arXiv subjects

Helios Sanchis-Alepuz

Publications and source records attributed to Helios Sanchis-Alepuz.

34 records · Page 2Linked to original sources

arXiv2013

Delta and Omega electromagnetic form factors in a three-body covariant Bethe-Salpeter approach

The electromagnetic form factors of the Delta and Omega baryons are calculated in the framework of Poincare-covariant bound-state equations. The quark-quark interaction is truncated to a single dressed-gluon exchange where for the dressings we use two different models and compare the results. The calculation predicts an oblate shape for the electric charge distribution and a prolate shape for the magnetic dipole distribution. We also identify the necessity of including pion-cloud corrections at low photon-momentum transfer.

hep-ph↗

arXiv2013

Delta and Omega electromagnetic form factors in a covariant three-body approach

We study baryons as three-body systems using the QCD degrees of freedom in the framework of covariant Bethe-Salpeter equations. The interaction among quarks is reduced to a vector-vector interaction via a single dressed-gluon exchange (Rainbow-Ladder truncation). The formalism allows for the study of the hadron spectrum as well as their internal properties. We will present the calculation of electromagnetic properties of spin-3/2 baryons. The model independent features of our results are assessed using two different models for the dressings.

hep-ph↗

arXiv2012

Stellar Structure Equations in Extended Palatini Gravity

We consider static spherically symmetric stellar configurations in Palatini theories of gravity in which the Lagrangian is an unspecified function of the form $f(R,R_{μν}R^{μν})$. We obtain the Tolman-Oppenheimer-Volkov equations corresponding to this class of theories and show that they recover those of $f(R)$ theories and General Relativity in the appropriate limits. We show that the exterior vacuum solutions are of Schwarzschild-de Sitter type and comment on the possible expected modifications, as compared to GR, of the interior solutions.

gr-qc↗

arXiv2012

Static Spherically Symmetric Solutions in Extended Palatini Gravity

We consider static spherically symmetric stellar configurations in Palatini theories of gravity in which the Lagrangian is an unspecified function of the form f(R,R_{μν}R^{μν}). We obtain the Tolman-Oppenheimer-Volkov equations corresponding to this class of theories and show that they recover those of f(R) theories and General Relativity in the appropriate limits. We compute exterior vacuum solutions and comment on the possible expected modifications, as compared to GR, of the interior solutions.

gr-qc↗

arXiv2012

Baryon properties from the covariant Faddeev equation

A calculation of the masses and electromagnetic properties of the Delta and Omega baryon together with their evolution with the current quark mass is presented. Hereby a generalized Bethe-Salpeter approach with the interaction truncated to a dressed one-gluon exchange is employed. The model dependence is explored by investigating two forms for the dressed gluon exchange.

hep-ph↗

arXiv2012

Baryon properties and glueballs from Poincare-covariant bound-state equations

In this thesis the covariant Bethe-Salpeter equation formalism is used to study some properties of ground-state baryons. This formalism relies on the knowledge of the interaction kernel among quarks and of the full quark propagator. For the interaction kernel, which is in principle a sum of infinitely many diagrams, I use the Ladder truncation. It amounts to reduce the interaction to a flavor-blind quark-mass independent vector-vector interaction between two quarks, mediated by a dressed gluon. The irreducible three-body interactions are neglected. The full quark propagator is obtained as a solution of the quark Dyson-Schwinger equation which is truncated such that, together with the truncation in the interaction kernel, chiral symmetry is correctly implemented. It is called Rainbow truncation, and together with the truncated kernel equation it constitutes the Rainbow-Ladder truncation of the Bethe-Salpeter equation. Any truncation induces the introduction of a model to account for the properties of the full system. The main goal of this thesis is to evaluate the model dependence of corrsponding results and, as a consequence, to isolate the features related to the truncation itself. To this end two non-related models are used to calculate the baryon spectra, from light to heavy quarks, and the electromagnetic properties of the Delta(1232). From these results one concludes a qualitative model independence, and that a Rainbow-Ladder truncated bound-state calculation reproduces the physical results with an accuracy of ~10%. Covariant bound-state equations are not limited to the study of hadronic bound states. As an example, in a further chapter of this thesis a Bethe-Salpeter equation for glueballs is proposed, and the main steps for a consistent resolution of the equation are described. However, to obtain this solution is beyond the scope of this work.

hep-ph↗

arXiv2011

Model comparison of Delta and Omega masses in a covariant Faddeev approach

We compute the vector-meson, nucleon and delta/omega-baryon masses and their evolution with the current-quark mass using a covariant generalized Bethe-Salpeter equation approach. The interaction kernel is truncated to a dressed gluon exchange. We study the model dependence of our results with the quark-gluon dressing to assess the validity of the truncation.

hep-ph↗

arXiv2011

Delta and Omega masses in a three-quark covariant Faddeev approach

We present the solution of the Poincare-covariant Faddeev equation for the Delta(1232) and Omega(1672) baryons. The covariant structure of the corresponding baryon amplitudes and their decomposition in terms of internal spin and orbital angular momentum is explicitly derived. The interaction kernel is truncated to a rainbow-ladder dressed-gluon exchange such that chiral symmetry and its dynamical breaking are correctly implemented. The resulting physical masses agree reasonably with experiment and their evolution with the pion mass compares favorably with lattice calculations. Evidence for the non-sphericity of the Delta-resonance is discussed as well.

hep-ph↗

arXiv2011

Hamiltonian Formulation of Palatini f(R) theories a la Brans-Dicke

We study the Hamiltonian formulation of f(R) theories of gravity both in metric and in Palatini formalism using their classical equivalence with Brans-Dicke theories with a non-trivial potential. The Palatini case, which corresponds to the w=-3/2 Brans-Dicke theory, requires special attention because of new constraints associated with the scalar field, which is non-dynamical. We derive, compare, and discuss the constraints and evolution equations for the ww=-3/2 and w\neq -3/2 cases. Based on the properties of the constraint and evolution equations, we find that, contrary to certain claims in the literature, the Cauchy problem for the w=-3/2 case is well-formulated and there is no reason to believe that it is not well-posed in general.

gr-qc↗

arXiv2010

On baryon properties from a covariant Faddeev approach

The Poincare-covariant Faddeev equation and its solution in rainbow-ladder truncation, i.e., with a generalized gluon exchange as irreducible two-particle-interaction, is presented. The covariant decomposition of baryon amplitudes, representing relativistic three-quark states, is discussed and explicitly given for the Delta-baryon. The calculated Delta mass compares favourably with the experimental one.

hep-ph↗

arXiv2010

Avoiding the Big Bang Singularity with Palatini f(R) Theories

We show that there exist modified theories of gravity in which the metric satisfies second-order equations and in which the Big Bang singularity is replaced by a cosmic bounce without violating any energy condition. In fact, the bounce is possible even for presureless dust. We give a characterization of such theories, which are formulated in the Palatini formalism, and discuss their dynamics in the region near the bounce. We consider spatially flat and non-flat homogeneous and isotropic universes.

gr-qc↗

arXiv2010

Enriched Phenomenology in Extended Palatini Theories

We show that extended theories of gravity with Lagrangian f(R,R_{μν}R^{μν}) in the Palatini formulation possess a phenomenology much richer than the simpler f(R) or f(R_{μν}R^{μν}) theories. In fact, we find that the scalars R and Q=R_{μν}R^{μν} can be written as algebraic functions of the energy density and pressure of the energy momentum tensor. In the simpler cases of f(R) or f(R_{μν}R^{μν}) theories, R and Q are just functions of the trace T of the energy-momentum tensor. As a result, in radiation dominated universes f(R) and f(R_{μν}R^{μν}) theories exhibit the same dynamics as general relativity with an effective cosmological constant. This is not the case of f(R,R_{μν}R^{μν}) models, in which R=R(ρ,P) and Q=Q(ρ,P) and, therefore, modified dynamics exists even for traceless sources.

gr-qc↗

arXiv2009

Bouncing Cosmologies in Palatini $f(R)$ Gravity

We consider the early time cosmology of f(R) theories in Palatini formalism and study the conditions that guarantee the existence of homogeneous and isotropic models that avoid the Big Bang singularity. We show that for such models the Big Bang singularity can be replaced by a cosmic bounce without violating any energy condition. In fact, the bounce is possible even for pressureless dust. We give a characterization of such models and discuss their dynamics in the region near the bounce. We also find that power-law lagrangians with a finite number of terms may lead to non-singular universes, which contrasts with the infinite-series Palatini f(R) lagrangian that one needs to fully capture the effective dynamics of Loop Quantum Cosmology. We argue that these models could also avoid the formation of singularities during stellar gravitational collapse.

gr-qc↗

arXiv2009

Dynamical Aspects of Generalized Palatini Theories of Gravity

We study the field equations of modified theories of gravity in which the lagrangian is a general function of the Ricci scalar and Ricci-squared terms in Palatini formalism. We show that the independent connection can be expressed as the Levi-Civita connection of an auxiliary metric which, in particular cases of interest, is related with the physical metric by means of a disformal transformation. This relation between physical and auxiliary metric boils down to a conformal transformation in the case of f(R) theories. We also show with explicit models that the inclusion of Ricci squared terms in the action can impose upper bounds on the accessible values of pressure and density, which might have important consequences for the early time cosmology and black hole formation scenarios. Our results indicate that the phenomenology of f(R_{ab}R^{ab}) theories is much richer than that of f(R) and f(R_{ab}R^{ab}) theories and that they also share some similarities with Bekenstein's relativistic theory of MOND.

gr-qc↗

arXiv2007

Shannon Entropy as Characterization Tool in Acoustics

We introduce Shannon's information entropy to characterize the avoided crossing appearing in the resonant Zener-like phenomenon in ultrasonic superlattices made of two different fluidlike meta- materials. We show that Shannon's entropy gives a correct physical insight of the localization effects taking place and manifest the informational exchange of the involved acoustic states in the narrow region of parameters where the avoided crossing occurs. Results for ultrasonic structures consisting of alternating layers of methyl-metacrylate and water cavities, in which the acoustic Zener effect were recently demonstrated, are also reported.

cond-mat.other↗

arXiv2007

Acoustic Analogue of Bloch Oscillations and Resonant Zener Tunneling in Ultrasonic Superlattices

We demonstrate the existence of Bloch oscillations of acoustic fields in sound propagation through a superlattice of water cavities and layers of methyl methacrylate. To obtain the acoustic equivalent of a Wannier-Stark ladder, we have employed a set of cavities with different thicknesses. Bloch oscillations are observed as time-resolved oscillations of transmission in a direct analogy to electronic Bloch oscillations in biased semiconductor superlattices. Moreover, for a particular gradient of cavity thicknesses, an overlap of two acoustic minibands occurs, which results in resonant Zener-like transmission enhancement.

cond-mat.other↗