36,066 research outputs found
Noncommutative Einstein-Maxwell pp-waves
The field equations coupling a Seiberg-Witten electromagnetic field to
noncommutative gravity, as described by a formal power series in the
noncommutativity parameters , is investigated. A large
family of solutions, up to order one in , describing
Einstein-Maxwell null pp-waves is obtained. The order-one contributions can be
viewed as providing noncommutative corrections to pp-waves. In our solutions,
noncommutativity enters the spacetime metric through a conformal factor and is
responsible for dilating/contracting the separation between points in the same
null surface. The noncommutative corrections to the electromagnetic waves,
while preserving the wave null character, include constant polarization, higher
harmonic generation and inhomogeneous susceptibility. As compared to pure
noncommutative gravity, the novelty is that nonzero corrections to the metric
already occur at order one in .Comment: 19 revtex pages. One refrence suppressed, two references added. Minor
wording changes in the abstract, introduction and conclusio
Mapping the circumstellar SiO maser emission in R Leo
The study of the innermost circumstellar layers around AGB stars is crucial
to understand how these envelopes are formed and evolve. The SiO maser emission
occurs at a few stellar radii from the central star, providing direct
information on the stellar pulsation and on the chemical and physical
properties of these regions. Our data also shed light on several aspects of the
SiO maser pumping theory that are not well understood yet. We aim to determine}
the relative spatial distribution of the 43 GHz and 86 GHz SiO maser lines in
the oxygen-rich evolved star R Leo. We have imaged with milliarcsecond
resolution, by means of Very Long Baseline Interferometry, the 43 GHz (28SiO
v=1, 2 J=1-0 and 29SiO v=0 J=1-0) and 86 GHz (28SiO v=1 J=2-1 and 29SiO v=0
J=2-1) masing regions. We confirm previous results obtained in other
oxygen-rich envelopes. In particular, when comparing the 43 GHz emitting
regions, the 28SiO v=2 transition is produced in an inner layer, closer to the
central star. On the other hand, the 86 GHz line arises in a clearly farther
shell. We have also mapped for the first time the 29SiO v=0 J=1-0 emission in R
Leo. The already reported discrepancy between the observed distributions of the
different maser lines and the theoretical predictions is also found in R Leo.Comment: accepted for publication in A&
Pion transition form factor in the Regge approach and incomplete vector-meson dominance
The concept of incomplete vector-meson dominance and Regge models is applied
to the transition form factor of the pion. First, we argue that variants of the
chiral quark model fulfilling the chiral anomaly may violate the Terazawa-West
unitarity bounds, as these bounds are based on unverified assumptions for the
real parts of the amplitudes, precluding a possible presence of polynomial
terms. A direct consequence is that the transition form factor need not
necessarily vanish at large values of the photon virtuality. Moreover, in the
range of the BaBar experiment, the Terazawa-West bound is an order of magnitude
above the data, thus is of formal rather than practical interest. Then we
demonstrate how the experimental data may be properly explained with incomplete
vector-meson dominance in a simple model with one state, as well as in more
sophisticated Regge models. Generalizations of the simple Regge model along the
lines of Dominguez result in a proper description of the data, where one may
adjust the parameters in such a way that the Terazawa-West bound is satisfied
or violated. We also impose the experimental constraint from the Z -> pi0 gamma
decay. Finally, we point out that the photon momentum asymmetry parameter may
noticeably influence the precision analysis.Comment: 11 pages, 7 figure
The impact of priors and observables on parameter inferences in the Constrained MSSM
We use a newly released version of the SuperBayeS code to analyze the impact
of the choice of priors and the influence of various constraints on the
statistical conclusions for the preferred values of the parameters of the
Constrained MSSM. We assess the effect in a Bayesian framework and compare it
with an alternative likelihood-based measure of a profile likelihood. We employ
a new scanning algorithm (MultiNest) which increases the computational
efficiency by a factor ~200 with respect to previously used techniques. We
demonstrate that the currently available data are not yet sufficiently
constraining to allow one to determine the preferred values of CMSSM parameters
in a way that is completely independent of the choice of priors and statistical
measures. While b->s gamma generally favors large m_0, this is in some contrast
with the preference for low values of m_0 and m_1/2 that is almost entirely a
consequence of a combination of prior effects and a single constraint coming
from the anomalous magnetic moment of the muon, which remains somewhat
controversial. Using an information-theoretical measure, we find that the
cosmological dark matter abundance determination provides at least 80% of the
total constraining power of all available observables. Despite the remaining
uncertainties, prospects for direct detection in the CMSSM remain excellent,
with the spin-independent neutralino-proton cross section almost guaranteed
above sigma_SI ~ 10^{-10} pb, independently of the choice of priors or
statistics. Likewise, gluino and lightest Higgs discovery at the LHC remain
highly encouraging. While in this work we have used the CMSSM as particle
physics model, our formalism and scanning technique can be readily applied to a
wider class of models with several free parameters.Comment: Minor changes, extended discussion of profile likelihood. Matches
JHEP accepted version. SuperBayeS code with MultiNest algorithm available at
http://www.superbayes.or
A Coverage Study of the CMSSM Based on ATLAS Sensitivity Using Fast Neural Networks Techniques
We assess the coverage properties of confidence and credible intervals on the
CMSSM parameter space inferred from a Bayesian posterior and the profile
likelihood based on an ATLAS sensitivity study. In order to make those
calculations feasible, we introduce a new method based on neural networks to
approximate the mapping between CMSSM parameters and weak-scale particle
masses. Our method reduces the computational effort needed to sample the CMSSM
parameter space by a factor of ~ 10^4 with respect to conventional techniques.
We find that both the Bayesian posterior and the profile likelihood intervals
can significantly over-cover and identify the origin of this effect to physical
boundaries in the parameter space. Finally, we point out that the effects
intrinsic to the statistical procedure are conflated with simplifications to
the likelihood functions from the experiments themselves.Comment: Further checks about accuracy of neural network approximation, fixed
typos, added refs. Main results unchanged. Matches version accepted by JHE
Symmetry breaking and clustering in a vibrated granular gas with several macroscopically connected compartments
The spontaneous symmetry breaking in a vibro-fluidized low-density granular
gas in three connected compartments is investigated. When the total number of
particles in the system becomes large enough, particles distribute themselves
unequally among the three compartments. Particles tend to concentrate in one of
the compartments, the other two having the (relatively small) same average
number of particles. A hydrodynamical model that accurately predicts the
bifurcation diagram of the system is presented. The theory can be easily
extended to the case of an arbitrary number of connected compartments
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