2,103 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
Resolving Power of 2-D Pion Interferometry
A analysis is performed to test the resolving power of
two-dimensional pion interferometry using for illustration the preliminary E802
data on at 14.6 AGeV/c. We find that the resolving power to distinguish
two decoupling geometries of different dynamical models is enhanced by studying
the variation of the mean per degrees of freedom with respect to the
range of the analysis in the plane. The preliminary data seem to rule
out dynamical models with significant resonance formation yields.Comment: latex file, 7 pages plus 2 figure
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
Comparison of Viscosities from the Chapman-Enskog and Relaxation Time Methods
A quantitative comparison between the results of shear viscosities from the
Chapman-Enskog and relaxation time methods is performed for selected test cases
with specified elastic differential cross sections: (i) the non-relativistic,
relativistic and ultra-relativistic hard sphere gas with angle and energy
independent differential cross section, (ii) the Maxwell gas, (iii) chiral
pions and (iv) massive pions. Our quantitative results reveal that the extent
of agreement (or disagreement) depends very sensitively on the energy
dependence of the differential cross sections employed.Comment: Submitted to Cent. Eur. J.Phy
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
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.
Searches for gamma ray emission from radio pulsars
Searches were made for pulsed high energy (E 35 MeV) gamma radiation from 43 pulsars using the SAS-2 data base and radio parameters. No positive results were found, and the upper limits are consistent with the concept that gamma ray production efficiency increases with increasing apparent age. Two limits suggest that efficiency cannot be a simple function of apparent age beyond 10,000,000 years
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