Trapped phonons

We analyze the effect of restricted geometries on the contribution of Nambu-Goldstone bosons (phonons) to the shear viscosity, $\eta$, of a superfluid. For illustrative purpose we examine a simplified system consisting of a circular boundary of radius $R$, confining a two-dimensional rarefied gas of phonons. Considering the Maxwell-type conditions, we show that phonons that are not in equilibrium with the boundary and that are not specularly reflected exert a shear stress on the boundary. In this case it is possible to define an effective (ballistic) shear viscosity coefficient $\eta \propto \rho_{\rm ph} \chi R$, where $\rho_{\rm ph}$ is the density of phonons and $\chi$ is a parameter which characterizes the type of scattering at the boundary. For an optically trapped superfluid our results corroborate the findings of Refs. \cite{Mannarelli:2012su, Mannarelli:2012eg}, which imply that at very low temperature the shear viscosity correlates with the size of the optical trap and decreases with decreasing temperature.Comment: 8 pages, 5 figures, talk given at the conference "Xth Quark Confinement and the Hadron Spectrum", October 8-12, 2012, TUM Campus Garching, Munich, German

Torsional oscillations of strange stars

Strange stars are one of the hypothetical compact stellar objects that can be formed after a supernova explosion. The existence of these objects relies on the absolute stability of strange {\it collapsed} quark matter with respect to standard nuclear matter. We discuss simple models of strange stars with a bare quark matter surface, thus standard nuclear matter is completely absent. In these models an electric dipole layer a few hundreds Fermi thick should exist close to the star surface. Studying the torsional oscillations of the electrically charged layer we estimate the emitted power, finding that it is of the order of $10^{45}$ erg/s, meaning that these objects would be among the brightest compact sources in the heavens. The associated relaxation times are very uncertain, with values ranging between microseconds and minutes, depending on the crust thickness. Although part of the radiated power should be absorbed by the electrosphere surrounding the strange star, a sizable fraction of photons should escape and be detectable.Comment: 8 pages, 6 figures, proceedings of QCD@work-International Workshop on QCD - Theory and Experiment 201

The improved Ginzburg-Landau technique

We discuss an innovative method for the description of inhomogeneous phases designed to improve the standard Ginzburg-Landau expansion. The method is characterized by two key ingredients. The first one is a moving average of the order parameter designed to account for the long-wavelength modulations of the condensate. The second one is a sum of the high frequency modes, to improve the description of the phase transition to the restored phase. The method is applied to compare the free energies of 1D and 2D inhomogeneous structures arising in the chirally symmetric broken phase.Comment: 7 pages, 5 figures. Presented at QCD@Work - International Workshop on QCD Theory and Experiment 25-28 June 2018, Matera, Ital

Properties of charmonia in a hot equilibrated medium

We investigate the properties of charmonia in a thermal medium, showing that with increasing temperature the decay widths of these mesons behave in a non-trivial way. Our analysis is based on a potential model with interaction potential extracted from thermal lattice QCD calculations of the free-energy of a static quark-antiquark pair. We find that in the crossover region some decay widths are extremely enhanced, which might be interpreted as a signature of the transition from hadronic matter to the deconfined phase. In particular, at temperatures $T\sim T_c$ the decay widths of the $J/\Psi$ that depend on the value of the wave function at the origin are enhanced with respect to the values in vacuum by about a factor 2. In the same temperature range the decay width of the process $\chi_{c J} \to J/\Psi + \gamma$ is enhanced by approximately a factor 6 with respect to the value in vacuum. At higher temperatures the charmonia states dissociate and the widths of both decay processes become vanishing small.Comment: minor changes, references added, almost matches published versio

Jet-induced gauge field instabilities in the quark-gluon plasma

We discuss the properties of the collective modes of a system composed by a thermalized quark-gluon plasma traversed by a relativistic jet of partons. The transport equations obeyed by the components of the plasma and of the jet are studied in the Vlasov approximation. Assuming that the partons in the jet can be described with a tsunami-like distribution function we derive the expressions of the dispersion law of the collective modes. Then the behavior of the unstable gauge modes of the system is analyzed for various values of the velocity of the jet, of the momentum of the collective modes and of the angle between these two quantities. We find that the most unstable modes are those with momentum orthogonal to the velocity of the jet, and the effect is stronger for ultrarelativistic jet velocities. Our results suggest a new possible collective mechanism for the description of the jet quenching phenomena in heavy ion collisions.Comment: 7 pages; invited talk at the YITP symposium on "Fundamental Problems in Hot and/or dense QCD", Kyoto, Japan, 3-6 March 200