637 research outputs found
nPI Resummation in 3D SU(N) Higgs Theory
We test the utility of the nPI formalism for solving nonperturbative dynamics
of gauge theories by applying it to study the phase diagram of SU(N) Higgs
theory in 3 Euclidean spacetime dimensions. Solutions reveal standard
signatures of a first order phase transition with a critical endpoint leading
to a crossover regime, in qualitative agreement with lattice studies. The
location of the critical endpoint, x sim 0.14 for SU(2) with a fundamental
Higgs, is in rough but not tight quantitative agreement with the lattice. We
end by commenting on the overall effectiveness and limitations of an nPI
effective action based study. In particular, we have been unable to find an nPI
gauge-fixing procedure which can simultaneously display the right phase
structure and correctly handle the large-VEV Higgs region. We explain why doing
so appears to be a serious challenge.Comment: 24 pages plus appendices, 8 figure
UV Cascade in Classical Yang-Mills via Kinetic Theory
We show that classical Yang-Mills theory with statistically homogeneous and
isotropic initial conditions has a kinetic description and approaches a scaling
solution at late times. We find the scaling solution by explicitly solving the
Boltzmann equations, including all dominant processes (elastic and
number-changing). Above a scale the occupancy falls
exponentially in . For asymptotically late times and sufficiently small
momenta the occupancy scales as , but this behavior sets in
only at very late time scales. We find quantitative agreement of our results
with lattice simulations, for times and momenta within the range of validity of
kinetic theory.Comment: 18 pages, 4 figure
Limits on Lorentz Violation from the Highest Energy Cosmic Rays
We place several new limits on Lorentz violating effects, which can modify
particles' dispersion relations, by considering the highest energy cosmic rays
observed. Since these are hadrons, this involves considering the partonic
content of such cosmic rays. We get a number of bounds on differences in
maximum propagation speeds, which are typically bounded at the 10^{-21} level,
and on momentum dependent dispersion corrections of the form v = 1 +-
p^2/Lambda^2, which typically bound Lambda > 10^{21} GeV, well above the Planck
scale. For (CPT violating) dispersion correction of the form v = 1 + p/Lambda,
the bounds are up to 15 orders of magnitude beyond the Planck scale.Comment: 24 pages, no figures. Added references, very slight changes. Version
published in Physical Review
Energy Loss of Leading Hadrons and Direct Photon production in Evolving Quark-Gluon Plasma
We calculate the nuclear modification factor of neutral pions and the photon
yield at high p_T in central Au-Au collisions at RHIC (\sqrt{s}=200 GeV) and
Pb-Pb collisions at the LHC (\sqrt{s}=5500 GeV). A leading-order accurate
treatment of jet energy loss in the medium has been convolved with a physical
description of the initial spatial distribution of jets and a (1+1) dimensional
expansion. We reproduce the nuclear modification factor of pion R_{AA} at RHIC,
assuming an initial temperature T_i=370 MeV and a formation time \tau_i=0.26
fm/c, corresponding to dN/dy=1260. The resulting suppression depends on the
particle rapidity density dN/dy but weakly on the initial temperature. The jet
energy loss treatment is also included in the calculation of high p_T photons.
Photons coming from primordial hard N-N scattering are the dominant
contribution at RHIC for p_T > 5 GeV, while at the LHC, the range 8<p_T<14 GeV
is dominated by jet-photon conversion in the plasma.Comment: 21 pages, 16 figures. Discussions and references added. New figure
includind photon dat
Philanthropy and COVID-19 in 2020: Measuring One Year of Giving
It has been a year since the global outbreak of COVID-19, and the world is still recovering and operating in what we have come to accept as the "new normal." In 2020, we saw funders react swiftly, not only directing emergency funds to organizations on the ground but also committing to changes in their grantmaking practices and priorities to better help nonprofits face the myriad challenges brought on by the pandemic. In this report, Candid and the Center for Disaster Philanthropy look at the global philanthropic response to COVID-19 in 2020
Magnetism and magnetocaloric properties of CoMnCrO
CoMnCrO crystallizes as a normal spinel in the cubic space group, and the end members have been reported to display a
region of collinear ferrimagnetism as well as a low-temperature spin-spiral
state with variable coherence lengths from 3 nm to 10 nm in polycrystalline
samples. Here, we present the synthesis of the entire solid solution, and data
showing that the ferrimagnetic ordering temperature as well as the spin-spiral
lock-in temperature are tunable with the Co/Mn ratio. The peak magnetocaloric
entropy change was determined to be = -5.63 J kg K
in an applied magnetic field change of = 0 T to 5 T for the Mn
end-member at the ferrimagnetic ordering temperature. Using density functional
theory (DFT), we explore the shortcomings of the magnetic deformation proxy to
identify trends in across composition in this spinel system, and
explore future extensions of theory to address these discrepancies
High-throughput determination of Hubbard U and Hund J values for transition metal oxides via linear response formalism
DFT+U provides a convenient, cost-effective correction for the
self-interaction error (SIE) that arises when describing correlated electronic
states using conventional approximate density functional theory (DFT). The
success of a DFT+U(+J) calculation hinges on the accurate determination of its
Hubbard U and Hund's J parameters, and the linear response (LR) methodology has
proven to be computationally effective and accurate for calculating these
parameters. This study provides a high-throughput computational analysis of the
U and J values for transition metal d-electron states in a representative set
of over 2000 magnetic transition metal oxides (TMOs), providing a frame of
reference for researchers who use DFT+U to study transition metal oxides. In
order to perform this high-throughput study, an atomate workflow is developed
for calculating U and J values automatically on massively parallel
supercomputing architectures. To demonstrate an application of this workflow,
the spin-canting magnetic structure and unit cell parameters of the
multiferroic olivine LiNiPO4 are calculated using the computed Hubbard U and
Hund J values for Ni-d and O-p states, and are compared with experiment. Both
the Ni-d U and J corrections have a strong effect on the Ni-moment canting
angle. Additionally, including a O-p U value results in a significantly
improved agreement between the computed lattice parameters and experiment.Comment: 18 pages, 6 figure
Transition temperature of a dilute homogeneous imperfect Bose gas
The leading-order effect of interactions on a homogeneous Bose gas is
theoretically predicted to shift the critical temperature by an amount
\Delta\Tc = # a_{scatt} n^{1/3} T_0 from the ideal gas result T_0, where
a_{scatt} is the scattering length and n is the density. There have been
several different theoretical estimates for the numerical coefficient #. We
claim to settle the issue by measuring the numerical coefficient in a lattice
simulation of O(2) phi^4 field theory in three dimensions---an effective theory
which, as observed previously in the literature, can be systematically matched
to the dilute Bose gas problem to reproduce non-universal quantities such as
the critical temperature. We find # = 1.32 +- 0.02.Comment: 4 pages, submitted to Phys. Rev. Lett; minor changes due to
improvement of analysis in the longer companion pape
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