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Register a new userShear viscosity of hot nuclear matter by the mean free path method
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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.
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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 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.
We calculate the shear viscosity $eta$ and thermal conductivity $kappa$ of a nuclear pasta phase in neutron star crusts. This involves complex non-spherical shapes. We use semiclassical molecular dynamics simulations involving 40,000 to 100,000 nucleons. The viscosity $eta$ can be simply expressed in terms of the height $Z^*$ and width $Delta q$ of the peak in the static structure factor $S_p(q)$. We find that $eta$ increases somewhat, compared to a lower density phase involving spherical nuclei, because $Z^*$ decreases from form factor and ion screening effects. However, we do not find a dramatic increase in $eta$ from non-spherical shapes, as may occur in conventional complex fluids.
We study cold and hot nuclear matter effects on charmonium production in p+Pb collisions at $sqrt{s_text{NN}}=5.02$ TeV in a transport approach. At the forward rapidity, the cold medium effect on all the $cbar c$ states and the hot medium effect on the excited $cbar c$ states only can explain well the $J/psi$ and $psi$ yield and transverse momentum distribution measured by the ALICE collaboration, and we predict a significantly larger $psi$ $p_text{T}$ broadening in comparison with $J/psi$. However, we can not reproduce the $J/psi$ and $psi$ data at the backward rapidity with reasonable cold and hot medium effects.
The phenomenon of low-temperature superconductivity is intimately associated with the condensation of weakly bound, very extended, strongly overlapping Cooper pairs, and systematic experimental studies of the associated mean square radius (coherence length) have been made. While the extension of BCS theory to the atomic nucleus has been successful beyond expectation, to our knowledge, no measurement of the nuclear coherence length (expected to be much larger than nuclear dimensions) has been reported in the literature. Recent studies of Cooper pair transfer across a Josephson-like junction, transiently established in a heavy ion collision between superfluid nuclei, have likely changed the situation, providing the experimental input for a quantitative estimate of the nuclear coherence length, as well as the basis for a nuclear analogue of the (ac) Josephson effect.
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