570 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
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.
Quantum Shape-Phase Transitions in Finite Nuclei
Quantum shape-phase transitions in finite nuclei are considered in the
framework of the interacting boson model. Critical-point Hamiltonians for
first- and second-order transitions are identified by resolving them into
intrinsic and collective parts. Suitable wave functions and finite-N estimates
for observables at the critical-points are derived.Comment: 6 pages, 2 figures, Proc. 2nd Int. Conf. on "Collective Motion in
Nuclei under Extreme Conditions" (COMEX 2), June 20-23, 2006, Sankt Goar,
German
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 -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
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