on the flow coefficients of thermal dileptons, originating from a temperature-dependent specific shear viscosity $eta/s (T)$ at temperatures beyond 180 MeV formed at the Relativistic Heavy-Ion Collider (RHIC). We show that the elliptic flow of thermal dileptons can resolve the magnitude of $eta/s$ at the high temperatures, where partonic degrees of freedom become relevant, whereas discriminating between different specific functional forms will likely not be possible at RHIC using this observable.
Shear viscosity $eta$ is calculated for the nuclear matter described as a system of interacting nucleons with the van der Waals (VDW) equation of state. The Boltzmann-Vlasov kinetic equation is solved in terms of the plane waves of the collective overdamped motion. In the frequent-collision regime, the shear viscosity depends on the particle-number density $n$ through the mean-field parameter $a$, which describes attractive forces in the VDW equation. In the temperature region $T=15 - 40$~MeV, a ratio of the shear viscosity to the entropy density $s$ is smaller than 1 at the nucleon number density $n =(0.5 - 1.5),n^{}_0$, where $n^{}_0=0.16,$fm$^{-3}$ is the particle density of equilibrium nuclear matter at zero temperature. A minimum of the $eta/s$ ratio takes place somewhere in a vicinity of the critical point of the VDW system. Large values of $eta/sgg 1$ are, however, found in both the low-density, $nll n^{}_0$, and high-density, $n>2n^{}_0$, regions. This makes the ideal hydrodynamic approach inapplicable for these densities.
The $mathrm{n^{th}}$-order linear flow coefficients $mathrm{v^L_n , (n=2,3,4,5)}$, and the corresponding nonlinear mode-coupled ($mathrm{mc}$) coefficients $mathrm{v^{mc}_{4,(2,2)}}$, $mathrm{v^{mc}_{5,(2,3)}}$, $mathrm{v^{mc}_{6,(3,3)}}$ and $mathrm{v^{mc}_{6,(2,2,2)}}$, are studied for Pb+Pb collisions at $sqrt{s_{_{rm NN}}} = 2.76$ TeV. Both sets of coefficients indicate a common acoustic scaling pattern of exponential viscous modulation, with a rate proportional to the square of the harmonic numbers and the mean transverse momenta (respectively), and inversely proportional to the cube root of the charged particle multiplicity ($mathrm{(N_{ch})^{1/3}}$), that characterizes the dimensionless size of the systems produced in the collisions. These patterns and their associated scaling parameters, provide new stringent constraints for eccentricity independent estimates of the specific shear viscosity ($eta/s$) and the viscous correction to the thermal distribution function for the matter produced in the collisions. They also give crucial constraints for extraction of the initial-state eccentricity spectrum.
We calculate two transport coefficients -- the shear viscosity over entropy ratio $eta/s$ and the ratio of the electric conductivity to the temperature $sigma_0/T$ -- of strongly interacting quark matter within the extended $N_f=3$ Polyakov Nambu-Jona-Lasinio (PNJL) model along the crossover transition line for moderate values of baryon chemical potential $0 leq mu_B leq 0.9$ GeV as well as in the vicinity of the critical endpoint (CEP) and at large baryon chemical potential $mu_B=1.2$ GeV, where the first-order phase transition takes place. The evaluation of the transport coefficients is performed on the basis of the effective Boltzmann equation in the relaxation time approximation. We employ two different methods for the calculation of the quark relaxation times: i) using the averaged transition rate defined via thermal averaged quark-quark and quark-antiquark PNJL cross sections and ii) using the weighted thermal averaged quark-quark and quark-antiquark PNJL cross sections. The $eta/s$ and $sigma_0/T$ transport coefficients have a similar temperature and chemical potential behavior when approaching the chiral phase transition for the both methods for the quark relaxation time, however, the differences grow with increasing temperature. We demonstrate the effect of the first-order phase transition and of the CEP on the transport coefficients in the deconfined QCD medium.
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