Persona: Fernández Ramírez, César
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Fernández Ramírez
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Publicación Hints on the quadrupole deformation of the Δ(1232)(American Physical Society, 2006-04-18) Fernández Ramírez, César; Moya de Guerra, E.; Udías, J. M.The 𝐸2/𝑀1 ratio (EMR) of the Δ(1232) is extracted from the world data in pion photoproduction by means of an effective Lagrangian approach (ELA). This quantity has been derived within a crossing symmetric, gauge invariant, and chiral symmetric Lagrangian model which also contains a consistent modern treatment of the Δ(1232) resonance. The bare 𝑠-channel Δ(1232) contribution is well isolated and final state interactions (FSI) are effectively taken into account fulfilling Watson's theorem. The obtained EMR value, EMR =(−1.30±0.52)%, is in good agreement with the latest lattice QCD calculations [Phys. Rev. Lett. 94, 021601 (2005)] and disagrees with results of current quark model calculations.Publicación A precision measurement of the 𝑝(𝑒,𝑒′𝑝)𝜋0 reaction at threshold(American Physical Society, 2015-05-13) Chirapatpimol, K,; Shabestari, M. H.; Lindgren, R. A.; Fernández Ramírez, CésarNew results are reported from a measurement of 𝜋0 electroproduction near threshold using the 𝑝(𝑒,𝑒′𝑝)𝜋0 reaction. The experiment was designed to determine precisely the energy dependence of 𝑠- and 𝑝-wave electromagnetic multipoles as a stringent test of the predictions of chiral perturbation theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the 𝜙*𝜋 and 𝜃*𝜋 angles in the 𝑝𝜋0 center of mass was obtained for invariant energies above threshold from 0.5 up to 15 MeV. The 4-momentum transfer 𝑄2 coverage ranges from 0.05 to 0.155 (GeV/𝑐)2 in fine steps. A simple phenomenological analysis of our data shows strong disagreement with 𝑝-wave predictions from ChPT for 𝑄2>0.07 (GeV/𝑐)2, while the 𝑠-wave predictions are in reasonable agreement.Publicación Measurements of the Electric Form Factor of the Neutron up to 𝑄2 =3.4 GeV2 Using the Reaction 3 −→ He ( → 𝑒 ,𝑒′𝑛)𝑝𝑝(American Physical Society, 2010-12-30) Riordan, S.; Abrahamyan, S.; Craver, B.; Kelleher, A.; Fernández Ramírez, CésarThe electric form factor of the neutron was determined from studies of the reaction 3−→He(→𝑒,𝑒′𝑛)𝑝𝑝 in quasielastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the 𝑄2 range over which it is known, we find 𝐺𝑛𝐸 =0.0236 ±0.0017(stat) ±0.0026(syst), 0.0208 ±0.0024 ±0.0019, and 0.0147 ±0.0020 ±0.0014 for 𝑄2 =1.72, 2.48, and 3.41 GeV2, respectively.Publicación Equivalence of three-particle scattering formalisms(American Physical Society, 2019-08-13) Jackura, A. W.; Dawid, S. M.; Fernández Ramírez, César; Mathieu, V.; Mikhasenko, M.; Pilloni, A.; Sharpe, S. R.; Szczepaniak, A. P.In recent years, different on-shell 𝟑→𝟑 scattering formalisms have been proposed to be applied to both lattice QCD and infinite-volume scattering processes. We prove that the formulation in the infinite volume presented by Hansen and Sharpe in [M. T. Hansen and S. R. Sharpe, Phys. Rev. D 92, 114509 (2015).] and subsequently Briceño et al. in [R. A. Briceño, M. T. Hansen, and S. R. Sharpe, Phys. Rev. D 95, 074510 (2017).] can be recovered from the 𝐵-matrix representation, derived on the basis of 𝑆-matrix unitarity, presented by Mai et al. in [M. Mai, B. Hu, M. Döring, A. Pilloni, and A. Szczepaniak, Eur. Phys. J. A 53, 177 (2017).] and Jackura et al. in [A. Jackura, C. Fernández-Ramírez, V. Mathieu, M. Mikhasenko, J. Nys, A. Pilloni, K. Saldaña, N. Sherrill, and A. P. Szczepaniak (JPAC Collaboration), Eur. Phys. J. C 79, 56 (2019).] Therefore, both formalisms in the infinite volume are equivalent and the physical content is identical. Additionally, the Faddeev equations are recovered in the nonrelativistic limit of both representations.Publicación Charmonium spectrum with a Dirac potential model in the momentum space(American Physical Society, 2017-05-26) Molina, David; Sanchis, Maurizio De; Fernández Ramírez, CésarWe study the charmonium spectrum using a complete one gluon exchange approach based on a phenomenological relativistic 𝑞¯𝑞 potential model with Dirac spinors in momentum space. We use phenomenological screening factors to include vacuum quantum effects. Our formulation does not rely on nonrelativistic approximations. We fit the lowest-lying charmonia (below the 𝐷¯𝐷 threshold) and predict the higher-lying resonances of the spectrum. In general, we reproduce the overall structure of the charmonium spectrum and, in particular, we can reasonably describe the 𝑋(3872) resonance mass as (mostly) a 𝑐 ¯𝑐state. The numerical values of the free parameters of the model are determined taking into account also the experimental uncertainties of the resonance energies. In this way, we are able to obtain the uncertainties of the theoretical resonance masses and the correlation among the free parameters of the model.Publicación Eta Photoproduction as a Test of the Extended Chiral Symmetry(Elsevier, 2007-08-09) Fernández Ramírez, César; Moya de Guerra, E.; Udías, J. M.We analyze the γp→ηp process from threshold up to 1.2 GeV, employing an effective Lagrangian approach that allows for a mixing of eta couplings of pseudoscalar and pseudovector nature. The mixing ratio of the couplings may serve as a quantitative estimation of the SUL(3)×SUR(3) extended chiral symmetry violation in this energy regime. The data analyzed (differential cross sections and asymmetries) show a preference for the pseudoscalar coupling—91% of pseudoscalar coupling component for the best fit. We stress that a more conclusive answer to this question requires a more complete electromagnetic multipole database than the presently available one.Publicación Spectral-fluctuations test of the quark-model baryon spectrum(American Physical Society, 2007-02-08) Fernández Ramírez, César; Relaño, A.We study the low-lying baryon spectrum (up to 2.2 GeV) provided by experiments and different quark models using statistical tools which allow us to postulate the existence of missing levels in spectra. We confirm that the experimental spectrum is compatible with random matrix theory, the paradigmatic model of quantum chaos, and we find that the quark models are more similar to a Poisson distribution, which is not compatible with what should be expected in a correlated spectrum. From our analysis it stems that the spectral fluctuation properties of quark-model spectra are incompatible with experimental data. This result can be used to enlighten the problem of missing resonances.Publicación Spectral-statistics properties of the experimental and theoretical light meson spectra(Elsevier, 2012-03-29) Muñoz Muñoz, Laura; Fernández Ramírez, César; Relaño Pérez, Armando; Retamosa Granado, JoaquínWe present a robust analysis of the spectral fluctuations exhibited by the light meson spectrum. This analysis provides information about the degree of chaos in light mesons and may be useful to get some insight on the underlying interactions. Our analysis unveils that the statistical properties of the light meson spectrum are close, but not exactly equal, to those of chaotic systems. In addition, we have analyzed several theoretical spectra including the latest lattice QCD calculation. With a single exception, their statistical properties are close to those of a generic integrable system, and thus incompatible with the experimental spectrum.Publicación Effective Lagrangian Approach to pion photoproduction from the nucleon(Elsevier, 2006-06) Fernández Ramírez, César; Moya de Guerra, E.; Udías, J. M.We present a pion photoproduction model on the free nucleon based on an Effective Lagrangian Approach (ELA) which includes the nucleon resonances (Δ(1232), N(1440), N(1520), N(1535), Δ(1620), N(1650), and Δ(1700)), in addition to Born and vector meson exchange terms. The model incorporates a new theoretical treatment of spin-3/2 resonances, first introduced by Pascalutsa, avoiding pathologies present in previous models. Other main features of the model are chiral symmetry, gauge invariance, and crossing symmetry. We use the model combined with modern optimization techniques to assess the parameters of the nucleon resonances on the basis of world data on electromagnetic multipoles. We present results for electromagnetic multipoles, differential cross-sections, asymmetries, and total cross-sections for all one pion photoproduction processes on free nucleons. We find overall agreement with data from threshold up to 1 GeV in laboratory frame.Publicación Properties of Nucleon Resonances by Means of a Genetic Algorithm(American Physical Society, 2008-06-24) Fernández Ramírez, César; Moya de Guerra, E.; Udías, A; Udías, J. M.We present an optimization scheme that employs a genetic algorithm (GA) to determine the properties of low-lying nucleon excitations within a realistic photo-pion production model based upon an effective Lagrangian. We show that with this modern optimization technique it is possible to reliably assess the parameters of the resonances and the associated error bars as well as to identify weaknesses in the models. To illustrate the problems the optimization process may encounter, we provide results obtained for the nucleon resonances 𝛥(1230) and 𝛥(1700). The former can be easily isolated and thus has been studied in depth, while the latter is not as well known experimentally.