arXiv subjects

Carla Carvalho

Publications and source records attributed to Carla Carvalho.

10 recordsLinked to original sources

arXiv2013

Experimental evidence of twin fast metastable H(2S) atoms from dissociation of cold H_2 induced by electrons

We report the direct detection of two metastable H(2S) atoms coming from the dissociation of a single cold H_2 molecule, in coincidence measurements. The molecular dissociation was induced by electron impact in order to avoid limitations by the selection rules governing radiative transitions. Two detectors, placed close from the collision center, measure the neutral metastable H(2S) through a localized quenching process, which mixes the H(2S) state with the H(2P), leading to a Lyman-αdetection. Our data show the accomplishment of a coincidence measurement which proves for the first time the existence of the H(2S)-H(2S) dissociation channel.

physics.atom-ph↗

arXiv2008

A new source for a braneworld cosmological constant from a modified gravity model in the bulk

We show that a four-dimensional equation of state for a cosmological constant term arises from a perfect fluid in the bulk in the context of a gravity model where the scalar curvature is non-minimally coupled to the perfect fluid Lagrangian density. The four-dimensional theory is fully determined from the induced equations on the brane, subject to the boundary conditions derived across the brane.

hep-th↗

arXiv2007

Non-minimal coupling as a mechanism for spontaneous symmetry breaking on the brane

Motivated by the dimensional asymmetry characteristic of braneworlds, we populate the bulk spacetime with matter scalar fields, both real and complex, and couple them non-minimally to gravity. We derive the effective equations of motion on the brane and realize the case when the fields acquire a non-vanishing vacuum expectation value. This entails a change in the effective cosmological constant and in the effective mass of the scalar fields. We find that the non-minimal coupling provides a mechanism for generating spontaneous symmetry breaking at Planck energies on the brane. This mechanism is however rendered unviable for the generation of masses at the energy scales of the standard model.

hep-th↗

arXiv2007

Model for Osteosarcoma-9 as a Potent Factor in Cell Survivor and Resistance to Apoptosis

In this paper we use a simple toy model to explore the function of the gene Osteosarcoma-9. We are in particular interested in understanding the role of this gene as a potent anti-apoptotic factor. The theoretical description is constrained by experimental data from induction of apoptosis in cells where OS-9 is overexpressed. The data available suggest that OS-9 promotes cell viability and confers resistance to apoptosis, potentially implicating OS-9 in the survival of cancer cells. Three different apoptosis inducing mechanisms were tested and are here modelled. More complex and realistic models are also discussed.

q-bio.SC↗

arXiv2007

Spontaneous Symmetry Breaking in the Bulk as a Localization Mechanism of Fields on the Brane

We consider real and complex scalar fields non-minimally coupled to gravity in the bulk spacetime and study their impact on the brane upon acquiring a non-vanishing vacuum expectation value. When examining the case of the complex scalar field, a minimally coupled U(1) gauge field is also considered so that spontaneous symmetry breaking can take place. Particular attention is paid to the terms arising from the junction conditions, which act towards reinforcing the mechanism of spontaneous symmetry breaking. We find that the embedding of a braneworld universe in a bulk spacetime endowed with matter fields can induce this mechanism at very high energies, which implies the localization on the brane of the bulk fields.

hep-th↗

arXiv2007

Lorentz symmetry from Lorentz violation in the bulk

We consider the mechanism of spontaneous symmetry breaking of a bulk vector field to study signatures of bulk dimensions invisible to the standard model confined to the brane. By assigning a non-vanishing vacuum expectation value to the vector field, a direction is singled out in the bulk vacuum, thus breaking the bulk Lorentz symmetry. We present the condition for induced Lorentz symmetry on the brane, as phenomenologically required, noting that it is related to the value of the observed cosmological constant.

astro-ph↗

arXiv2006

Lorentz Symmetry Derived from Lorentz Violation in the Bulk

We consider bulk fields coupled to the graviton in a Lorentz violating fashion. We expect that the overly tested Lorentz symmetry might set constraints on the induced Lorentz violation in the brane, and hence on the dynamics of the interaction of bulk fields on the brane. We also use the requirement for Lorentz symmetry to constrain the cosmological constant observed on the brane.

gr-qc↗

arXiv2004

Linearized Israel Matching Conditions for Cosmological Perturbations in a Moving Brane Background

With a view toward the problem of computing the linearized cosmological perturbations in Randall-Sundrum cosmological models where a Z2 symmetry has been imposed about a boundary brane, we derive the form of the linearized Israel matching conditions and the auxiliary gauge conditions on the boundary when Lorentz gauge has been imposed in the bulk. This gauge is completely covariant and local, manifestly respecting all the AdS5 symmetries in the bulk and not relying on a decomposition into pure scalar, vector, and tensor sectors, which is necessarily nonlocal. We demonstrate that the auxiliary gauge conditions on the boundary ensure that bulk gravitons upon reflection off the brane do not emit polarizations that violate the bulk Lorentz gauge condition. We also characterize the residual gauge freedom, embodied by five longitudinal (pure gauge) graviton polarizations in the bulk, four of which correspond to reparameterizations of the induced brane metric and one of which corresponds to normal displacements of the brane.

hep-th↗

arXiv2004

Models for the Brane-Bulk Interaction: Toward Understanding Braneworld Cosmological Perturbation

Using some simple toy models, we explore the nature of the brane-bulk interaction for cosmological models with a large extra dimension. We are in particular interested in understanding the role of the bulk gravitons, which from the point of view of an observer on the brane will appear to generate dissipation and nonlocality, effects which cannot be incorporated into an effective (3+1)-dimensional Lagrangian field theoretic description. We explicitly work out the dynamics of several discrete systems consisting of a finite number of degrees of freedom on the boundary coupled to a (1+1)-dimensional field theory subject to a variety of wave equations. Systems both with and without time translation invariance are considered and moving boundaries are discussed as well. The models considered contain all the qualitative feature of quantized linearized cosmological perturbations for a Randall-Sundrum universe having an arbitrary expansion history, with the sole exception of gravitational gauge invariance, which will be treated in a later paper.

hep-th↗

arXiv2002

Separation Distribution of Vacuum Bubbles in de Sitter Space

We compute the probability distribution of the invariant separation between nucleation centers of colliding true vacuum bubbles arising from the decay of a false de Sitter space vacuum. We find that even in the limit of a very small nucleation rate per unit Hubble volume the production of widely separated bubble pairs is suppressed. This distribution is of particular relevance for the recently proposed ``colliding bubble braneworld'' scenario, in which the value of Omega_k (the contribution of negative spatial curvature to the cosmological density parameter) is determined by the invariant separation of the colliding bubble pair. We also consider the probability of a collision with a `third' bubble.

hep-ph↗