217 research outputs found
CMB Constraints on WIMP Annihilation: Energy Absorption During the Recombination Epoch
We compute in detail the rate at which energy injected by dark matter
annihilation heats and ionizes the photon-baryon plasma at z ~ 1000, and
provide accurate fitting functions over the relevant redshift range for a broad
array of annihilation channels and DM masses. The resulting perturbations to
the ionization history can be constrained by measurements of the CMB
temperature and polarization angular power spectra. We show that models which
fit recently measured excesses in 10-1000 GeV electron and positron cosmic rays
are already close to the 95% confidence limits from WMAP. The recently launched
Planck satellite will be capable of ruling out a wide range of DM explanations
for these excesses. In models of dark matter with Sommerfeld-enhanced
annihilation, where sigma v rises with decreasing WIMP velocity until some
saturation point, the WMAP5 constraints imply that the enhancement must be
close to saturation in the neighborhood of the Earth.Comment: 17 pages, 6 figures, v2 extends discussion of constraints on
Sommerfeld-enhanced model
Variance Calculations and the Bessel Kernel
In the Laguerre ensemble of N x N (positive) hermitian matrices, it is of
interest both theoretically and for applications to quantum transport problems
to compute the variance of a linear statistic, denoted var_N f, as N->infinity.
Furthermore, this statistic often contains an additional parameter alpha for
which the limit alpha->infinity is most interesting and most difficult to
compute numerically. We derive exact expressions for both lim_{N->infinity}
var_N f and lim_{alpha->infinity}lim_{N->infinity} var_N f.Comment: 7 pages; resubmitted to make postscript compatibl
Universal Parametric Correlations of Eigenvalues of Random Matrix Ensemble
Eigenvalue correlations of random matrix ensembles as a function of an
external perturbation are investigated vis the Dyson Brownian Motion Model in
the situation where the level density has a hard edge singularity. By solving a
linearized hydrodynamical equation, a universal dependence of the
density-density correlator on the external field is found. As an application we
obtain a formula for the variance of linear statistics with the parametric
dependence exhibited as a Laplace transform.Comment: 23 pages, late
Transitions In Spectral Statistics
We present long range statistical properties of a recently introduced unitary
random matrix ensemble, whose short range correlations were found to describe a
transition from Wigner to Poisson type as a function of a single parameter.Comment: 12 pp. late
Frailty and Risk of Falls, Fracture, and Mortality in Older Women: The Study of Osteoporotic Fractures
Background. A standard phenotype of frailty was associated with an increased risk of adverse outcomes including mortality in a recent study of older adults. However, the predictive validity of this phenotype for fracture outcomes and across risk subgroups is uncertain. Methods. To determine whether a standard frailty phenotype was independently associated with risk of adverse health outcomes in older women and to evaluate the consistency of associations across risk subgroups defined by age and body mass index (BMI), we ascertained frailty status in a cohort of 6724 women ≥ 69 years and followed them prospectively for incident falls, fractures, and mortality. Frailty was defined by the presence of three or more of the following criteria: unintentional weight loss, weakness, self-reported poor energy, slow walking speed, and low physical activity. Incident recurrent falls were defined as at least two falls during the subsequent year. Incident fractures (confirmed with x-ray reports), including hip fractures, and deaths were ascertained during an average of 9 years of follow-up. Results. After controlling for multiple confounders such as age, health status, medical conditions, functional status, depressive symptoms, cognitive function, and bone mineral density, frail women were subsequently at increased risk of recurrent falls (multivariate odds ratio = 1.38, 95% confidence interval [CI], 1.02-1.88), hip fracture (multivariate hazards ratio [MHR] = 1.40, 95% CI, 1.03-1.90), any nonspine fracture (MHR = 1.25, 95% CI, 1.05-1.49), and death (MHR = 1.82, 95% CI, 1.56-2.13). The associations between frailty and these outcomes persisted among women ≥ 80 years. In addition, associations between frailty and an increased risk of falls, fracture, and mortality were consistently observed across categories of BMI, including BMI ≥ 30 kg/m2. Conclusion. Frailty is an independent predictor of adverse health outcomes in older women, including very elderly women and older obese wome
Thermally Driven Electronic Topological Transition in FeTi
Ab initio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic x-ray scattering, showed an anomalous thermal softening of the M^−_5 phonon mode in B2-ordered FeTi that could not be explained by phonon-phonon interactions or electron-phonon interactions calculated at low temperatures. A computational investigation showed that the Fermi surface undergoes a novel thermally driven electronic topological transition, in which new features of the Fermi surface arise at elevated temperatures. The thermally induced electronic topological transition causes an increased electronic screening for the atom displacements in the M^−_5 phonon mode and an adiabatic electron-phonon interaction with an unusual temperature dependence
A Brownian Motion Model of Parametric Correlations in Ballistic Cavities
A Brownian motion model is proposed to study parametric correlations in the
transmission eigenvalues of open ballistic cavities. We find interesting
universal properties when the eigenvalues are rescaled at the hard edge of the
spectrum. We derive a formula for the power spectrum of the fluctuations of
transport observables as a response to an external adiabatic perturbation. Our
formula correctly recovers the Lorentzian-squared behaviour obtained by
semiclassical approaches for the correlation function of conductance
fluctuations.Comment: 19 pages, written in RevTe
Ground-State Magnetization for Interacting Fermions in a Disordered Potential : Kinetic Energy, Exchange Interaction and Off-Diagonal Fluctuations
We study a model of interacting fermions in a disordered potential, which is
assumed to generate uniformly fluctuating interaction matrix elements. We show
that the ground state magnetization is systematically decreased by off-diagonal
fluctuations of the interaction matrix elements. This effect is neglected in
the Stoner picture of itinerant ferromagnetism in which the ground-state
magnetization is simply determined by the balance between ferromagnetic
exchange and kinetic energy, and increasing the interaction strength always
favors ferromagnetism. The physical origin of the demagnetizing effect of
interaction fluctuations is the larger number of final states available for
interaction-induced scattering in the lower spin sectors of the Hilbert space.
We analyze the energetic role played by these fluctuations in the limits of
small and large interaction . In the small limit we do second-order
perturbation theory and identify explicitly transitions which are allowed for
minimal spin and forbidden for higher spin. These transitions then on average
lower the energy of the minimal spin ground state with respect to higher spin.
For large interactions we amplify on our earlier work [Ph. Jacquod and A.D.
Stone, Phys. Rev. Lett. 84, 3938 (2000)] which showed that minimal spin is
favored due to a larger broadening of the many-body density of states in the
low-spin sectors. Numerical results are presented in both limits.Comment: 35 pages, 24 figures - final, shortened version, to appear in
Physical Review
Thermally Driven Electronic Topological Transition in FeTi
Ab initio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic x-ray scattering, showed an anomalous thermal softening of the M^−_5 phonon mode in B2-ordered FeTi that could not be explained by phonon-phonon interactions or electron-phonon interactions calculated at low temperatures. A computational investigation showed that the Fermi surface undergoes a novel thermally driven electronic topological transition, in which new features of the Fermi surface arise at elevated temperatures. The thermally induced electronic topological transition causes an increased electronic screening for the atom displacements in the M^−_5 phonon mode and an adiabatic electron-phonon interaction with an unusual temperature dependence
Frailty and risk of falls, fracture, and mortality in older women: the study of osteoporotic fractures
BACKGROUND: A standard phenotype of frailty was associated with an increased risk of adverse outcomes including mortality in a recent study of older adults. However, the predictive validity of this phenotype for fracture outcomes and across risk subgroups is uncertain. METHODS: To determine whether a standard frailty phenotype was independently associated with risk of adverse health outcomes in older women and to evaluate the consistency of associations across risk subgroups defined by age and body mass index (BMI), we ascertained frailty status in a cohort of 6724 women>or=69 years and followed them prospectively for incident falls, fractures, and mortality. Frailty was defined by the presence of three or more of the following criteria: unintentional weight loss, weakness, self-reported poor energy, slow walking speed, and low physical activity. Incident recurrent falls were defined as at least two falls during the subsequent year. Incident fractures (confirmed with x-ray reports), including hip fractures, and deaths were ascertained during an average of 9 years of follow-up. RESULTS: After controlling for multiple confounders such as age, health status, medical conditions, functional status, depressive symptoms, cognitive function, and bone mineral density, frail women were subsequently at increased risk of recurrent falls (multivariate odds ratio=1.38, 95% confidence interval [CI], 1.02-1.88), hip fracture (multivariate hazards ratio [MHR]=1.40, 95% CI, 1.03-1.90), any nonspine fracture (MHR=1.25, 95% CI, 1.05-1.49), and death (MHR=1.82, 95% CI, 1.56-2.13). The associations between frailty and these outcomes persisted among women>or=80 years. In addition, associations between frailty and an increased risk of falls, fracture, and mortality were consistently observed across categories of BMI, including BMI>or=30 kg/m2. CONCLUSION: Frailty is an independent predictor of adverse health outcomes in older women, including very elderly women and older obese women
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