Multiple Hard Partonic Collisions with Correlations in Proton-Proton Scattering

We propose a simple method for incorporating correlations into the impact parameter space description of multiple (semi-)hard partonic collisions in high energy hadron-hadron scattering. The perturbative QCD input is the standard factorization theorem for inclusive dijet production with a lower cutoff on transverse momentum. The width of the transverse distribution of hard partons is fixed by parameterizations of the two-gluon form factor. We then reconstruct the hard contribution to the total inelastic profile function and obtain corrections due to correlations to the more commonly used eikonal description. Estimates of the size of double correlation corrections are based on the rate of double collisions measured at the Tevatron. We find that, if typical values for the lower transverse momentum cutoff are used in the calculation of the inclusive hard dijet cross section, then the correlation corrections are necessary for maintaining consistency with expectations for the total inelastic proton-proton cross section at LHC energies.Comment: Typos fixed, Figures 2,9 and 10 added, matches version published in Phys. Rev.

Observational signatures of convectively driven waves in massive stars

We demonstrate observational evidence for the occurrence of convectively driven internal gravity waves (IGW) in young massive O-type stars observed with high-precision CoRoT space photometry. This evidence results from a comparison between velocity spectra based on 2D hydrodynamical simulations of IGW in a differentially-rotating massive star and the observed spectra.We also show that the velocity spectra caused by IGW may lead to detectable line-profile variability and explain the occurrence of macroturbulence in the observed line profiles of OB stars. Our findings provide predictions that can readily be tested by including a sample of bright slowly and rapidly rotating OB-type stars in the scientific programme of the K2 mission accompanied by high-precision spectroscopy and their confrontation with multi-dimensional hydrodynamic simulations of IGW for various masses and ages.Comment: 4 pages, 3 figures, accepted for publication in The Astrophysical Journal Letter

Magnetohydrodynamic Simulations of the Atmosphere of HD 209458b

We present the first three-dimensional magnetohydrodynamic (MHD) simulations of the atmosphere of HD 209458b which self-consistently include reduction of winds due to the Lorentz force and Ohmic heating. We find overall wind structures similar to that seen in previous models of hot Jupiter atmospheres, with strong equatorial jets and meridional flows poleward near the day side and equatorward near the night side. Inclusion of magnetic fields slows those winds and leads to Ohmic dissipation. We find wind slowing ranging from 10%-40% for reasonable field strengths. We find Ohmic dissipation rates ~10^17 W at 100 bar, orders of magnitude too small to explain the inflated radius of this planet. Faster wind speeds, not achievable in these anelastic calculations, may be able to increase this value somewhat, but likely will not be able to close the gap necessary to explain the inflated radius. We demonstrate that the discrepancy between the simulations presented here and previous models is due to inadequate treatment of magnetic field geometry and evolution. Induced poloidal fields become much larger than those imposed, highlighting the need for a self-consistent MHD treatment of these hot atmospheres.Comment: 6 pages, 4 figures, accepted at ApJ

Semi-parametric analysis of multi-rater data

Datasets that are subjectively labeled by a number of experts are becoming more common in tasks such as biological text annotation where class definitions are necessarily somewhat subjective. Standard classification and regression models are not suited to multiple labels and typically a pre-processing step (normally assigning the majority class) is performed. We propose Bayesian models for classification and ordinal regression that naturally incorporate multiple expert opinions in defining predictive distributions. The models make use of Gaussian process priors, resulting in great flexibility and particular suitability to text based problems where the number of covariates can be far greater than the number of data instances. We show that using all labels rather than just the majority improves performance on a recent biological dataset

Direct numerical simulation of buoyantly driven turbulence

Numerical simulations of homogeneous turbulence subject to buoyant forcing were performed. The presence of a mean temperature gradient combined with a gravitational field results in a forcing term in the momentum equations. The development of the turbulence was studied and compared to the decay of similar fields in the absence of gravity. In the buoyantly driven field, the vorticity is preferentially aligned with the intermediate eigenvector of the strain-rate tensor and the local temperature gradient is more likely to be aligned with the most compressive eigenvector. These relationships are qualitatively similar to those observed in previous shear flow results studied by Ashurst (1987). A tensor diffusivity model for passive scalar transport developed from shear flow results in Rogers, Moin, and Reynolds (1986) also predicts this buoyant scalar transport, indicating that the relationship between the scalar flux and the Reynolds stress is similar in both flows