326 research outputs found
Electroweak Baryogenesis and Colored Scalars
We consider the 2-loop finite temperature effective potential for a Standard
Model-like Higgs boson, allowing Higgs boson couplings to additional scalars.
If the scalars transform under color, they contribute 2-loop diagrams to the
effective potential that include gluons. These 2-loop effects are perhaps
stronger than previously appreciated. For a Higgs boson mass of 115 GeV, they
can increase the strength of the phase transition by as much as a factor of
3.5. It is the analogue of this effect that is responsible for the survival of
the tenuous electroweak baryogenesis window of the Minimal Supersymmetric
Standard Model. We further illuminate the importance of these 2-loop diagrams
by contrasting models with colored scalars to models with singlet scalars. We
conclude that baryogenesis favors models with light colored scalars. This
motivates searches for pair-produced di-jet resonances or jet(s) + missing
energy.Comment: 12 pages, 5 figures; v2: minor changes, journal versio
Measurements of Top Quark Properties at the Tevatron
The top quark is the most recently discovered of the standard model quarks,
and studies of its properties are important tests of the standard model. Many
measurements of top properties have been produced by the CDF and D0
collaborations, which study top quarks produced in proton-antiproton collisions
at the Fermilab Tevatron with a center-of-mass energy sqrt{s} = 1.96 TeV. We
describe recent results from top properties measurements at the Tevatron using
datasets corresponding to integrated luminosities up to 8.7/fb.Comment: Presented at the 2012 Rencontres de Moriond, QCD and High Energy
Interactions, La Thuile, Italy, March 10-17, 2012. 5 pages, 1 figur
CW high intensity non-scaling FFAG proton drivers
Accelerators are playing increasingly important roles in basic science,
technology, and medicine including nuclear power, industrial irradiation,
material science, and neutrino production. Proton and light-ion accelerators in
particular have many research, energy and medical applications, providing one
of the most effective treatments for many types of cancer. Ultra high-intensity
and high-energy (GeV) proton drivers are a critical technology for
accelerator-driven sub-critical reactors (ADS) and many HEP programs (Muon
Collider). These high-intensity GeV-range proton drivers are particularly
challenging, encountering duty cycle and space-charge limits in the synchrotron
and machine size concerns in the weaker-focusing cyclotrons; a 10-20 MW proton
driver is not presently considered technically achievable with conventional
re-circulating accelerators. One, as-yet, unexplored re-circulating
accelerator, the Fixed-field Alternating Gradient, or FFAG, is an attractive
alternative to the cyclotron. Its strong focusing optics are expected to
mitigate space charge effects, and a recent innovation in design has coupled
stable tunes with isochronous orbits, making the FFAG capable of
fixed-frequency, CW acceleration, as in the classical cyclotron. This paper
reports on these new advances in FFAG accelerator technology and references
advanced modeling tools for fixed-field accelerators developed for and unique
to the code COSY INFINITY.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1
Apr 2011. New York, US
Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering
For over a century, x-ray scattering has been the most powerful tool for
determining the equilibrium structure of crystalline materials. Deviations from
perfect periodicity, for example due to thermal motion of the atoms, reduces
the intensity of the Bragg peaks as well as produces structure in the diffuse
scattering background. Analysis of the thermal diffuse scattering (TDS) had
been used to determine interatomic force constants and phonon dispersion in
relatively simple cases before inelastic neutron scattering became the
preferred technique to study lattice dynamics. With the advent of intense
synchrotron x-ray sources, there was a renewed interest in TDS for measuring
phonon dispersion. The relatively short x-ray pulses emanating from these
sources also enables the measurement of phonon dynamics in the time domain.
Prior experiments on nonequilibrium phonons were either limited by
time-resolution and/or to relatively long wavelength excitations. Here we
present the first images of nonequilibrium phonons throughout the Brillouin
zone in photoexcited III-V semiconductors, indium-phosphide and
indium-antimonide, using picosecond time-resolved diffuse scattering. In each
case, we find that the lattice remain out of equilibrium for several hundred
picoseconds up to nanoseconds after laser excitation. The non-equilibrium
population is dominated by transverse acoustic phonons which in InP are
directed along high-symmetry directions. The results have wide implications for
the detailed study of electron-phonon and phonon-phonon coupling in solids.Comment: 10 pages, 3 figure
A Multi-Code Analysis Toolkit for Astrophysical Simulation Data
The analysis of complex multiphysics astrophysical simulations presents a
unique and rapidly growing set of challenges: reproducibility, parallelization,
and vast increases in data size and complexity chief among them. In order to
meet these challenges, and in order to open up new avenues for collaboration
between users of multiple simulation platforms, we present yt (available at
http://yt.enzotools.org/), an open source, community-developed astrophysical
analysis and visualization toolkit. Analysis and visualization with yt are
oriented around physically relevant quantities rather than quantities native to
astrophysical simulation codes. While originally designed for handling Enzo's
structure adaptive mesh refinement (AMR) data, yt has been extended to work
with several different simulation methods and simulation codes including Orion,
RAMSES, and FLASH. We report on its methods for reading, handling, and
visualizing data, including projections, multivariate volume rendering,
multi-dimensional histograms, halo finding, light cone generation and
topologically-connected isocontour identification. Furthermore, we discuss the
underlying algorithms yt uses for processing and visualizing data, and its
mechanisms for parallelization of analysis tasks.Comment: 18 pages, 6 figures, emulateapj format. Resubmitted to Astrophysical
Journal Supplement Series with revisions from referee. yt can be found at
http://yt.enzotools.org
Rhapsody. I. Structural Properties and Formation History From a Statistical Sample of Re-simulated Cluster-size Halos
We present the first results from the Rhapsody cluster re-simulation project:
a sample of 96 "zoom-in" simulations of dark matter halos of 10^14.8 +- 0.05
Msun/h, selected from a 1 (Gpc/h)^3 volume. This simulation suite is the first
to resolve this many halos with ~5x10^6 particles per halo in the cluster-mass
regime, allowing us to statistically characterize the distribution of and
correlation between halo properties at fixed mass. We focus on the properties
of the main halos and how they are affected by formation history, which we
track back to z=12, over five decades in mass. We give particular attention to
the impact of the formation history on the density profiles of the halos. We
find that the deviations from the Navarro-Frenk-White (NFW) model and the
Einasto model depend on formation time. Late-forming halos tend to have
considerable deviations from both models, partly due to the presence of massive
subhalos, while early-forming halos deviate less but still significantly from
the NFW model and are better described by the Einasto model. We find that the
halo shapes depend only moderately on formation time. Departure from spherical
symmetry impacts the density profiles through the anisotropic distribution of
massive subhalos. Further evidence of the impact of subhalos is provided by
analyzing the phase-space structure. A detailed analysis of the properties of
the subhalo population in Rhapsody is presented in a companion paper.Comment: 20 pages, 13 figures, replaced to match published versio
Rhapsody. II. Subhalo Properties and the Impact of Tidal Stripping From a Statistical Sample of Cluster-Size Halos
We discuss the properties of subhalos in cluster-size halos, using a
high-resolution statistical sample: the Rhapsody simulations introduced in Wu
et al. (2012). We demonstrate that the criteria applied to select subhalos have
significant impact on the inferred properties of the sample, including the
scatter in the number of subhalos, the correlation between the subhalo number
and formation time, and the shape of subhalos' spatial distribution and
velocity structure. We find that the number of subhalos, when selected using
the peak maximum circular velocity in their histories (a property expected to
be closely related to the galaxy luminosity), is uncorrelated with the
formation time of the main halo. This is in contrast to the previously reported
correlation from studies where subhalos are selected by the current maximum
circular velocity; we show that this difference is a result of the tidal
stripping of the subhalos. We also find that the dominance of the main halo and
the subhalo mass fraction are strongly correlated with halo concentration and
formation history. These correlations are important to take into account when
interpreting results from cluster samples selected with different criteria. Our
sample also includes a fossil cluster, which is presented separately and placed
in the context of the rest of the sample.Comment: 15 pages, 10 figures; Paper I: arXiv:1209.3309; replaced to match
published versio
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