23,559 research outputs found
Polarization Measurements and the Pairing Gap in the Universal Regime
We analyze recent cold-atom experiments on imbalanced Fermi systems using a
minimal model with a BCS-like superfluid phase coexisting with a normal phase.
This model is used to extract the T=0 pairing gap in the fully paired
superfluid state. The recently measured particle density profiles are in good
agreement with the theoretical predictions obtained from the universal
parameters from previous Quantum Monte Carlo calculations. We find that the T=0
pairing gap is greater than 0.4 times the Fermi energy , with a preferred
value of . The ratio of the pairing gap to the
Fermi Energy is larger here than in any other system of strongly-paired
fermions in which individual pairs are unbound.Comment: 4 pages, 3 figures. Revised verison includes cosmetic changes to the
text and figures. One reference adde
Comparison of 3-D viscous flow computations of Mach 5 inlet with experimental data
A time marching 3-D full Navier-Stokes code, called PARC3D, is validated for an experimental Mach 5 inlet configuration using the data obtained in the 10 x 10 ft supersonic wind tunnel at the NASA Lewis Research Center. For the first time, a solution is obtained for this configuration with the actual geometry, the tunnel conditions, and all the bleed zones modeled in the computation. Pitot pressure profiles and static pressures at various locations in the inlet are compared with the corresponding experimental data. The effect of bleed zones, located in different places on the inlet walls, in eliminating the low energy vortical flow generated from the 3-D shock-boundary layer interaction is simulated very well even though some approximations are used in applying the bleed boundary conditions and in the turbulence model. A further detailed study of the effect of individual bleed ports is needed to understand fully the actual mechanism of efficiently eliminating the vortical flow from the inlet. A better turbulence model would help to improve the accuracy even further in predicting the corner flow boundary layer profiles
Asymmetric Two-component Fermion Systems in Strong Coupling
We study the phase structure of a dilute two-component Fermi system with
attractive interactions as a function of the coupling and the polarization or
number difference between the two components. In weak coupling, a finite number
asymmetry results in phase separation. A mixed phase containing symmetric
superfluid matter and an asymmetric normal phase is favored. With increasing
coupling strength, we show that the stress on the superfluid phase to
accommodate a number asymmetry increases. Near the infinite-scattering length
limit, we calculate the single-particle excitation spectrum and the
ground-state energy at various polarizations. A picture of weakly-interacting
quasi-particles emerges for modest polarizations. In this regime near infinite
scattering length, and for modest polarizations, a homogeneous phase with a
finite population of excited quasi-particle states characterized by a gapless
spectrum should be favored over the phase separated state. These states may be
realized in cold atom experiments.Comment: 4 pages, 3 figur
Spin Response and Neutrino Emissivity of Dense Neutron Matter
We study the spin response of cold dense neutron matter in the limit of zero
momentum transfer, and show that the frequency dependence of the
long-wavelength spin response is well constrained by sum-rules and the
asymptotic behavior of the two-particle response at high frequency. The
sum-rules are calculated using Auxiliary Field Diffusion Monte Carlo technique
and the high frequency two-particle response is calculated for several
nucleon-nucleon potentials. At nuclear saturation density, the sum-rules
suggest that the strength of the spin response peaks at 40--60
MeV, decays rapidly for 100 MeV, and has a sizable strength below
40 MeV. This strength at relatively low energy may lead to enhanced neutrino
production rates in dense neutron-rich matter at temperatures of relevance to
core-collapse supernova.Comment: 11 pages, 4 figures. Minor change. Published versio
Three Li-rich K giants: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952
We report on spectroscopic analyses of three K giants previously suggested to
be Li-rich: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952.
High-resolution optical spectra and the LTE model atmospheres are used to
derive the stellar parameters: (, log , [Fe/H]), elemental
abundances, and the isotopic ratio C/C. IRAS 13539-4153 shows an
extremely high Li abundance of (Li) 4.2, a value ten
times more than the present Li abundance in the local interstellar medium. This
is the third highest Li abundance yet reported for a K giant. IRAS 12327-6523
shows a Li abundances of (Li) 1.4. IRAS 17596-3952 is a
rapidly rotating ( 35 km s) K giant with
(Li) 2.2. Infrared photometry which shows the presence
of an IR excess suggesting mass-loss. A comparison is made between these three
stars and previously recognized Li-rich giants.Comment: 17 pages, 6 figures, accepted for A
Mass, radius, and composition of the outer crust of nonaccreting cold neutron stars
The properties and composition of the outer crust of nonaccreting cold
neutron stars are studied by applying the model of Baym, Pethick, and
Sutherland, which was extended by including higher order corrections of the
atomic binding, screening, exchange and zero-point energy. The most recent
experimental nuclear data from the atomic mass table of Audi, Wapstra, and
Thibault from 2003 is used. Extrapolation to the drip line is utilized by
various state-of-the-art theoretical nuclear models (finite range droplet,
relativistic nuclear field and non-relativistic Skyrme Hartree-Fock
parameterizations). The different nuclear models are compared with respect to
the mass and radius of the outer crust for different neutron star
configurations and the nuclear compositions of the outer crust.Comment: 5 pages, 2 figures, submitted to J. Phys. G, part of the proceedings
of the Nuclear Physics in Astrophysics III conference in Dresde
Virus Sharing, Genetic Sequencing, and Global Health Security
The WHO’s Pandemic Influenza Preparedness (PIP) Framework was a milestone global agreement designed to promote the international sharing of biological samples to develop vaccines, while that ensuring poorer countries would have access to those vaccines. Since the PIP Framework was negotiated, scientists have developed the capacity to use genetic sequencing data (GSD) to develop synthetic viruses rapidly for product development of life-saving technologies in a time-sensitive global emergency—threatening to unravel the Framework. Access to GSD may also have major implications for biosecurity, biosafety, and intellectual property (IP).
By rendering the physical transfer of viruses antiquated, GSD may also undermine the effectiveness of the PIP Framework itself, with disproportionate impacts on poorer countries. We examine the changes that need to be made to the PIP Framework to address the growing likelihood that GSD might be shared instead of physical virus samples. We also propose that the international community harness this opportunity to expand the scope of the PIP Framework beyond only influenza viruses with pandemic potential.
In light of non-influenza pandemic threats such as the Middle East Respiratory Syndrome (MERS) and Ebola, we call for an international agreement on the sharing of the benefits of research – such as vaccines and treatments – for other infectious diseases to ensure not only a more secure and healthy world, but also a more just world, for humanity
Neutrino scattering off pair-breaking and collective excitations in superfluid neutron matter and in color-flavor locked quark matter
We calculate the correlation functions needed to describe the linear response
of superfluid matter, and go on to calculate the differential cross section for
neutral-current neutrino scattering in superfluid neutron matter and in
color-flavor locked quark matter (CFL). We report the first calculation of
scattering rates that includes neutrino interactions with both pair-breaking
excitations and low-lying collective excitations (Goldstone modes). Our results
apply both above and below the critical temperature, allowing use in
simulations of neutrino transport in supernovae and neutron stars.Comment: 22 pages, 9 figure
Isospin asymmetry and type-I superconductivity in neutron star matter
It has been argued by Buckley et. al.(Phys. Rev. Lett. 92, 151102, 2004) that
nuclear matter is a type-I rather than a type-II superconductor. The suggested
mechanism is a strong interaction between neutron and proton Cooper pairs,
which arises from an assumed U(2) symmetry of the effective potential, which is
supposed to originate in isospin symmetry of the underlying nuclear
interactions. To test this claim, we perform an explicit mean-field calculation
of the effective potential of the Cooper pairs in a model with a simple
four-point pairing interaction. In the neutron star context, matter is very
neutron rich with less than 10% protons, so there is no neutron-proton pairing.
We find that under these conditions our model shows no interaction between
proton Cooper pairs and neutron Cooper pairs at the mean-field level. We
estimate the leading contribution beyond mean field and find that it is is
small and attractive at weak coupling.Comment: 7 pages, 2 figure
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