arXiv subjects

Antonio Riotto

Publications and source records attributed to Antonio Riotto.

230 records · Page 13Linked to original sources

arXiv1997

Evolution of the Order Parameter after Bubble Collisions

If a first-order phase transition is terminated by collisions of new-phase bubbles, there will exist a period of nonequilibrium between the time bubbles collide and the time thermal equilibrium is established. We study the behavior of the order parameter during this phase. We find that large nonthermal fluctuations at this stage tend to restore symmetry, i.e., the order parameter is smaller than its eventual thermal equilibrium value. We comment on possible consequences for electroweak baryogenesis.

astro-ph↗

arXiv1997

Hybrid Inflation in Supergravity

We propose a simple realization of hybrid inflation in supergravity, which does not require any unnaturally small parameters. Inflation in this scenario is relatively short, and inflationary density perturbations fall down on large scales, which corresponds to a blue spectrum with n > 1. Superheavy strings which are formed after inflation in this scenario produce density perturbations of comparable magnitude. In this mixed perturbation (MP) model one may have microwave anisotropy produced by strings and galaxy formation due to inflationary perturbations.

hep-ph↗

arXiv1996

Propagation of Majorana fermions in hot plasma

The properties of Majorana fermions in hot plasma are studied. One-loop resummed propagator, dispersion relations and their interpretation are discussed. It is shown that particle and hole -like solutions appear as in Dirac/chiral fermion case. The dispersion relations are, however, crucially different. We find that, in presence of a large zero temperature bare mass, hole -like excitations posses a negligible effective mass. As an example of real application, we consider the neutralinos in the minimal supersymmetric extension of the standard model and argue that for realistic values of the soft supersymmetry breaking masses the existence of practically massless hole -like excitations have a considerable effect on the thermal properties, e.g. the thermalization rate, of particles interacting with these Majorana excitations.

hep-ph↗

arXiv1996

GUT Baryogenesis after Preheating

At the end of inflation the universe is frozen in a near zero-entropy state with energy density in a coherent scalar field and must be ``defrosted'' to produce the observed entropy and baryon number. We propose that the baryon asymmetry is generated by the decay of supermassive Grand Unified Theory (GUT) bosons produced non-thermally in a preheating phase after inflation. We show that baryogenesis is possible for an inflaton masses of order 10^{13} GeV and a GUT Higgs boson mass of order 10^{14} GeV, thus solving many drawbacks facing GUT baryogenesis in the old reheating scenario.

hep-ph↗

arXiv1996

Naturalness Lowers the Upper Bound on the Lightest Higgs Boson Mass in Supersymmetry

We quantify the extent to which naturalness is lost as experimental lower bounds on the Higgs boson mass increase, and we compute the natural upper bound on the lightest supersymmetric Higgs boson mass. We find that it would be unnatural for the mass of the lightest supersymmetric Higgs boson to saturate it's maximal upper bound. In the absence of significant fine-tuning, the lightest Higgs boson mass should lie below $120$ GeV, and in the most natural cases it should be lighter than $108$ GeV. For modest $tan β$, these bounds are significantly lower. Our results imply that a failure to observe a light Higgs boson in pre-LHC experiments could provide a serious challenge to the principal motivation for weak-scale supersymmetry.

hep-ph↗

arXiv1996

Anomalous $U(1)$ Mediated SUSY Breaking, Fermion Masses and Natural Suppression of FCNC and CP Violating Effects

We construct realistic supergravity models where supersymmetry breaking arises from the $D$-terms of an anomalous $U(1)$ gauge symmetry broken at the Planck scale. The model has the attractive feature that the gaugino masses, the $A$ and $B$ terms and the mass splittings between the like-charged squarks of the first two generations compared to their average masses are all suppressed. As a result, the electric dipole moment of the neutron as well as the flavor changing neutral current effects are predicted to be naturally small. These models predict naturally the expected value of the $μ$-term and also have the potential to qualitatively explain the observed mass hierarchy among quarks and leptons.

hep-ph↗

arXiv1996

Preheating, Supersymmetry Breaking and Baryogenesis

Fluctuations of scalar fields produced at the stage of preheating after inflation are so large that they can break supersymmetry much stronger than inflation itself. These fluctuations may lead to symmetry restoration along flat directions of the effective potential even in the theories where the usual high temperature corrections are exponentially suppressed. Our results show that nonthermal phase transitions after preheating may play a crucial role in the generation of the primordial baryon asymmetry by the Affleck-Dine mechanism. In particular, the baryon asymmetry may be generated at the very early stage of the evolution of the Universe, at the preheating era, and not when the Hubble parameter becomes of order the gravitino mass.

hep-ph↗

arXiv1996

Preheating and symmetry restoration in collisions of vacuum bubbles

In first-order inflation a phase transition is completed by the collisions of expanding true-vacuum bubbles. If bubble collisions produce large numbers of soft scalar particles carrying quantum numbers associated with a spontaneously broken symmetry, then symmetry restoration may occur in a ``pre-heating'' phase in a manner similar to symmetry restoration in the pre-heating phase of slow-roll inflation. Since bubble collisions lead to inhomogeneities, there is the possibility of inhomogeneous symmetry restoration where restoration occurs only in the regions of wall collisions.

astro-ph↗

arXiv1996

Eternal annihilations of light photinos

In a class of low-energy supersymmetry models the photino is a natural dark matter candidate. We investigate the effects of post-freeze-out photino annihilations which can generate electromognetic cascades and lead to photo-destruction of $^4$He and subsequent overproduction of D and $^3$He. We also generalize our analysis to a generic dark matter component whose relic abundance is {\it not} determined by the cross section of the self-annihilations giving rise to electromagnetic showers.

astro-ph↗

arXiv1995

Towards a Nonequilibrium Quantum Field Theory Approach to Electroweak Baryogenesis

We propose a general method to compute $CP$-violating observables from extensions of the standard model in the context of electroweak baryogenesis. It is alternative to the one recently developed by Huet and Nelson and relies on a nonequilibrium quantum field theory approach. The method is valid for all shapes and sizes of the bubble wall expanding in the thermal bath during a first-order electroweak phase transition. The quantum physics of $CP$-violation and its suppression coming from the incoherent nature of thermal processes are also made explicit.

hep-ph↗

arXiv1993

Thermal Background Corrections to the Neutrino Electromagnetic Vertex in Models with Charged Scalar Bosons

We calculate the correction to the neutrino electromagnetic vertex due to background of electrons in a large class of models, as the supersymmetric model with explicit breaking of R-parity, where charged scalar bosons couple to leptons and which are able to provide an astrophysically interesting value for the neutrino magnetic (electric) moment, $μ_ν\sim 10^{-12}\:μ_B$. We show that the medium contribution to the chirality flipping magnetic (electric) dipole moment is not significant, however a new chirality flipping, but helicity conserving, term arises. It signals the presence of ${\cal CP}$ and ${\cal CPT}$ asymmetries in the medium and is associated to the longitudinal photon and therefore disappears in the vacuum. We estimate the contribution of this new term to the rate of the plasmon decay process $γ_{pl}\rightarrow νν$ in the core of degenerate stars, showing that it can be comparable with the contribution coming from the vacuum magnetic (dipole) moment. We also calculate the correction to the effective potential of a propagating neutrino in presence of a magnetic field due to a chirality preserving contribution to the diagonal magnetic moment from the medium. This contribution is identical for particles and antiparticles and so need not to vanish for Majorana neutrinos.

hep-ph↗

arXiv1993

The Singlet Majoron Model with Hidden Scale Invariance

We investigate an extension of the Singlet Majoron Model in which the breaking of dilatation symmetry by the mass parameters of the scalar potential is removed by means of a dilaton field. Starting from the one-loop renormalization group improved potential, we discuss the ground state of the theory. The flat direction in the classical potential is lifted by quantum corrections and the true vacua are found. Studying the finite temperature potential, we analyze the cosmological consequences of a Jordan-Brans-Dicke dilaton and show that the lepton number is spontaneously broken after the electroweak phase transition, thus avoiding any constraint coming from the requirement of the preservation of the baryon asymmetry in the early Universe. We also find that, contrary to the Standard Model case, the dilaton cosmology does not impose any upper bound on the scale of the spontaneous breaking of scale invariance.

hep-ph↗

arXiv1992

Dynamics of the Cosmological Quark-Hadron Transition in a Matter Dominated Universe: Distribution and Evolution of Baryon Inhomogeneities

We study the dynamics of the quark-hadron transition for a scenario in which the Universe is matter dominated and a large amount of entropy is generated by decaying particles of mass 1--10 TeV, as suggested by a large class of superstring--inspired models. We estimate the nucleation rate and compute the mean separation between baryon fluctuations generated during the transition following their evolution up to the onset of primordial nucleosyntheisis.

hep-ph↗

arXiv1992

Cosmic $ΔB$ from Lepton Violating Interactions at the Electroweak Phase Transition

We propose a new mechanism for late cosmological baryon asymmetry in models with first order electroweak phase transition. Lepton asymmetry arises through the decay of particles produced out of equilbrium in bubble collisions and is converted into baryon asymmetry by sphalerons. Supersymmetric models with explicitly broken R-parity may provide a suiatble framework for the implementation of this mechanism.

hep-ph↗