111 research outputs found
Fat Brane Phenomena
Gravitons could permeate extra space dimensions inaccessible to all other
particles, which would be confined to ``branes''. We point out that these
branes could be ``fat'' and have a non-vanishing width in the dimensions
reserved for gravitons. In this case the other particles, confined within a
finite width, should have ``branon'' excitations. Chiral fermions behave
differently from bosons under dimensional reduction, and they may --or may
not-- be more localized than bosons. All these possibilities are in principle
testable and distinguishable, they could yield spectacular signatures at
colliders, such as the production of the first branon excitation of 's
or 's, decaying into their ground state plus a quasi-continuum of graviton
recurrences. We explore these ideas in the realm of a future lepton collider
and we individuate a {\it dimensiometer}: an observable that would cleanly
diagnose the number of large ``extra'' dimensions.Comment: 12 Latex2e, 8 EPS figures, using epsfi
Dark Coupling and Gauge Invariance
We study a coupled dark energy-dark matter model in which the energy-momentum
exchange is proportional to the Hubble expansion rate. The inclusion of its
perturbation is required by gauge invariance. We derive the linear perturbation
equations for the gauge invariant energy density contrast and velocity of the
coupled fluids, and we determine the initial conditions. The latter turn out to
be adiabatic for dark energy, when assuming adiabatic initial conditions for
all the standard fluids. We perform a full Monte Carlo Markov Chain likelihood
analysis of the model, using WMAP 7-year data.Comment: 16 pages, 2 figures, version accepted for publication in JCA
Quenched Supersymmetry
We study the effects of quenching in Super-Yang-Mills theory. While
supersymmetry is broken, the lagrangian acquires a new flavour
symmetry. The anomaly structure thus differs from the unquenched case. We
derive the corresponding low-energy effective lagrangian. As a consequence, we
predict the mass splitting expected in numerical simulations for particles
belonging to the lowest-lying supermultiplet.Comment: LATTICE98(yukawa), minor change
To rescue a star
Massless neutrinos are exchanged in a neutron star, leading to long range
interactions. Many body forces of this type follow and we resum them. Their net
contribution to the total energy is negligible as compared to the star mass.
The stability of the star is not in danger, contrary to recent assertions.Comment: 10 pages, Latex2e, two figure
Renormalization of Lepton Mixing for Majorana Neutrinos
We discuss the one-loop electroweak renormalization of the leptonic mixing
matrix in the case of Majorana neutrinos, and establish its relationship with
the renormalization group evolution of the dimension five operator responsible
for the light Majorana neutrino masses. We compare our results in the effective
theory with those in the full seesaw theory.Comment: 28 pages. With axodra
A Way to Reopen the Window for Electroweak Baryogenesis
We reanalyse the sphaleron bound of electroweak baryogenesis when allowing
deviations to the Friedmann equation. These modifications are well motivated in
the context of brane cosmology where they appear without being in conflict with
major experimental constraints on four-dimensional gravity. While suppressed at
the time of nucleosynthesis, these corrections can dominate at the time of the
electroweak phase transition and in certain cases provide the amount of
expansion needed to freeze out the baryon asymmetry without requiring a
strongly first order phase transition. The sphaleron bound is substantially
weakened and can even disappear so that the constraints on the higgs and stop
masses do not apply anymore. Such modification of cosmology at early times
therefore reopens the parameter space allowing electroweak baryogenesis which
had been reduced substantially given the new bound on the higgs mass imposed by
LEP. In contrast with previous attempts to turn around the sphaleron bound
using alternative cosmologies, we are still considering that the electroweak
phase transition takes place in a radiation dominated universe. The universe is
expanding fast because of the modification of the Friedmann equation itself
without the need for a scalar field and therefore evading the problem of the
decay of this scalar field after the completion of the phase transition and the
risk that its release of entropy dilutes the baryon asymmetry produced at the
transition.Comment: 19 pages, 3 figures; v2: minor changes, remark added at end of
section 5 and in caption of figure 1; v3: references added, version to be
publishe
Neutrino Oscillations v.s. Leptogenesis in SO(10) Models
We study the link between neutrino oscillations and leptogenesis in the
minimal framework assuming an SO(10) see-saw mechanism with 3 families. Dirac
neutrino masses being fixed, the solar and atmospheric data then generically
induce a large mass-hierarchy and a small mixing between the lightest
right-handed neutrinos, which fails to produce sufficient lepton asymmetry by 5
orders of magnitudes at least. This failure can be attenuated for a very
specific value of the mixing sin^2(2\theta_{e3})=0.1, which interestingly lies
at the boundary of the CHOOZ exclusion region, but will be accessible to future
long baseline experiments.Comment: 23 pages, 8 eps figures, JHEP3 format; more accurate effect of
dilution reduces previous results, inclusion of all phases, added reference
Critical Analysis of Theoretical Estimates for to Light Meson Form Factors and the Data
We point out that current estimates of form factors fail to explain the
non-leptonic decays and that the combination of data
on the semi-leptonic decays and on the non-leptonic
decays (in particular recent po\-la\-ri\-za\-tion
data) severely constrain the form (normalization and dependence) of the
heavy-to-light meson form factors, if we assume the factorization hypothesis
for the latter. From a simultaneous fit to \bpsi and \dk data we find that
strict heavy quark limit scaling laws do not hold when going from to
and must have large corrections that make softer the dependence on the masses.
We find that should increase slower with \qq than .
We propose a simple parametrization of these corrections based on a quark
model or on an extension of the \hhs laws to the \hl case, complemented with an
approximately constant . We analyze in the light of these data and
theoretical input various theoretical approaches (lattice calculations, QCD sum
rules, quark models) and point out the origin of the difficulties encountered
by most of these schemes. In particular we check the compatibility of several
quark models with the heavy quark scaling relations.Comment: 48 pages, DAPNIA/SPP/94-24, LPTHE-Orsay 94/1
Standard Model CP-violation and Baryon asymmetry
Simply based on CP arguments, we argue against a Standard Model explanation
of the baryon asymmetry of the universe in the presence of a first order phase
transition. A CP-asymmetry is found in the reflection coefficients of quarks
hitting the phase boundary created during the electroweak transition. The
problem is analyzed both in an academic zero temperature case and in the
realistic finite temperature one. The building blocks are similar in both
cases: Kobayashi-Maskawa CP-violation, CP-even phases in the reflection
coefficients of quarks, and physical transitions due to fermion self-energies.
In both cases an effect is present at order in rate. A standard
GIM behaviour is found as intuitively expected. In the finite temperature case,
a crucial role is played by the damping rate of quasi-particles in a hot
plasma, which is a relevant scale together with and the temperature. The
effect is many orders of magnitude below what observation requires, and
indicates that non standard physics is indeed needed in the cosmological
scenario.Comment: 15p, LaTeX (3figs incl.), CERN 93/7081,LPTHE
Orsay-93/48,HUTP-93/A036,HD-THEP-93-4
Minimal Flavour Seesaw Models
We explore realizations of minimal flavour violation (MFV) for the lepton
sector. We find that it can be realized within those seesaw models where a
separation of the lepton number and lepton flavour violating scales can be
achieved, such as type II and inverse seesaw models. We present in particular a
simple implementation of the MFV hypothesis which differs in nature from those
previously discussed. It allows to reconstruct the flavour structure of the
model from the values of the light neutrino masses and mixing parameters, even
in the presence of CP-violating phases. Experimentally reachable predictions
for rare processes such as mu --> e gamma are given.Comment: Misprints corrected. A reference added. A subtle point in the
reconstruction of the flavour parameters is clarifie
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