arXiv subjects

G. Toledo Sanchez

Publications and source records attributed to G. Toledo Sanchez.

At least 19 recordsLinked to original sources

arXiv2016

Properties of vector mesons in four pseudoscalars electroproduction

Vector mesons and the $W$ gauge boson share some common features, such as the spin and a very short lifetime. The electromagnetic properties of the $W$ are linked to its gauge nature and thus a big effort has been devoted to measure them, while accounting for its instability. In this work, we elaborate on how these ideas can be translated to study vector mesons. We focus in the unstable property of such states and the restrictions from electromagnetic gauge invariance, describing the differences and similarities. Then, we describe the four pseudoscalars electroproduction as an analog to the $e^+e^- \to 4 fermions$ process, used to study the electromagnetic properties of the $W$ boson. We point out that the current experimental capabilities are reaching the possibility to measure the magnetic dipole moment of light vector mesons.

hep-ph↗

arXiv2016

Some aspects on four quarks recombination

We have performed a 3-D Monte Carlo simulation of a system composed of two identical light quarks ($qq$) and two identical antiquarks ($\bar Q\bar Q$) and determined whether it is energetically more favorable to form a tetraquark or two mesons, as a function of the interparticle separation distance which, for a fixed number of particles, can be identified as a particle density. In this proceedings, we highlight the main results and elaborate on the implications in properties like the correlation function for two-mesons and characterize the isolated diquark correlation function. We analize the four-body potential evolution and exhibit its linear behavior as a function of the invariant distance. We track the dynamical flipping among configurations to determine the recombination probability, exhibiting the importance of the tetraquark state.

nucl-th↗

arXiv2016

Dynamical transition between two mesons and a tetraquark

We consider a system composed of two identical light quarks ($qq$) and two identical antiquarks ($\bar Q\bar Q$) that can be linked either as two mesons or as a tetraquark, incorporating quantum correlations between identical particles and an effective many-body potential between particles. We perform a 3-D Monte Carlo simulation of the system, considering the configurations allowed to form: i) Only two mesons, ii) Only tetraquark and iii) two mesons and tetraquark . We characterize each case and determine whether it is energetically more favorable to form a tetraquark or two mesons, as a function of the interparticle separation distance which, for a fixed number of particles, can be identified as a particle density. We determine how the two mesons, which dominate the low density regime, mixes with a tetraquark state as the density increases. Properties like the mean square radius and the two-particle correlation function are found to reflect such transition, and we provide a parameterization of the diquark correlation function in the isolated case. We track the dynamical flipping among configurations to determine the recombination probability, exhibiting the importance of the tetraquark state. We analize the four-body potential evolution and show that its linear behavior is preserved, although the slope can reflect the presence of a mixed state. Results are shown for several light-quarks to heavy-antiquarks mass ratios whenever they are found to be relevant.

hep-ph↗

arXiv2014

About the inclusion of an infinite number of resonances in anomalous decays

The extracted value for the $g^{eff}_{ωρπ}$ effective coupling from experimental data, considering only the $ρ$ meson, resumes not only the $ρ$ meson effect but also all its additional radial excitation modes. By explicitly adding the radial excitations of the $ρ$ meson, considering a particular form of the spectrum and relations among the couplings, we identify the single $g_{ωρπ}$ and the $ρ$ radial excitations effect in the $ω\rightarrow π^0 γ$ decay. We obtain that the individual coupling is in the range $g_{ωρπ}= 8.2 - 8.6 {\text GeV}^{-1}$, which is about 40% smaller than the effective $g^{eff}_{ωρπ}$. We verify the consistency with the chiral approach in the $π^0 \rightarrow γγ$ and $γ^* \rightarrow 3π$ processes. Besides the model dependence, our description succeeds in exhibiting how each contribution came into the game. In particular, we show that for the $γ^* \rightarrow 3 π$ decay, the usual relation $\mathcal{A}^{VMD}_{\gamma3π}=(3/2)\mathcal{A}^{WZW}_{\gamma3π}$, encodes all the vector contributions and not only the $ρ$ meson one. In addition, we find that there is an almost exact (accidental) cancelation between the radial excitations and the contact term contributions.

nucl-th↗

arXiv2012

The omega rho pi coupling in the VMD model revisited

We determine the value of the ω-ρ- πmesons coupling (g_{ωρπ}), in the context of the vector meson dominance model, from radiative decays, the ω\rightarrow 3πdecay width and the e^+e^- \rightarrow 3πcross section. For the last two observables we consider the effect of either a heavier resonance (ρ'(1450)) or a contact term. A weighted average of the results from the set of observables yields g_{ωρπ} =14.7 \pm 0.1 GeV^{-1} in absence of those contributions, and g_{ωρπ} =11.9 \pm 0.2 GeV^{-1} or g_{ωρπ} =11.7 \pm 0.1 GeV^{-1} when including the ρ' or contact term respectively. The inclusion of these additional terms makes the estimates from the different observables to lay in a more reduced range. Improved measurements of these observables and the ρ'(1450) meson parameters are needed to give a definite answer on the pertinence of the inclusion of this last one in the considered processes.

hep-ph↗

arXiv2011

Heavy flavor nuclear modification factor: more baryons than mesons less energy loss

The suppression of the nuclear modification factor for heavy flavor hadrons is usually attributed to the energy loss of heavy quarks propagating in a QCD plasma. Nevertheless it is puzzling that the suppression is as strong as for light flavors. We show that when accounting for the quark momentum shift associated to the opening of the recombination/coalescence channel for hadron production in the plasma, it is not necessary to invoke such strong energy loss. This shift is expressed in terms of an increase of the heavy baryon to meson ratio in nuclear with respect to proton collisions. When this mechanism is included along with a moderate energy loss, data from RHIC and LHC for the nuclear modification factor of electrons coming from heavy flavor decays as well as of charm mesons, can be reasonably described.

nucl-th↗

arXiv2011

Proton fraction in the inner neutron-star crust

Monte Carlo simulations of neutron-rich matter of relevance to the inner neutron-star crust are performed for a system of A=5,000 nucleons. To determine the proton fraction in the inner crust, numerical simulations are carried out for a variety of densities and proton fractions. We conclude---as others have before us using different techniques---that the proton fraction in the inner stellar crust is very small. Given that the purported "nuclear pasta" phase in stellar crusts develops as a consequence of the long-range Coulomb interaction among protons, we question whether pasta formation is possible in such proton-poor environments. To answer this question, we search for physical observables sensitive to the transition between spherical nuclei and exotic pasta structures. Of particular relevance is the static structure factor S(k)---an observable sensitive to density fluctuations. However, no dramatic behavior was observed in S(k). We regard the identification of physical observables sensitive to the existence---or lack-thereof---of a pasta phase in proton-poor environments as an open problem of critical importance.

nucl-th↗

arXiv2010

Finite width induced modification to the electromagnetic form factors of spin-1 particles

The inclusion of the unstable features of a spin-1 particle, without breaking the electromagnetic gauge invariance, can be properly accomplished by including higher order contributions as done in the so-called fermion loop scheme (for the W gauge boson), and the boson loop scheme (for vector mesons). This induces a non trivial modification to the electromagnetic vertex of the particle, which must be considered in addition to any other contribution computed as stable particles. Considering the modified electromagnetic vertex, we obtain general expressions for the corresponding corrections to the multipoles as a function of the mass of the particles in the loop. For the W gauge boson no substantial deviations from the stable case is observed. For the rho and K* mesons the mass of the particles in the loop makes a significant effect, and can be comparable with corrections of different nature .

hep-ph↗

arXiv2010

Hadron production in electron-positron annihilation computed from the gauge gravity correspondence

We provide a non-perturbative expression for the hadron production in electron-positron annihilation at zero temperature in a strongly coupled, large-Nc SU(Nc) field theory with Nf << Nc quark flavors. The resulting expressions are valid to leading order in the electromagnetic coupling constant but non-perturbatively in the SU(Nc) interactions and the mass of the quark. We obtain this quantity by computing the imaginary part of the hadronic vacuum polarization function Pi_q using holographic techniques, providing an alternative to the known method that uses the spectrum of infinitely stable mesons determined by the normalizable modes of the appropriated fields in the bulk. Our result exhibits a structure of poles localized at specific real values of q^2, which coincide with the ones found using the normalizable modes, and extends it offering the unique analytic continuation of this distribution to a function defined for values of q^2 over the complex plane. This analytic continuation permits to include a finite decay width for the mesons. By comparison with experimental data we find qualitatively good agreement on the shape of the first pole, when using the rho meson parameters and choosing a proper normalization factor. We then estimate the contribution to the anomalous magnetic moment of the muon finding an agreement within 25%, for this choice of parameters.

hep-th↗

arXiv2009

The Discrepancy Between tau and e+e- Spectral Functions Revisited and the Consequences for the Muon Magnetic Anomaly

We revisit the procedure for comparing the pi pi spectral function measured in tau decays to that obtained in e+e- annihilation. We re-examine the isospin-breaking corrections using new experimental and theoretical input, and find improved agreement between the tau- --> pi- pi0 nu_tau branching fraction measurement and its prediction using the isospin-breaking-corrected e+e- --> pi+pi- spectral function, though not resolving all discrepancies. We recompute the lowest order hadronic contributions to the muon g-2 using e+e- and tau data with the new corrections, and find a reduced difference between the two evaluations. The new tau-based estimate of the muon magnetic anomaly is found to be 1.9 standard deviations lower than the direct measurement.

hep-ph↗

arXiv2009

Dynamical heavy-quark recombination and the non-photonic single electron puzzle at RHIC

We show that the single, non-photonic electron nuclear modification factor $R_{AA}^e$ is affected by the thermal enhancement of the heavy-baryon to heavy-meson ratio in relativistic heavy-ion collisions with respect to proton-proton collisions. We make use of the dynamical quark recombination model to compute such ratio and show that this produces a sizable suppression factor for $R_{AA}^e$ at intermediate transverse momenta. We argue that such suppression factor needs to be considered, in addition to the energy loss contribution, in calculations of $R_{AA}^e$

nucl-th↗

arXiv2008

Simulation of an inhomogeneous Fermi gas through the BCS-BEC crossover

We perform a variational quantum Monte Carlo simulation of the transition from a Bardeen-Cooper-Schrieffer superfluid (BCS) to a Bose-Einstein condensate (BEC) at zero temperature. The model Hamiltonian involves an attractive short range two body interaction and the atoms number $2N =330$ is chosen so that, in the non-interacting limit, the ground state function corresponds to a closed shell configuration. The system is then characterized by the s-wave scattering length $a$ of the two-particle collisions in the gas, which is varied from negative to positive values, and the Fermi wave number $k_F$. Based on an extensive analysis of the s-wave two-body problem, one parameter variational many-body wave functions are proposed to describe the ground state of the interacting Fermi gas from BCS to BEC states. We exploit properties of antisymmetrized many-body functions to develop efficient techniques that permit variational calculations for a large number of particles. It is shown that a virial relation between the energy per particle and the trapping energy is approximately valid for $-0.1<1/k_Fa<3.4$. The influence of the harmonic trap and the interaction potential as exhibited in two-body correlation functions is also analyzed.

cond-mat.other↗

arXiv2008

Dynamical quark recombination in ultrarelativistic heavy-ion collisions and the proton to pion ratio

We study quark thermal recombination as a function of energy density during the evolution of a heavy-ion collision in a numerical model that reproduces aspects of QCD phenomenology. We show that starting with a set of free quarks (or quarks and antiquarks) the probability to form colorless clusters of three quarks differs from that to form colorless clusters of quark-antiquark and that the former has a sharp jump at a critical energy density whereas the latter transits smoothly from the low to the high energy density domains. We interpret this as a quantitative difference in the production of baryons and mesons with energy density. We use this approach to compute the proton and pion spectra in a Bjorken scenario that incorporates the evolution of these probabilities with energy density, and therefore with proper time. From the spectra, we compute the proton to pion ratio and compare to data at the highest RHIC energies. We show that for a standard choice of parameters, this ratio reaches one, though the maximum is very sensitive to the initial evolution proper time.

hep-ph↗

arXiv2007

The width difference of rho vector mesons

We compute the difference in decay widths of charged and neutral ρ(770) vector mesons. The isospin breaking arising from mass differences of neutral and charged πand ρmesons, radiative corrections to ρ-> ππ, and the ρ-> ππγdecays are taken into account. It is found that the width difference ΔΓ_rho is very sensitive ot the isospin breaking in the $ρ$ meson mass Δm_ρ. This result can be useful to test the correlations observed between the values of these parameters extracted from experimental data.

hep-ph↗

arXiv2007

BEC-BCS crossover of a trapped Fermi gas without using the local density approximation

We perform a variational quantum Monte Carlo simulation of an interacting Fermi gas confined in a three dimensional harmonic potential. This gas is considered as the precursor system from which a molecular bosonic gas is formed. Based on the results of two-body calculations for trapped atoms, we propose a family of variational many-body wave functions that takes into account the qualitative different nature of the BCS and BEC regimes as a function of the scattering length. Energies, densities and correlation functions are calculated and compared with previous results for homogeneous gases. Universality tests at the unitarity limit are performed including the verification of the virial relation and the evaluation of the $β$ parameter.

cond-mat.other↗

arXiv2007

Role of resonances in rho^0 -> pi^+ pi^- gamma

We study the effect of the sigma(600) and a_1(1260) resonances in the rho^0 -> pi^+ pi^- gamma decay, within the meson dominance model. Major effects are driven by the mass and width parameters of the sigma(600), and the usually neglected contribution of the a_1(1260), although small by itself, may become sizable through its interference with pion bremsstrahlung, and the proper relative sign can favor the central value of the experimental branching ratio. We present a procedure, using the gauge invariant structure of the resonant amplitudes, to kinematically enhance the resonant effects in the angular and energy distribution of the photon. We also elaborate on the coupling constants involved.

hep-ph↗

arXiv2006

Model-dependent radiative corrections to tau- -> pi- pi0 nu revisited

The long-distance electromagnetic radiative corrections to tau- -> pi- pi0 nu are re-evaluated. A meson dominance model is used to describe the emission of real photons in this decay. Results obtained for the hadronic spectrum and the decay rate in photon inclusive reactions are compared with previous calculations based on the chiral resonance theory. Independent tests in tau -> pi pi nu gamma that can help to validate the predictions of one of the two models are briefly discussed.

hep-ph↗

arXiv2006

Long-distance radiative corrections to the di-pion tau lepton decay

We evaluate the model-dependent piece of O(alpha) long-distance radiative corrections to tau^- \to π^- π^0ν_τ decays by using a meson dominance model. We find that these corrections to the di-pion invariant mass spectrum are smaller than in previous calculations based on chiral perturbation theory. The corresponding correction to the photon inclusive rate is tiny (-0.15%) but it can be of relevance when new measurements reach better precision.

hep-ph↗