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

Photodisintegration of the deuteron in the few GeV region using asymptotic amplitudes

Exclusive photodisintegration of the deuteron in the 1-4GeV range is described in terms of a simple covariant and gauge invariant approach using an effective counting rule as the hard part of the d-np vertex. At a scattering angle of theta_cm=90 degrees a prescaling behavior of the differential cross section proportional to 1/s^(n-2) with n ~ 12 is obtained; going away from 90 degrees the value of n decreases slowly, in qualitative agreement with the recent data.Comment: 10 pages, 3 figures, to appear in Physics Letters

The Spectral Function for Finite Nuclei in the Local Density Approximation

The spectral function for finite nuclei is computed within the framework of the Local Density Approximation, starting from nuclear matter spectral functions obtained with a realistic nucleon-nucleon interaction. The spectral function is decomposed into a single-particle part and a ''correlated'' part; the latter is treated in the local density approximation. As an application momentum distributions, quasi-particle strengths and overlap functions for valence hole states, and the light-cone momentum distribution in finite nuclei are computed.Comment: 21 pages + 9 figures available upon request, RevTex, preprint KVI-108

On the nuclear symmetry energy and the neutron skin in neutron-rich nuclei

The symmetry energy for nuclear matter and its relation to the neutron skin in finite nuclei is discussed. The symmetry energy as a function of density obtained in a self-consistent Green function approach is presented and compared to the results of other recent theoretical approaches. A partial explanation of the linear relation between the symmetry energy and the neutron skin is proposed. The potential of several experimental methods to extract the neutron skin is examined.Comment: to appear in Phys. Rev.

Bulk Viscosity in Neutron Stars from Hyperons

The contribution from hyperons to the bulk viscosity of neutron star matter is calculated. Compared to previous works we use for the weak interaction the one-pion exchange model rather than a current-current interaction, and include the neutral current $nn \leftrightarrow n\Lambda$ process. Also the sensitivity to details of the equation of state is examined. Compared to previous works we find that the contribution from hyperons to the bulk viscosity is about two orders of magnitude smaller.Comment: 18 pages, to appear in Physical Review

Center-of-mass effects on the quasi-hole spectroscopic factors in the 16O(e,e'p) reaction

The spectroscopic factors for the low-lying quasi-hole states observed in the 16O(e,e'p)15N reaction are reinvestigated with a variational Monte Carlo calculation for the structure of the initial and final nucleus. A computational error in a previous report is rectified. It is shown that a proper treatment of center-of-mass motion does not lead to a reduction of the spectroscopic factor for $p$-shell quasi-hole states, but rather to a 7% enhancement. This is in agreement with analytical results obtained in the harmonic oscillator model. The center-of-mass effect worsens the discrepancy between present theoretical models and the experimentally observed single-particle strength. We discuss the present status of this problem, including some other mechanisms that may be relevant in this respect.Comment: 14 pages, no figures, uses Revtex, to be published in Phys. Rev. C 58 (1998