250 research outputs found
Coherent neutrino radiation in supernovae at two loops
We develop a neutrino transport theory, in terms of the real-time
non-equilibrium Green's functions, which is applicable to physical conditions
arbitrary far from thermal equilibrium. We compute the coherent neutrino
radiation in cores of supernovae by evaluating the two-particle-two-hole
(2p-2h) polarization function with dressed propagators. The propagator dressing
is carried out in the particle-particle channel to all orders in the
interaction. We show that at two loops there are two distinct sources of
coherence effects in the bremsstrahlung. One is the generically off-shell
intermediate state propagation, which leads to the Landau-Pomeranchuk-Migdal
type suppression of radiation. We extend previous perturbative results,
obtained in the leading order in quasiparticle width, by deriving the exact
non-perturbative expression. A new contribution due to off-shell finial/initial
baryon states is treated in the leading order in the quasiparticle width. The
latter contribution corresponds to processes of higher order than second order
in the virial expansion in the number of quasiparticles. At 2p-2h level, the
time component of the polarization tensor for the vector transitions vanishes
identically in the soft neutrino limit. Vector current thereby is conserved.
The contraction of the neutral axial vector current with tensor interaction
among the baryons leads to a non-vanishing contribution to the bremsstrahlung
rate. These rates are evaluated numerically for finite temperature pure neutron
matter at and above the nuclear saturation density.Comment: 26 pages, 5 figures, uses Revte
Spontaneous breaking of rotational symmetry in superconductors
We show that homogeneous superconductors with broken spin/isospin symmetry
lower their energy via a transition to a novel superconducting state where the
Fermi-surfaces are deformed to a quasi-ellipsoidal form at zero total momentum
of Cooper pairs. In this state, the gain in the condensation energy of the
pairs dominates over the loss in the kinetic energy caused by the lowest order
(quadrupole) deformation of Fermi-surfaces from the spherically symmetric form.
There are two energy minima in general, corresponding to the deformations of
the Fermi-spheres into either prolate or oblate forms. The phase transition
from spherically symmetric state to the superconducting state with broken
rotational symmetry is of the first order.Comment: 5 pages, including 3 figures, published versio
Response functions of cold neutron matter: density fluctuations
We compute the finite temperature density response function of
nonrelativistic cold fermions with an isotropic condensate. The pair-breaking
contribution to the response function is evaluated in the limit of small
three-momentum transfers q within an effective theory which exploits series
expansion in powers of small q/p_F, where p_F is the Fermi momentum. The
leading order O(q^2) contribution is universal and depends only on two
fundamental scales, the Fermi energy and the pairing gap. The particle-hole
Landau Fermi-liquid interaction contributes first at the next-to-leading-order
O(q^4). The scattering contribution to the polarization tensor is
nonperturbative (in the above sense) and is evaluated numerically. The spectral
functions of density fluctuations are constructed and the relevance of the q^2
scaling for the pair-breaking neutrino emission from neutron stars is
discussed.Comment: v2: 11 pages, 4 figures, matches published version
Color-magnetic flux tubes in quark matter cores of neutron stars
We argue that if color-superconducting quark matter exists in the core of a
neutron star, it may contain a high density of flux tubes, carrying flux that
is mostly color-magnetic, with a small admixture of ordinary magnetic flux. We
focus on the two-flavor color-superconducting ("2SC") phase, and assume that
the flux tubes are energetically stable, although this has not yet been
demonstrated. The density of flux tubes depends on the nature of the transition
to the color-superconducting phase, and could be within an order of magnitude
of the density of magnetic flux tubes that would be found if the core were
superconducting nuclear matter. We calculate the cross-section for
Aharonov-Bohm scattering of gapless fermions off the flux tubes, and the
associated collision time and frictional force on a moving flux tube. We
discuss the other forces on the flux tube, and find that if we take in to
account only the forces that arise within the 2SC core region then the
timescale for expulsion of the color flux tubes from the 2SC core is of order
10^10 years.Comment: 28 pages, LaTeX, 1 figure, 2 appendices; added discussion of
energetic stability of flux tube
Damping of differential rotation in neutron stars
We derive the transport relaxation times for quasiparticle-vortex scattering
processes via nuclear force, relevant for the damping of differential rotation
of superfluids in the quantum liquid core of a neutron star. The proton
scattering off the neutron vortices provides the dominant resistive force on
the vortex lattice at all relevant temperatures in the phase where neutrons
only are in the paired state. If protons are superconducting, a small fraction
of hyperons and resonances in the normal state would be the dominant source of
friction on neutron and proton vortex lattices at the core temperatures K.Comment: 5 pages, Revtex, Phys. Rev. D 58, Rapid Communication, in pres
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