9 research outputs found
Superscaling in electroweak excitation of nuclei
Superscaling properties of 12C, 16O and 40Ca nuclear responses, induced by
electron and neutrino scattering, are studied for momentum transfer values
between 300 and 700 MeV/c. We have defined two indexes to have quantitative
estimates of the scaling quality. We have analyzed experimental responses to
get the empirical values of the two indexes. We have then investigated the
effects of finite dimensions, collective excitations, meson exchange currents,
short-range correlations and final state interactions. These effects strongly
modify the relativistic Fermi gas scaling functions, but they conserve the
scaling properties. We used the scaling functions to predict electron and
neutrino cross sections and we tested their validity by comparing them with the
cross sections obtained with a full calculation. For electron scattering we
also made a comparison with data. We have calculated the total charge-exchange
neutrino cross sections for neutrino energies up to 300 MeV.Comment: 19 pages, 12 figures, 1 table; to be published in Physical Review
Building Atomic Nuclei with the Dirac Equation
The relevance of the Dirac equation for computations of nuclear structure is
motivated and discussed. Quantitatively successful results for medium- and
heavy-mass nuclei are described, and modern ideas of effective field theory and
density functional theory are used to justify them.Comment: 9 pages, REVTeX 4.0 with 12pt.rtx, aps.rtx, amssymb.tex, bm.sty,
ntgdefs.tex. Contribution to the Dirac Centennial Symposium (FSU, 12/6-7/02
Generalized Hartree-Fock Theory for Interacting Fermions in Lattices: Numerical Methods
We present numerical methods to solve the Generalized Hartree-Fock theory for
fermionic systems in lattices, both in thermal equilibrium and out of
equilibrium. Specifically, we show how to determine the covariance matrix
corresponding to the Fermionic Gaussian state that optimally approximates the
quantum state of the fermions. The methods apply to relatively large systems,
since their complexity only scales quadratically with the number of lattice
sites. Moreover, they are specially suited to describe inhomogenous systems, as
those typically found in recent experiments with atoms in optical lattices, at
least in the weak interaction regime. As a benchmark, we have applied them to
the two-dimensional Hubbard model on a 10x10 lattice with and without an
external confinement.Comment: 16 pages, 22 figure
Relativistic Treatment of Hypernuclear Decay
We compute for the first time the decay width of lambda-hypernuclei in a
relativistic mean-field approximation to the Walecka model. Due to the small
mass difference between the lambda-hyperon and its decay products---a nucleon
and a pion---the mesonic component of the decay is strongly Pauli blocked in
the nuclear medium. Thus, the in-medium decay becomes dominated by the
non-mesonic, or two-body, component of the decay. For this mode, the
lambda-hyperon decays into a nucleon and a spacelike nuclear excitation. In
this work we concentrate exclusively on the pion-like modes. By relying on the
analytic structure of the nucleon and pion propagators, we express the
non-mesonic component of the decay in terms of the spin-longitudinal response
function. This response has been constrained from precise quasielastic (p,n)
measurements done at LAMPF. We compute the spin-longitudinal response in a
relativistic random-phase-approximation model that reproduces accurately the
quasielastic data. By doing so, we obtain hypernuclear decay widths that are
considerably smaller---by factors of two or three---relative to existing
nonrelativistic calculations.Comment: Revtex: 18 pages and 4 postscript figure
Phase Transitions in Warm, Asymmetric Nuclear Matter
A relativistic mean-field model of nuclear matter with arbitrary proton
fraction is studied at finite temperature. An analysis is performed of the
liquid-gas phase transition in a system with two conserved charges (baryon
number and isospin) using the stability conditions on the free energy, the
conservation laws, and Gibbs' criteria for phase equilibrium. For a binary
system with two phases, the coexistence surface (binodal) is two-dimensional.
The Maxwell construction through the phase-separation region is discussed, and
it is shown that the stable configuration can be determined uniquely at every
density. Moreover, because of the greater dimensionality of the binodal
surface, the liquid-gas phase transition is continuous (second order by
Ehrenfest's definition), rather than discontinuous (first order), as in
familiar one-component systems. Using a mean-field equation of state calibrated
to the properties of nuclear matter and finite nuclei, various phase-separation
scenarios are considered. The model is then applied to the liquid-gas phase
transition that may occur in the warm, dilute matter produced in energetic
heavy-ion collisions. In asymmetric matter, instabilities that produce a
liquid-gas phase separation arise from fluctuations in the proton concentration
(chemical instability), rather than from fluctuations in the baryon density
(mechanical instability).Comment: Postscript file, 50 pages including 23 figure
Chiral Effective Field Theory for Nuclear Matter with long- and short-range Multi-Nucleon Interactions
We derive a novel chiral power counting scheme for in-medium chiral
perturbation theory with explicit nucleonic and pionic degrees of freedom
coupled to external sources. It allows for a systematic expansion taking into
account local as well as pion-mediated inter-nucleon interactions. Based on
this power counting, one can identify classes of non-perturbative diagrams that
require a resummation. Within this scheme, the pion self-energy in asymmetric
nuclear matter is analyzed and calculated up-to-and-including next-to-leading
order (NLO). It is shown that the corrections involving in-medium
nucleon-nucleon interactions cancel between each other at NLO. As a result,
there are no corrections up to this order to the linear density approximation
for the in-medium pion self-energy.Comment: 16 pages, 7 figures. Final version to match the published on