arXiv subjects

J. Rodriguez-Quintero

Publications and source records attributed to J. Rodriguez-Quintero.

At least 19 recordsLinked to original sources

arXiv2025

Light-Front Transverse Nucleon Charge and Magnetisation Densities

Nucleon elastic electromagnetic form factors obtained using both the three-body and quark + fully-interacting-diquark pictures of nucleon structure are employed to calculate an array of light-front transverse densities for the proton and neutron and their dressed valence-quark constituents, viz. flavour separations of the proton and neutron results. These two complementary descriptions of nucleon structure deliver mutually compatible predictions, which match expectations based on modern parametrisations of available data, where such are available. Amongst other things, it is found that transverse-plane valence $u$- and $d$-quark Dirac radii are practically indistinguishable; but regarding kindred Pauli radii, the $d$ quark value is roughly 10% greater than that of the $u$-quark. Moreover, magnetically, the valence $d$ quark is far more active than the valence $u$ quark, probably because it has much greater orbital angular momentum. Both pictures of nucleon structure agree in predicting that, in a polarised nucleon, the transverse-plane charge densities are no longer rotationally invariant. Instead, for a $+\hat x$ polarised nucleon, positive charge is displaced in the $+\hat y$ direction, with the opposite effect for negative charge.

hep-ph↗

arXiv2016

On the zero crossing of the three-gluon vertex

We report on new results on the infrared behaviour of the three-gluon vertex in quenched Quantum Chormodynamics, obtained from large-volume lattice simulations. The main focus of our study is the appearance of the characteristic infrared feature known as 'zero crossing', the origin of which is intimately connected with the nonperturbative masslessness of the Faddeev-Popov ghost. The appearance of this effect is clearly visible in one of the two kinematic configurations analyzed, and its theoretical origin is discussed in the framework of Schwinger-Dyson equations. The effective coupling in the momentum subtraction scheme that corresponds to the three-gluon vertex is constructed, revealing the vanishing of the effective interaction at the exact location of the zero crossing.

hep-ph↗

arXiv2016

From Bethe-Salpeter Wave Functions to Generalised Parton Distributions

We review recent works on the modelling of Generalised Parton Distributions within the Dyson-Schwinger formalism. We highlight how covariant computations, using the impulse approximation, allows one to fulfil most of the theoretical constraints of the GPDs. Specific attention is brought to chiral properties and especially the so-called soft pion theorem, and its link with the Axial-Vector Ward-Takahashi identity. The limitation of the impulse approximation are also explained. Beyond impulse approximation computations are reviewed in the forward case. Finally, we stress the advantages of the overlap of lightcone wave functions, and possible ways to construct covariant GPD models within this framework, in a two-body approximation.

nucl-th↗

arXiv2015

mu-Squared Dependent Deviation of the Non Perturbative ZA,MOM from the True Axial Renormalisation Constant, Implied by Ward Identity

It is recalled why, as already stated in a previous paper, there seems to be an inconsistency in identifying the non perturbative ZA,MOM as the renormalisation of the axial current, or equivalently, in setting as normalisation condition that the renormalised vertex=1 at p^2 = mu^2 at some renormalisation scale mu, where p is the momentum in the legs. Indeed, unlike the vector case, the Ward-Takahashi (WT) identity for the axial current is shown to imply both the renormalisation scale independence of ZA and a mu2 dependence of ZA,MOM. This mu^2 dependence is simply related to certain invariants in the pseudoscalar vertex and can persist in the chiral limit due to the spontaneous breaking of chiral symmetry (pion pole). It is seen clearly in the mu^2 dependence of some lattice calculations of ZA,MOM/ZV,MOM near the chiral limit.

hep-lat↗

arXiv2014

Sketching the pion's valence-quark generalised parton distribution

In order to learn effectively from measurements of generalised parton distributions (GPDs), it is desirable to compute them using a framework that can potentially connect empirical information with basic features of the Standard Model. We sketch an approach to such computations, based upon a rainbow-ladder (RL) truncation of QCD's Dyson-Schwinger equations and exemplified via the pion's valence dressed-quark GPD, $H_π^{\rm v}(x,ξ,t)$. Our analysis focuses primarily on $ξ=0$, although we also capitalise on the symmetry-preserving nature of the RL truncation by connecting $H_π^{\rm v}(x,ξ=\pm 1,t)$ with the pion's valence-quark parton distribution amplitude. We explain that the impulse-approximation used hitherto to define the pion's valence dressed-quark GPD is generally invalid owing to omission of contributions from the gluons which bind dressed-quarks into the pion. A simple correction enables us to identify a practicable improvement to the approximation for $H_π^{\rm v}(x,0,t)$, expressed as the Radon transform of a single amplitude. Therewith we obtain results for $H_π^{\rm v}(x,0,t)$ and the associated impact-parameter dependent distribution, $q_π^{\rm v}(x,|\vec{b}_\perp|)$, which provide a qualitatively sound picture of the pion's dressed-quark structure at an hadronic scale. We evolve the distributions to a scale $ζ=2\,$GeV, so as to facilitate comparisons in future with results from experiment or other nonperturbative methods.

nucl-th↗

arXiv2013

Lattice measurement of α_s with a realistic charm quark

We report on an estimate of α_s, renormalised in the MSbar scheme at the tau and Z^0 mass scales, by means of lattice QCD. Our major improvement compared to previous lattice calculations is that, for the first time, no perturbative treatment at the charm threshold has been required since we have used statistical samples of gluon fields built by incorporating the vacuum polarisation effects of u/d, s and c sea quarks. Extracting α_s in the Taylor scheme from the lattice measurement of the ghost-ghost-gluon vertex, we obtain α_s^{MSbar}(m^2_Z)=0.1200(14) and α_s^{MSbar}(m^2_tau)=0.339(13).

hep-lat↗

arXiv2012

Vacuum Polarization and Dynamical Chiral Symmetry Breaking: Phase Diagram of QED with Four-Fermion Contact Interaction

We study chiral symmetry breaking for fundamental charged fermions coupled electromagnetically to photons with the inclusion of four-fermion contact self-interaction term. We employ multiplicatively renormalizable models for the photon dressing function and the electron-photon vertex which minimally ensures mass anomalous dimension = 1. Vacuum polarization screens the interaction strength. Consequently, the pattern of dynamical mass generation for fermions is characterized by a critical number of massless fermion flavors above which chiral symmetry is restored. This effect is in diametrical opposition to the existence of criticality for the minimum interaction strength necessary to break chiral symmetry dynamically. The presence of virtual fermions dictates the nature of phase transition. Miransky scaling laws for the electromagnetic interaction strength and the four-fermion coupling, observed for quenched QED, are replaced by a mean-field power law behavior corresponding to a second order phase transition. These results are derived analytically by employing the bifurcation analysis, and are later confirmed numerically by solving the original non-linearized gap equation. A three dimensional critical surface is drawn to clearly depict the interplay of the relative strengths of interactions and number of flavors to separate the two phases. We also compute the beta-function and observe that it has ultraviolet fixed point. The power law part of the momentum dependence, describing the mass function, reproduces the quenched limit trivially. We also comment on the continuum limit and the triviality of QED.

hep-ph↗

arXiv2010

Quark pseudoscalar vertex and quark mass function with clover fermions : spontaneous symmetry breaking, OPE, symmetry restoration at small volume

We study the quark mass function on hypercubic lattices, in a large range of physical volumes and cutoffs. To avoid the very large Wilson term artefact, we exploit the relation between the quark mass function and the pseudoscalar vertex in the continuum. We extrapolate to the chiral limit. In function of the physical volume, we observe a striking discontinuity in the properties of chiral extrapolation around a physical volume $L_c 6 (GeV}^{-1}=1.2 fm$. It is present in the quark mass function, which collapses to zero, as well as in the pion mass and the quark condensate as directly calculated from the pseudoscalar correlator. It is strongly reminiscent of the phenomenon of chiral symmetry restoration observed by Neuberger and Narayanan at $N_C=\infty$ around the same physical length. In the case of spontaneous symmetry breaking, we confirm that the OPE of the quark mass function, involving the quark condensate, is not operative at the available momenta, even taking into account the unusually large high order corrections to the Wilson coefficient calculated by Chetyrkin and Maier ; the gap remains large, around a factor 2, even at the largest momenta available to us (p \simeg GeV)

hep-lat↗

arXiv2010

$\tau$-Flavour Violation at the LHC

We study the conditions required for $\chi_2 \to \chi + \tau^\pm \mu^\mp$ decays to yield observable tau flavour violation at the LHC, for cosmologically interesting values of the neutralino relic density. These condition can be achieved in the framework of a SU(5) model with a see-saw mechanism that allows a possible coexistence of a LHC signal a low prediction for radiative LFV decays.

hep-ph↗

arXiv2009

The case for a directional dark matter detector and the status of current experimental efforts

We present the case for a dark matter detector with directional sensitivity. This document was developed at the 2009 CYGNUS workshop on directional dark matter detection, and contains contributions from theorists and experimental groups in the field. We describe the need for a dark matter detector with directional sensitivity; each directional dark matter experiment presents their project's status; and we close with a feasibility study for scaling up to a one ton directional detector, which would cost around $150M.

astro-ph.CO↗

arXiv2009

Quintessential Kination and Thermal Production of Gravitinos and Axinos

The impact of a kination-dominated phase generated by a quintessential exponential model on the thermal abundance of gravitinos and axinos is investigated. We find that their abundances become proportional to the transition temperature from the kination to the radiation era; since this temperature is significantly lower than the initial ("reheating") temperature, the abundances decrease with respect to their values in the standard cosmology. For values of the quintessential energy-density parameter close to its upper bound, at the eve of nucleosynthesis, we find the following: (i) for unstable gravitinos, the gravitino constraint is totally evaded; (ii) If the gravitino is stable, its thermal abundance is not sufficient to account for the cold dark matter of the universe; (iii) the thermal abundance of axinos can satisfy the cold dark matter constraint for values of the initial temperature well above those required in the standard cosmology. A novel calculation of the axino production rate by scatterings at low temperature is also presented.

hep-ph↗

arXiv2009

Non-perturbative comparison of QCD effective charges

We study the non-perturbative behavior of two versions of the QCD effective charge, one obtained from the pinch technique gluon self-energy, and one from the ghost-gluon vertex. Despite their distinct theoretical origin, due to a fundamental identity relating various of the ingredients appearing in their respective definitions, the two effective charges are almost identical in the entire range of physical momenta, and coincide exactly in the deep infrared, where they freeze at a common finite value. Specifically, the dressing function of the ghost propagator is related to the two form factors in the Lorentz decomposition of a certain Green's function, appearing in a variety of field-theoretic contexts. The central identity, which is valid only in the Landau gauge, is derived from the Schwinger-Dyson equations governing the dynamics of the aforementioned quantities. The renormalization procedure that preserves the validity of the identity is carried out, and various relevant kinematic limits and physically motivated approximations are studied in detail. A crucial ingredient in this analysis is the infrared finiteness of the gluon propagator, which is inextricably connected with the aforementioned freezing of the effective charges. Some important issues related to the consistent definition of the effective charge in the presence of such a gluon propagator are resolved. We finally present a detailed numerical study of a special set of Schwinger-Dyson equations, whose solutions determine the non-perturbative dynamics of the quantities composing the two effective charges.

hep-ph↗

arXiv2009

Dark Matter and Lepton Flavour Violation in Yukawa Unification with Massive Neutrinos

The WMAP dark matter constraints on b-$τ$ Yukawa Unification in the presence of massive neutrinos is revisited. The predictions for the bottom quark mass are observed to be modified by the large neutrino mixing suggested by the data. This enables Yukawa unification also for large $\tanβ$, and for positive $μ$ that were previously disfavoured. Consequently, the allowed parameter space for neutralino dark matter differs from the MSSM one. We also find that the parameter space being compatible with dark matter also predicts detectable rates for Lepton Flavour Violation at the LHC.

hep-ph↗

arXiv2009

Dark Matter and Yukawa Unification with Massive Neutrinos

We revisit the WMAP dark matter constraints on Yukawa Unification in the presence of massive neutrinos. The large neutrino mixing indicated by the data modifies the predictions for the bottom quark mass, and enables Yukawa also for large $\tanβ$, and for positive $μ$ that were previously disfavoured. As a result, the allowed parameter space for neutralino dark matter also increases, particularly for areas with resonant enhancement of the neutralino relic density.

hep-ph↗

arXiv2009

WMAP Dark Matter Constraints on Yukawa Unification with Massive Neutrinos

We revisit the WMAP dark matter constraints on Yukawa Unification in the presence of massive neutrinos. The large lepton mixing indicated by the data may modify the predictions for the bottom quark mass, enabling Yukawa unification also for large $\tanβ$, and for positive $μ$ that was previously disfavoured. As a result, the allowed parameter space for neutralino dark matter increases for positive $μ$, particularly for areas with resonant enhancement of the neutralino relic density. On the contrary, a negative $μ$ is not easily compatible with large lepton mixing and Dirac neutrino Yukawa couplings, and the WMAP allowed parameter space is in this case strongly constrained.

hep-ph↗

arXiv2008

Search for Tau Flavour Violation at the LHC

We explore the prospects for searches at the LHC for sparticle decays that violate $τ$ lepton number, in the light of neutrino oscillation data and the seesaw model for neutrino masses and mixing. We analyse the theoretical and phenomenological conditions required for tau flavour violation to be observable in χ_2 \to χ+ τ^\pm μ^\mp decays, for cosmologically interesting values of the relic neutralino LSP density. We study the relevant supersymmetric parameter space in the context of the Constrained Minimal Supersymmetric Extension of the Standard Model (CMSSM) and in SU(5) extensions of the theory. We pay particular attention to the possible signals from hadronic tau decays, that we analyse using PYTHIA event simulation. We find that a signal for τflavour-violating χ_2 decays may be observable if the branching ratio exceeds about 10%. This may be compatible with the existing upper limit on τ\to μγdecays if there is mixing between right-handed sleptons, as could be induced in non-minimal SU(5) GUTs.

hep-ph↗

arXiv2008

Quintessential Kination and Thermal Production of SUSY e-WIMPs

The impact of a kination-dominated phase generated by a quintessential exponential model on the thermal abundance of Supersymmetric (SUSY) extremely Weekly Interacting Massive Particles (e-WIMPs) is investigated. For values of the quintessential energy-density parameter on the eve of nucleosynthesis close to its upper bound, we find that: (i) the gravitino constraint is totally evaded for unstable gravitinos; (ii) the thermal abundance of stable gravitinos is not sufficient to account for the cold dark matter of the universe; (iii) the thermal abundance of axinos can satisfy the cold dark matter constraint for values of the initial (``reheating'') temperature well above those required in the standard cosmology.

hep-ph↗

arXiv2006

The Infrared Behaviour of the Pure Yang-Mills Green Functions

We study the infrared behaviour of the pure Yang-Mills correlators using relations that are well defined in the non-perturbative domain. These are the Slavnov-Taylor identity for three-gluon vertex and the Schwinger-Dyson equation for ghost propagator in the Landau gauge. We also use several inputs from lattice simulations. We show that lattice data are in serious conflict with a widely spread analytical relation between the gluon and ghost infrared critical exponents. We conjecture that this is explained by a singular behaviour of the ghost-ghost-gluon vertex function in the infrared. We show that, anyhow, this discrepancy is not due to some lattice artefact since lattice Green functions satisfy the ghost propagator Schwinger-Dyson equation. We also report on a puzzle concerning the infrared gluon propagator: lattice data seem to favor a constant non vanishing zero momentum gluon propagator, while the Slavnov-Taylor identity (complemented with some regularity hypothesis of scalar functions) implies that it should diverge.

hep-ph↗