927 research outputs found
The statistical mechanics of multi-index matching problems with site disorder
We study the statistical mechanics of multi-index matching problems where the
quenched disorder is a geometric site disorder rather than a link disorder. A
recently developed functional formalism is exploited which yields exact results
in the finite temperature thermodynamic limit. Particular attention is paid to
the zero temperature limit of maximal matching problems where the method allows
us to obtain the average value of the optimal match and also sheds light on the
algorithmic heuristics leading to that optimal matchComment: 11 pages 11 figures, RevTe
Shell-model Monte Carlo studies of nuclei far from stability
The shell-model Monte Carlo (SMMC) technique transforms the traditional nuclear shell-model problem into a path-integral over auxiliary fields. The authors describe below the method and its applications to two physics issues: calculations of electron-capture rates, and exploration of pairing correlations in unstable nuclei
Recommended from our members
Shell-model Monte Carlo studies of nuclei
The pair content and structure of nuclei near N = Z are described in the frwnework of shell-model Monte Carlo (SMMC) calculations. Results include the enhancement of J=0 T=1 proton-neutron pairing at N=Z nuclei, and the maxked difference of thermal properties between even-even and odd-odd N=Z nuclei. Additionally, a study of the rotational properties of the T=1 (ground state), and T=0 band mixing seen in {sup 74}Rb is presented
Pairing correlations in N~Z pf-shell nuclei
We perform Shell Model Monte Carlo calculations to study pair correlations in
the ground states of nuclei with masses A=48-60. We find that ,
proton-neutron correlations play an important, and even dominant
role, in the ground states of odd-odd nuclei, in agreement with
experiment. By studying pairing in the ground states of Fe, we
observe that the isovector proton-neutron correlations decrease rapidly with
increasing neutron excess. In contrast, both the proton, and trivially the
neutron correlations increase as neutrons are added.
We also study the thermal properties and the temperature dependence of pair
correlations for Mn and Fe as exemplars of odd-odd and even-even
nuclei. While for Fe results are similar to those obtained for
other even-even nuclei in this mass range, the properties of Mn at low
temperatures are strongly influenced by isovector neutron-proton pairing. In
coexistence with these isovector pair correlations, our calculations also
indicate an excess of isoscalar proton-neutron pairing over the mean-field
values. The isovector neutron-proton correlations rapidly decrease with
temperatures and vanish for temperatures above keV, while the isovector
correlations among like nucleons persist to higher temperatures. Related to the
quenching of the isovector proton-neutron correlations, the average isospin
decreases from 1, appropriate for the ground state, to 0 as the temperature
increases
Relativistic Heavy--Ion Collisions in the Dynamical String--Parton Model
We develop and extend the dynamical string parton model. This model, which is
based on the salient features of QCD, uses classical Nambu-Got\=o strings with
the endpoints identified as partons, an invariant string breaking model of the
hadronization process, and interactions described as quark-quark interactions.
In this work, the original model is extended to include a phenomenological
quantization of the mass of the strings, an analytical technique for treating
the incident nucleons as a distribution of string configurations determined by
the experimentally measured structure function, the inclusion of the gluonic
content of the nucleon through the introduction of purely gluonic strings, and
the use of a hard parton-parton interaction taken from perturbative QCD
combined with a phenomenological soft interaction. The limited number of
parameters in the model are adjusted to and -- data. Utilizing
these parameters, the first calculations of the model for -- and
-- collisions are presented and found to be in reasonable agreement with
a broad set of data.Comment: 26 pages of text with 23 Postscript figures placed in tex
Temperature dependence of the nuclear symmetry energy
We have studied the properties of A=54 and A=64 isobars at temperatures T
\leq 2 MeV via Monte Carlo shell model calculations with two different residual
interactions. In accord with empirical indications, we find that the symmetry
energy coefficient, b_{sym}, is independent of temperature to within 0.6 MeV
for T \leq 1 MeV. This is in contrast to a recent suggestion of a 2.5 MeV
increase of b_{sym} for this temperature, which would have significantly
altered the supernova explosion scenario.Comment: 7 pages, including 2 figures, Caltech preprint MAP-17
Shell model Monte Carlo calculations for Dy-170
We present the first auxiliary field Monte Carlo calculations for a rare
earth nucleus, Dy-170. A pairing plus quadrupole Hamiltonian is used to
demonstrate the physical properties that can be studied in this region. We
calculate various static observables for both uncranked and cranked systems and
show how the shape distribution evolves with temperature. We also introduce a
discretization of the path integral that allows a more efficient Monte Carlo
sampling.Comment: 11 pages, figures available upon request, Caltech Preprint No.
MAP-16
Neutral-current neutrino reactions in the supernova environment
We study the neutral-current neutrino scattering for four nuclei in the iron
region. We evaluate the cross sections for the relevant temperatures during the
supernova core collapse and derive Gamow-Teller distributions from large-scale
shell-model calculations. We show that the thermal population of the excited
states significantly enhances the cross sections at low neutrino energies.
Calculations of the outgoing neutrino spectra indicate the prospect of neutrino
upscattering at finite temperatures. Both results are particularly notable in
even-even nuclei.Comment: 14 pages, 4 figures, accepted in Phys. Lett. B
On the temperature dependence of the symmetry energy
We perform large-scale shell model Monte Carlo (SMMC) calculations for many
nuclei in the mass range A=56-65 in the complete pfg_{9/2}d_{5/2} model space
using an effective quadrupole-quadrupole+pairing residual interaction. Our
calculations are performed at finite temperatures between T=0.33-2 MeV. Our
main focus is the temperature dependence of the symmetry energy which we
determine from the energy differences between various isobaric pairs with the
same pairing structure and at different temperatures. Our SMMC studies are
consistent with an increase of the symmetry energy with temperature. We also
investigate possible consequences for core-collapse supernovae events
- âŠ