Phase Transitions in Neutron Stars and Maximum Masses

Using the most recent realistic effective interactions for nuclear matter with a smooth extrapolation to high densities including causality, we constrain the equation of state and calculate maximum masses of rotating neutron stars. First and second order phase transitions to, e.g., quark matter at high densities are included. If neutron star masses of $\sim 2.3M_\odot$ from quasi-periodic oscillations in low mass X-ray binaries are confirmed, a soft equation of state as well as strong phase transitions can be excluded in neutron star cores.Comment: Replaced with revised version, 7 pages, 3 figs. To appear in Ap. J. Let

Influence of Induced Interactions on the Superfluid Transition in Dilute Fermi Gases

We calculate the effects of induced interactions on the transition temperature to the BCS state in dilute Fermi gases. For a pure Fermi system with 2 species having equal densities, the transition temperature is suppressed by a factor $(4e)^{1/3}\approx 2.2$, and for $\nu$ fermion species, the transition temperature is increased by a factor $(4e)^{\nu /3-1} \approx 2.2^{\nu-3}$. For mixtures of fermions and bosons the exchange of boson density fluctuations gives rise to an effective interaction, and we estimate the increase of the transition temperature due to this effect.Comment: 4 pages, 3 figure

Sound modes at the BCS-BEC crossover

First and second sound speeds are calculated for a uniform superfluid gas of fermi atoms as a function of temperature, density and interaction strength. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature. The sound modes and their dependence on density, scattering length and temperature are calculated in the BCS, molecular BEC and unitarity limits and a smooth crossover is extrapolated. It is found that first and second sound undergo avoided crossing on the BEC side due to mixing. Consequently, they are detectable at crossover both as density and thermal waves in traps.Comment: To appear in Phys. Rev.

How to identify a Strange Star

Contrary to young neutron stars, young strange stars are not subject to the r-mode instability which slows rapidly rotating, hot neutron stars to rotation periods near 10 ms via gravitational wave emission. Young millisecond pulsars are therefore likely to be strange stars rather than neutron stars, or at least to contain significant quantities of quark matter in the interior.Comment: 4 pages, 1 figur

Inhomogeneous freeze-out in relativistic heavy-ion collisions

A QCD phase transition may reflect in a inhomogeneous decoupling surface of hadrons produced in relativistic heavy-ion collisions. We show that due to the non-linear dependence of the particle densities on the temperature and baryon-chemical potential such inhomogeneities should be visible even in the integrated, inclusive abundances. We analyze experimental data from Pb+Pb collisions at CERN-SPS and Au+Au collisions at BNL-RHIC to determine the amplitude of inhomogeneities.Comment: 8 pages, 5 figure

Viscosities of Quark-Gluon Plasmas

The quark and gluon viscosities are calculated in quark-gluon plasmas to leading orders in the coupling constant by including screening. For weakly interaction QCD and QED plasmas dynamical screening of transverse interactions and Debye screening of longitudinal interactions controls the infrared divergences. For strongly interacting plasmas other screening mechanisms taken from lattice calculations are employed. By solving the Boltzmann equation for quarks and gluons including screening the viscosity is calculated to leading orders in the coupling constant. The leading logarithmic order is calculated exactly by a full variational treatment. The next to leading orders are found to be very important for sizable coupling constants as those relevant for the transport properties relevant for quark-gluon plasmas created in relativistic heavy ion collisions and the early universe.Comment: 12 pages + 6 figures, report LBL-3492

Phase Transitions in Rotating Neutron Stars

As rotating neutron stars slow down, the pressure and the density in the core region increase due to the decreasing centrifugal forces and phase transitions may occur in the center. We extract the analytic behavior near the critical angular velocity $\Omega_0$, where the phase transitions occur in the center of a neutron star, and calculate the moment of inertia, angular velocity, rate of slow down, braking index, etc. For a first order phase transition these quantities have a characteristic behavior, e.g., the braking index diverges as $\sim(\Omega_0-\Omega)^{-1/2}$. Observational consequences for first, second and other phase transitions are discussed.Comment: 5 pages, one figure included, revtex latex styl

Fermi systems with long scattering lengths

Ground state energies and superfluid gaps are calculated for degenerate Fermi systems interacting via long attractive scattering lengths such as cold atomic gases, neutron and nuclear matter. In the intermediate region of densities, where the interparticle spacing $(\sim 1/k_F)$ is longer than the range of the interaction but shorter than the scattering length, the superfluid gaps and the energy per particle are found to be proportional to the Fermi energy and thus differs from the dilute and high density limits. The attractive potential increase linearly with the spin-isospin or hyperspin statistical factor such that, e.g., symmetric nuclear matter undergoes spinodal decomposition and collapses whereas neutron matter and Fermionic atomic gases with two hyperspin states are mechanically stable in the intermediate density region. The regions of spinodal instabilities in the resulting phase diagram are reduced and do not prevent a superfluid transition.Comment: extended and revised version, 7 pages including new phase diagra

Anisotropic J/ΨJ/\Psi suppression in nuclear collisions

The nuclear overlap zone in non-central relativistic heavy ion collisions is azimuthally very asymmetric. By varying the angle between the axes of deformation and the transverse direction of the pair momenta, the suppression of $J/\Psi$ and $\Psi'$ will oscillate in a characteristic way. Whereas the average suppression is mostly sensitive to the early and high density stages of the collision, the amplitude is more sensitive to the late stages. This effect provides additional information on the $J/\Psi$ suppression mechanisms such as direct absorption on participating nucleons, comover absorption or formation of a quark-gluon plasma. The behavior of the average $J/\Psi$ suppression and its amplitude with centrality of the collisions is discussed for SPS, RHIC and LHC energies with and without a phase transition.Comment: Revised and extended version, new figure