2,056 research outputs found
Chiral interactions up to next-to-next-to-next-to-leading order and nuclear saturation
We present an efficient Monte Carlo framework for perturbative calculations
of infinite nuclear matter based on chiral two-, three-, and four-nucleon
interactions. The method enables the incorporation of all many-body
contributions in a straightforward and transparent way, and makes it possible
to extract systematic uncertainty estimates by performing order-by-order
calculations in the chiral expansion as well as the many-body expansion. The
versatility of this new framework is demonstrated by applying it to chiral
low-momentum interactions, exhibiting a very good many-body convergence up to
fourth order. Following these benchmarks, we explore new chiral interactions up
to next-to-next-to-next-to-leading order (NLO). Remarkably, simultaneous
fits to the triton and to saturation properties can be achieved, while all
three-nucleon low-energy couplings remain natural. The theoretical
uncertainties of nuclear matter are significantly reduced when going from
next-to-next-to-leading order to NLO.Comment: published version, incl. supplemental materia
Microscopic calculations and energy expansions for neutron-rich matter
We investigate asymmetric nuclear matter with two- and three-nucleon
interactions based on chiral effective field theory, where three-body forces
are fit only to light nuclei. Focusing on neutron-rich matter, we calculate the
energy for different proton fractions and include estimates of the theoretical
uncertainty. We use our ab-initio results to test the quadratic expansion
around symmetric matter with the symmetry energy term, and confirm its validity
for highly asymmetric systems. Our calculations are in remarkable agreement
with an empirical parametrization for the energy density. These findings are
very useful for astrophysical applications and for developing new equations of
state.Comment: 15 pages, 9 figures, published versio
Supernova matter at subnuclear densities as a resonant Fermi gas: Enhancement of neutrino rates
At low energies nucleon-nucleon interactions are resonant and therefore
supernova matter at subnuclear densities has many similarities to atomic gases
with interactions dominated by a Feshbach resonance. We calculate the rates of
neutrino processes involving nucleon-nucleon collisions and show that these are
enhanced in mixtures of neutrons and protons at subnuclear densities due to the
large scattering lengths. As a result, the rate for neutrino pair
bremsstrahlung and absorption is significantly larger below 10^{13} g cm^{-3}
compared to rates used in supernova simulations.Comment: 5 pages, 4 figures, published version, NORDITA-2014-2
Neutron matter from chiral two- and three-nucleon calculations up to NLO
Neutron matter is an ideal laboratory for nuclear interactions derived from
chiral effective field theory since all contributions are predicted up to
next-to-next-to-next-to-leading order (NLO) in the chiral expansion. By
making use of recent advances in the partial-wave decomposition of three-
nucleon (3N) forces, we include for the first time NLO 3N interactions in
many-body perturbation theory (MBPT) up to third order and in self-consistent
Green's function theory (SCGF). Using these two complementary many-body
frameworks we provide improved predictions for the equation of state of neutron
matter at zero temperature and also analyze systematically the many-body
convergence for different chiral EFT interactions. Furthermore, we present an
extension of the normal-ordering framework to finite temperatures. These
developments open the way to improved calculations of neutron-rich matter
including estimates of theoretical uncertainties for astrophysical
applications.Comment: minor changes, published versio
Pairing in neutron matter: New uncertainty estimates and three-body forces
We present solutions of the BCS gap equation in the channels and
in neutron matter based on nuclear interactions derived
within chiral effective field theory (EFT). Our studies are based on a
representative set of nonlocal nucleon-nucleon (NN) plus three-nucleon (3N)
interactions up to next-to-next-to-next-to-leading order (NLO) as well as
local and semilocal chiral NN interactions up to NLO and NLO,
respectively. In particular, we investigate for the first time the impact of
subleading 3N forces at NLO on pairing gaps and also derive uncertainty
estimates by taking into account results for pairing gaps at different orders
in the chiral expansion. Finally, we discuss different methods for obtaining
self-consistent solutions of the gap equation. Besides the widely-used
quasi-linear method by Khodel et al. we demonstrate that the modified Broyden
method is well applicable and exhibits a robust convergence behavior. In
contrast to Khodel's method it is based on a direct iteration of the gap
equation without imposing an auxiliary potential and is straightforward to
implement
Age and Sex of Motor Vehicle Fatality Victims
To develop a system for improving methods of analysis of accident data, the following study was designed. It was decided to make the tabulation of fatality victims for 1955 by age and sex and also by classification as to whether the person was a driver, passenger, or pedestrian
Probing chiral interactions up to next-to-next-to-next-to-leading order in medium-mass nuclei
We study ground-state energies and charge radii of closed-shell medium-mass
nuclei based on novel chiral nucleon-nucleon (NN) and three-nucleon (3N)
interactions, with a focus on exploring the connections between finite nuclei
and nuclear matter. To this end, we perform in-medium similarity
renormalization group (IM-SRG) calculations based on chiral interactions at
next-to-leading order (NLO), NLO, and NLO, where the 3N interactions at
NLO and NLO are fit to the empirical saturation point of nuclear matter
and to the triton binding energy. Our results for energies and radii at NLO
and NLO overlap within uncertainties, and the cutoff variation of the
interactions is within the EFT uncertainty band. We find underbound
ground-state energies, as expected from the comparison to the empirical
saturation point. The radii are systematically too large, but the agreement
with experiment is better. We further explore variations of the 3N couplings to
test their sensitivity in nuclei. While nuclear matter at saturation density is
quite sensitive to the 3N couplings, we find a considerably weaker dependence
in medium-mass nuclei. In addition, we explore a consistent momentum-space SRG
evolution of these NN and 3N interactions, exhibiting improved many-body
convergence. For the SRG-evolved interactions, the sensitivity to the 3N
couplings is found to be stronger in medium-mass nuclei.Comment: 10 pages, 11 figures, published versio
Analysis of the 1956 Automobile Fatalities in Iowa by Age, Sex, and Other Factors
In 1956 there were 69 7 lives lost in motor vehicle accidents in Iowa as compared with the 606 recorded in 1955. Considering sex, the tabulation shows that 171 females and 43 5 males were killed in 1955, whereas 202 females and 495 males lost their lives in 1956 through automobile accidents, or an increase of 31 females and 60 males. Analysis of the results further shows that the age of drivers who lost their lives ranged from 7-86! The ratio of male to female drivers killed in accidents is approximately 7: 1, whereas the overall fatality list showed about 2½ males for each female. The peak age for fatalities among male drivers is from 18-23. This was about the same for both years studied. Whereas the percentage of male drivers above 60 is approximately 11 per cent, the percentage of fatalities runs slightly above 16 per cent. The percentage of licensed female drivers above 60 is approximately 2 per cent, yet their fatality percentage is 12.5 per cent
Dispersion and decay of collective modes in neutron star cores
We calculate the frequencies of collective modes of neutrons, protons and
electrons in the outer core of neutron stars. The neutrons and protons are
treated in a hydrodynamic approximation and the electrons are regarded as
collisionless. The coupling of the nucleons to the electrons leads to Landau
damping of the collective modes and to significant dispersion of the low-lying
modes. We investigate the sensitivity of the mode frequencies to the strength
of entrainment between neutrons and protons, which is not well characterized.
The contribution of collective modes to the thermal conductivity is evaluated.Comment: 10 pages, 4 figure
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