107 research outputs found
Hard Non-commutative Loops Resummation
The non-commutative version of the euclidean theory is
considered. By using Wilsonian flow equations the ultraviolet renormalizability
can be proved to all orders in perturbation theory. On the other hand, the
infrared sector cannot be treated perturbatively and requires a resummation of
the leading divergencies in the two-point function. This is analogous to what
is done in the Hard Thermal Loops resummation of finite temperature field
theory. Next-to-leading order corrections to the self-energy are computed,
resulting in contributions in the massless case, and
in the massive one.Comment: 4 pages, 3 figures. The resummation procedure is now discussed also
at finite ultraviolet cut-off. Minor changes in abstract and references.
Final version to be published in Physical Review Letter
Boltzmann Suppression of Interacting Heavy Particles
Matsumoto and Yoshimura have recently argued that the number density of heavy
particles in a thermal bath is not necessarily Boltzmann-suppressed for T << M,
as power law corrections may emerge at higher orders in perturbation theory.
This fact might have important implications on the determination of WIMP relic
densities. On the other hand, the definition of number densities in a
interacting theory is not a straightforward procedure. It usually requires
renormalization of composite operators and operator mixing, which obscure the
physical interpretation of the computed thermal average. We propose a new
definition for the thermal average of a composite operator, which does not
require any new renormalization counterterm and is thus free from such
ambiguities. Applying this definition to the model of Matsumoto and Yoshimura
we find that it gives number densities which are Boltzmann-suppressed at any
order in perturbation theory. We discuss also heavy particles which are
unstable already at T=0, showing that power law corrections do in general
emerge in this case.Comment: 7 pages, 5 figures. New section added, with the discussion of the
case of an unstable heavy particle. Version to appear on Phys. Rev.
Brane Worlds and the Cosmic Coincidence Problem
Brane world models with `large' extra dimensions with radii in the r_l ~
0.01- 0.1 mm range and smaller ones at r_s < (1 TeV)^(-1) have the potential to
solve the cosmic coincidence problem, i.e. the apparently fortuitous equality
between dark matter and dark energy components today. The main ingredient is
the assumption of a stabilization mechanism fixing the total volume of the
compact submanifold, but allowing for shape deformations. The latter are
associated with phenomenologically safe ultra-light scalar fields. Bulk fields
Casimir energy naturally plays the role of dark energy, which decreases in time
because of expanding r_l. Stable Kaluza Klein states may play the role of dark
matter with increasing, O(1/r_s), mass. The cosmological equations exhibit
attractor solutions in which the global equation of state is negative, the
ratio between dark energy and dark matter is constant and the observed value of
the ratio is obtained for two large extra dimensions. Experimental searches of
large extra dimensions should take into account that, due to the strong
coupling between dark matter and radii dynamics, the size of the large extra
dimensions inside the galactic halo may be smaller than the average value.Comment: 6 pages, enlarged discussion on the compact volume stabilization
mechanism. Version to appear on Phys. Rev.
On the Spontaneous CP Breaking at Finite Temperature in a Nonminimal Supersymmetric Standard Model
We study the spontaneous CP breaking at finite temperature in the Higgs
sector in the Minimal Supersymmetric Standard Model with a gauge singlet. We
consider the contribution of the standard model particles and that of stops,
charginos, neutralinos, charged and neutral Higgs boson to the one-loop
effective potential. Plasma effects for all bosons are also included. Assuming
CP conservation at zero temperature, so that experimental constraints coming
from, {\it e.g.}, the electric dipole moment of the neutron are avoided, and
the electroweak phase transition to be of the first order and proceeding via
bubble nucleation, we show that spontaneous CP breaking cannot occur inside the
bubble mainly due to large effects coming from the Higgs sector. However,
spontaneous CP breaking can be present in the region of interest for the
generation of the baryon asymmetry, namely inside the bubble wall. The
important presence of very tiny explicit CP violating phases is also commented.Comment: 28 pages, 4 figures available upon request, DFPD 94/TH/38 and SISSA
94/81-A preprint
Characterization of the boundary layer at Dome C (East Antarctica) during the OPALE summer campaign
The regional climate model MAR was run for the region of Dome C located on the East Antarctic plateau, during Antarctic summer 2011â2012, in order to refine our understanding of meteorological conditions during the OPALE observation campaign. A very high vertical resolution is set up in the lower troposphere, with a grid spacing of roughly 2 m. Comparisons are made with observed temperatures and winds near the surface and from a 45 m high tower as well as sodar and radiation data. MAR is generally in very good agreement with the observations but sometimes underestimates cloud formation, leading to an underestimation of the simulated downward long-wave radiation. Absorbed short-wave radiation may also be slightly overestimated due to an underestimation of the snow albedo and this influences the surface energy budget and atmospheric turbulence. Nevertheless the model provides sufficiently reliable information that represent key parameters when discussing the representativeness of chemical measurements made nearby the ground surface during field campaigns conducted at the Concordia site located at Dome C (3233 m a.s.l.)
Scalar-Tensor Gravity and Quintessence
Scalar fields with inverse power-law effective potentials may provide a
negative pressure component to the energy density of the universe today, as
required by cosmological observations. In order to be cosmologically relevant
today, the scalar field should have a mass
, thus potentially inducing sizable
violations of the equivalence principle and space-time variations of the
coupling constants. Scalar-tensor theories of gravity provide a framework for
accommodating phenomenologically acceptable ultra-light scalar fields. We
discuss non-minimally coupled scalar-tensor theories in which the scalar-matter
coupling is a dynamical quantity. Two attractor mechanisms are operative at the
same time: one towards the tracker solution, which accounts for the accelerated
expansion of the Universe, and one towards general relativity, which makes the
ultra-light scalar field phenomenologically safe today. As in usual
tracker-field models, the late-time behavior is largely independent on the
initial conditions. Strong distortions in the cosmic microwave background
anisotropy spectra as well as in the matter power spectrum are expected.Comment: 5 pages, 4 figure
Brane oscillations and the cosmic coincidence problem
We show that, under general assumptions, in six-dimensional brane-world
models with compactified large extra dimensions, the energy density of brane
oscillations scales as that of cold dark matter and its present value is
compatible with observations. Such value is obtained from the only dimensional
scale in the theory, namely, the fundamental scale of gravity in six dimensions
TeV, without any fine-tuning or the introduction of additional mass
scales apart from the large size of the extra dimensions. It has been suggested
that the same kind of models could provide also the correct magnitude of the
cosmological constant. This observation can be relevant for the resolution of
the cosmic coincidence problem in the brane-world scenario.Comment: 5 pages, RevTeX. Comments on the renormalization of the branon mass
included. Final version to appear in Phys.Rev.D (R
Dark Energy and Neutrino Mass Limits from Baryogenesis
In this brief report we consider couplings of the dark energy scalar, such as
Quintessence to the neutrinos and discuss its implications in studies on the
neutrino mass limits from Baryogenesis. During the evolution of the dark energy
scalar, the neutrino masses vary, consequently the bounds on the neutrino
masses we have here differ from those obtained before.Comment: 5 pages,3 figures. Version accepted for publication in Phys. Rev.
CFHTLS weak-lensing constraints on the neutrino masses
We use measurements of cosmic shear from CFHTLS, combined with WMAP-5 cosmic
microwave background anisotropy data, baryonic acoustic oscillations from SDSS
and 2dFGRS and supernovae data from SNLS and Gold-set, to constrain the
neutrino mass. We obtain a 95% confidence level upper limit of 0.54 eV for the
sum of the neutrino masses, and a lower limit of 0.03 eV. The preference for
massive neutrinos vanishes when shear-measurement systematics are included in
the analysis.Comment: 10 pages. Published versio
The Thermal Renormalization Group for Fermions, Universality, and the Chiral Phase-Transition
We formulate the thermal renormalization group, an implementation of the
Wilsonian RG in the real-time (CTP) formulation of finite temperature field
theory, for fermionic fields. Using a model with scalar and fermionic degrees
of freedom which should describe the two-flavor chiral phase-transition, we
discuss the mechanism behind fermion decoupling and universality at second
order transitions. It turns out that an effective mass-like term in the fermion
propagator which is due to thermal fluctuations and does not break chiral
symmetry is necessary for fermion decoupling to work. This situation is in
contrast to the high-temperature limit, where the dominance of scalar over
fermionic degrees of freedom is due to the different behavior of the
distribution functions. The mass-like contribution is the leading thermal
effect in the fermionic sector and is missed if a derivative expansion of the
fermionic propagator is performed. We also discuss results on the
phase-transition of the model considered where we find good agreement with
results from other methods.Comment: References added, minor typos correcte
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