1,456 research outputs found
Unitary Fermi Gas in a Harmonic Trap
We present an {\it ab initio} calculation of small numbers of trapped,
strongly interacting fermions using the Green's Function Monte Carlo method
(GFMC). The ground state energy, density profile and pairing gap are calculated
for particle numbers using the parameter-free "unitary"
interaction. Trial wave functions are taken of the form of correlated pairs in
a harmonic oscillator basis. We find that the lowest energies are obtained with
a minimum explicit pair correlation beyond that needed to exploit the
degeneracy of oscillator states. We find that energies can be well fitted by
the expression where is the
Thomas-Fermi energy of a noninteracting gas in the trap and is a
pairing gap. There is no evidence of a shell correction energy in the
systematics, but the density distributions show pronounced shell effects. We
find the value for the pairing gap. This is smaller
than the value found for the uniform gas at a density corresponding to the
central density of the trapped gas.Comment: 2 figures, 2 table
The information content of a new observable: the case of the nuclear neutron skin
We address two questions pertaining to the uniqueness and usefulness of a new
observable: (i) Considering the current theoretical knowledge, what novel
information does new measurement bring in? (ii) How can new data reduce
uncertainties of current theoretical models? We illustrate these points by
studying the radius of the neutron distribution of a heavy nucleus, a quantity
related to the equation of state for neutron matter that determines properties
of nuclei and neutron stars. By systematically varying parameters of two
theoretical models and studying the resulting confidence ellipsoid, we quantify
the relationships between the neutron skin and various properties of finite
nuclei and infinite nuclear matter. Using the covariance analysis, we identify
observables and pseudo-observables that correlate, and do not correlate, with
the neutron skin. By adding the information on the neutron radius to the pool
of observables determining the energy functional, we show how precise
experimental determination of the neutron radius in Pb would reduce
theoretical uncertainties on the neutron matter equation of state.Comment: 5 pages, 3 figure
Shell Model Monte Carlo Studies of -Soft Nuclei
We present Shell Model Monte Carlo calculations for nuclei within the full
major shell 50-82 for both protons and neutrons. The interaction is determined
solely by self-consistency and odd-even mass differences. The methods are
illustrated for Sn, Te and Xe. We calculate shape
distributions, moments of inertia and pairing correlations as functions of
temperature and angular velocity. Our calculations are the first microscopic
evidence of -softness of nuclei in this region.Comment: uuencoded postscript of manuscript with three figure
Extracting particle freeze-out phase-space densities and entropies from sources imaged in heavy-ion reactions
The space-averaged phase-space density and entropy per particle are both
fundamental observables which can be extracted from the two-particle
correlation functions measured in heavy-ion collisions. Two techniques have
been proposed to extract the densities from correlation data: either by using
the radius parameters from Gaussian fits to meson correlations or by using
source imaging, which may be applied to any like pair correlation. We show that
the imaging and Gaussian fits give the same result in the case of meson
interferometry. We discuss the concept of an equivalent instantaneous source on
which both techniques rely. We also discuss the phase-space occupancy and
entropy per particle. Finally, we propose an improved formula for the
phase-space occupancy that has a more controlled dependence on the uncertainty
of the experimentally measured source functions.Comment: 14 pages, final version, to appear PRC. Fixed typos, added refs. for
last section, added discussions of imaging and d/p ratio
Nuclear correlations and the r-process
We show that long-range correlations for nuclear masses have a significant
effect on the synthesis of heavy elements by the r-process. As calculated by
Delaroche et al. [1], these correlations suppress magic number effects
associated with minor shells. This impacts the calculated abundances before the
third r-process peak (at mass number A~195), where the abundances are low and
form a trough. This trough and the position of the third abundance peak are
strongly affected by the masses of nuclei in the transition region between
deformed and spherical. Based on different astrophysical environments, our
results demonstrate that a microscopic theory of nuclear masses including
correlations naturally smoothens the separation energies, thus reducing the
trough and improving the agreement with observed solar system abundances.Comment: 4 pages, 3 figures, submitted to PR
Source Dimensions in Ultrarelativistic Heavy Ion Collisions
Recent experiments on pion correlations, interpreted as interferometric
measurements of the collision zone, are compared with models that distinguish a
prehadronic phase and a hadronic phase. The models include prehadronic
longitudinal expansion, conversion to hadrons in local kinetic equilibrium, and
rescattering of the produced hadrons. We find that the longitudinal and outward
radii are surprisingly sensitive to the algorithm used for two-body collisions.
The longitudinal radius measured in collisions of 200 GeV/u sulfur nuclei on a
heavy target requires the existence of a prehadronic phase which converts to
the hadronic phase at densities around 0.8-1.0 GeV/fm. The transverse radii
cannot be reproduced without introducing more complex dynamics into the
transverse expansion.Comment: RevTeX 3.0, 28 pages, 6 figures, not included, revised version, major
change is an additional discussion of the classical two-body collision
algorithm, a (compressed) postscript file of the complete paper including
figures can be obtained from Authors or via anonymous ftp at
ftp://ftp_int.phys.washington.edu/pub/herrmann/pisource.ps.
Nonadiabatic generation of coherent phonons
The time-dependent density functional theory (TDDFT) is the leading
computationally feasible theory to treat excitations by strong electromagnetic
fields. Here the theory is applied to coherent optical phonon generation
produced by intense laser pulses. We examine the process in the crystalline
semimetal antimony (Sb), where nonadiabatic coupling is very important. This
material is of particular interest because it exhibits strong phonon coupling
and optical phonons of different symmetries can be observed. The TDDFT is able
to account for a number of qualitative features of the observed coherent
phonons, despite its unsatisfactory performance on reproducing the observed
dielectric functions of Sb. A simple dielectric model for nonadiabatic coherent
phonon generation is also examined and compared with the TDDFT calculations.Comment: 19 pages, 11 figures. This is prepared for a special issue of Journal
of Chemical Physics on the topic of nonadiabatic processe
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