570 research outputs found
The WISDOM Radar: Unveiling the Subsurface Beneath the ExoMars Rover and Identifying the Best Locations for Drilling
The search for evidence of past or present life on Mars is the principal objective of the 2020 ESA-Roscosmos ExoMars Rover mission. If such evidence is to be found anywhere, it will most likely be in the subsurface, where organic molecules are shielded from the destructive effects of ionizing radiation and atmospheric oxidants. For this reason, the ExoMars Rover mission has been optimized to investigate the subsurface to identify, understand, and sample those locations where conditions for the preservation of evidence of past life are most likely to be found. The Water Ice Subsurface Deposit Observation on Mars (WISDOM) ground-penetrating radar has been designed to provide information about the nature of the shallow subsurface over depth ranging from 3 to 10 m (with a vertical resolution of up to 3 cm), depending on the dielectric properties of the regolith. This depth range is critical to understanding the geologic evolution stratigraphy and distribution and state of subsurface H2O, which provide important clues in the search for life and the identification of optimal drilling sites for investigation and sampling by the Rover's 2-m drill. WISDOM will help ensure the safety and success of drilling operations by identification of potential hazards that might interfere with retrieval of subsurface samples
Natural Little Hierarchy from a Partially Goldstone Twin Higgs
We construct a simple theory in which the fine-tuning of the standard model
is significantly reduced. Radiative corrections to the quadratic part of the
scalar potential are constrained to be symmetric under a global U(4) x U(4)'
symmetry due to a discrete Z_2 "twin" parity, while the quartic part does not
possess this symmetry. As a consequence, when the global symmetry is broken the
Higgs fields emerge as light pseudo-Goldstone bosons, but with sizable quartic
self-interactions. This structure allows the cutoff scale, \Lambda, to be
raised to the multi-TeV region without significant fine-tuning. In the minimal
version of the theory, the amount of fine-tuning is about 15% for \Lambda = 5
TeV, while it is about 30% in an extended model. This provides a solution to
the little hierarchy problem. In the minimal model, the "visible" particle
content is exactly that of the two Higgs doublet standard model, while the
extended model also contains extra vector-like fermions with masses ~(1-2)TeV.
At the LHC, our minimal model may appear exactly as the two Higgs doublet
standard model, and new physics responsible for cutting off the divergences of
the Higgs mass-squared parameter may not be discovered. Several possible
processes that may be used to discriminate our model from the simple two Higgs
doublet model are discussed for the LHC and for a linear collider.Comment: 22 page
A genome-wide linkage and association scan reveals novel loci for autism
Member of the Autism Genome Project Consortium: Astrid M. VicenteAlthough autism is a highly heritable neurodevelopmental disorder, attempts to identify specific susceptibility genes have thus far met with limited success. Genome-wide association studies using half a million or more markers, particularly those with very large sample sizes achieved through meta-analysis, have shown great success in mapping genes for other complex genetic traits. Consequently, we initiated a linkage and association mapping study using half a million genome-wide single nucleotide polymorphisms (SNPs) in a common set of 1,031 multiplex autism families (1,553 affected offspring). We identified regions of suggestive and significant linkage on chromosomes 6q27 and 20p13, respectively. Initial analysis did not yield genome-wide significant associations; however, genotyping of top hits in additional families revealed an SNP on chromosome 5p15 (between SEMA5A and TAS2R1) that was significantly associated with autism (P = 2 x 10(-7)). We also demonstrated that expression of SEMA5A is reduced in brains from autistic patients, further implicating SEMA5A as an autism susceptibility gene. The linkage regions reported here provide targets for rare variation screening whereas the discovery of a single novel association demonstrates the action of common variants
A Symmetry for the Cosmological Constant
We study a symmetry, schematically Energy -> - Energy, which suppresses
matter contributions to the cosmological constant. The requisite negative
energy fluctuations are identified with a "ghost" copy of the Standard Model.
Gravity explicitly, but weakly, violates the symmetry, and naturalness requires
General Relativity to break down at short distances with testable consequences.
If this breakdown is accompanied by gravitational Lorentz-violation, the decay
of flat spacetime by ghost production is acceptably slow. We show that
inflation works in our scenario and can lead to the initial conditions required
for standard Big Bang cosmology.Comment: 18 pages, 3 figures, References correcte
The littlest Higgs model and Higgs boson associated production with top quark pair at high energy linear collider
In the parameter space allowed by the electroweak precision measurement data,
we consider the contributions of the new particles predicted by the littlest
Higgs() model to the Higgs boson associated production with top quark pair
in the future high energy linear collider(). We find that the
contributions mainly come from the new gauge bosons and . For
reasonable values of the free parameters, the absolute value of the relative
correction parameter can be significanly large,
which might be observed in the future experiment with .Comment: latex files, 13 pages, 3 figure
Recommended from our members
Microcraters in aluminum foils exposed by Stardust
We will present preliminary results on the nature and size frequency distribution of microcraters that formed in aluminum foils during the flyby of comet Wild 2 by the Stardust spacecraft
Multimessenger astronomy with the Einstein Telescope
Gravitational waves (GWs) are expected to play a crucial role in the
development of multimessenger astrophysics. The combination of GW observations
with other astrophysical triggers, such as from gamma-ray and X-ray satellites,
optical/radio telescopes, and neutrino detectors allows us to decipher science
that would otherwise be inaccessible. In this paper, we provide a broad review
from the multimessenger perspective of the science reach offered by the third
generation interferometric GW detectors and by the Einstein Telescope (ET) in
particular. We focus on cosmic transients, and base our estimates on the
results obtained by ET's predecessors GEO, LIGO, and Virgo.Comment: 26 pages. 3 figures. Special issue of GRG on the Einstein Telescope.
Minor corrections include
Particle Acceleration in Cosmic Sites - Astrophysics Issues in our Understanding of Cosmic Rays
Laboratory experiments to explore plasma conditions and stimulated particle
acceleration can illuminate aspects of the cosmic particle acceleration
process. Here we discuss the cosmic-ray candidate source object variety, and
what has been learned about their particle-acceleration characteristics. We
identify open issues as discussed among astrophysicists. -- The cosmic ray
differential intensity spectrum is a rather smooth power-law spectrum, with two
kinks at the "knee" (~10^15 eV) and at the "ankle" (~3 10^18 eV). It is unclear
if these kinks are related to boundaries between different dominating sources,
or rather related to characteristics of cosmic-ray propagation. We believe that
Galactic sources dominate up to 10^17 eV or even above, and the extragalactic
origin of cosmic rays at highest energies merges rather smoothly with Galactic
contributions throughout the 10^15--10^18 eV range. Pulsars and supernova
remnants are among the prime candidates for Galactic cosmic-ray production,
while nuclei of active galaxies are considered best candidates to produce
ultrahigh-energy cosmic rays of extragalactic origin. Acceleration processes
are related to shocks from violent ejections of matter from energetic sources
such as supernova explosions or matter accretion onto black holes. Details of
such acceleration are difficult, as relativistic particles modify the structure
of the shock, and simple approximations or perturbation calculations are
unsatisfactory. This is where laboratory plasma experiments are expected to
contribute, to enlighten the non-linear processes which occur under such
conditions.Comment: accepted for publication in EPJD, topical issue on Fundamental
physics and ultra-high laser fields. From review talk at "Extreme Light
Infrastructure" workshop, Sep 2008. Version-2 May 2009: adjust some wordings
and references at EPJD proofs stag
Gauging the Contribution of X-ray Sources to Reionization Through the Kinetic Sunyaev-Zel'dovich Effect
Measurements of the kinetic Sunyaev-Zel'dovich (kSZ) effect from instruments
such as the South Pole Telescope (SPT) and the Atacama Cosmology Telescope
(ACT) will soon put improved constraints on reionization. Popular models assume
that UV photons alone are responsible for reionization of the intergalactic
medium. We explore the effects of a significant contribution of X-rays to
reionization on the kSZ signal. Because X-rays have a large mean free path
through the neutral intergalactic medium, they introduce partial ionization in
between the sharp-edged bubbles created by UV photons. This smooth ionization
component changes the power spectrum of the cosmic microwave background (CMB)
temperature anisotropies. We quantify this effect by running semi-numerical
simulations of reionization. We test a number of different models of
reionization without X-rays that have varying physical parameters, but which
are constrained to have similar total optical depths to electron scattering.
These are then compared to models with varying levels of contribution to
reionization from X-rays. We find that models with more than a 10% contribution
from X-rays produce a significantly lower power spectrum of temperature
anisotropies than all the UV-only models tested. The expected sensitivity of
SPT and ACT may be insufficient to distinguish between our models, however, a
non-detection of the kSZ signal from the epoch of reionization could result
from the contribution of X-rays. It will be important for future missions with
improved sensitivity to consider the impact of X-ray sources on reionization.Comment: 11 pages, 4 figures, modified to reflect updated SPT error bars,
submitted to JCA
Low-Energy Brane-World Effective Actions and Partial Supersymmetry Breaking
As part of a programme for the general study of the low-energy implications
of supersymmetry breaking in brane-world scenarios, we study the nonlinear
realization of supersymmetry which occurs when breaking N=2 to N=1
supergravity. We consider three explicit realizations of this supersymmetry
breaking pattern, which correspond to breaking by one brane, by one antibrane
or by two (or more) parallel branes. We derive the minimal field content, the
effective action and supersymmetry transformation rules for the resulting N=1
theory perturbatively in powers of kappa = 1/M_{Planck}. We show that the way
the massive gravitino and spin-1 fields assemble into N=1 multiplets implies
the existence of direct brane-brane contact interactions at order O(kappa).
This result is contrary to the O(kappa^2) predicted by the sequestering
scenario but in agreement with recent work of Anisimov et al. Our low-energy
approach is model independent and is a first step towards determining the
low-energy implications of more realistic brane models which completely break
all supersymmetries.Comment: Latex, 29 Page
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