Persona: Fernández Ramírez, César
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cefera@ccia.uned.es
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0000-0001-8979-5660
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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 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.Publicación Determination of the Pole Position of the Lightest Hybrid Meson Candidate(American Physical Society, 2019-01-29) Rodas, A.; Albadalejo, M.; Jackura, A.; Fernández Ramírez, César; Mathieu, V.; Mikhasenko, M.; Nys, J.; Pauk, V.; Ketzer, B.; Szczepaniak, A. P.Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, 𝜋1(1400) and 𝜋1(1600), which couple separately to 𝜂𝜋 and 𝜂′𝜋. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the 𝜂(′)𝜋 system by the COMPASS Collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the 𝑆 matrix. We provide a robust extraction of a single exotic 𝜋1 resonant pole, with mass and width 1564 ±24 ±86 and 492 ±54 ±102 MeV, which couples to both 𝜂(′)𝜋 channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the 𝑎2(1320) and 𝑎′ 2(1700).Publicación Interpretation of the LHCb 𝑃𝑐(4312)+ Signal(American Physical Society, 2019-08-29) Fernández Ramírez, César; Pilloni, A; Albadalejo, M.; Jackura, A.; Mathieu, V.; Mikhasenko, M.; Silva castro, J.A.; Szczepaniak, A. P.We study the nature of the new signal reported by LHCb in the 𝐽/𝜓𝑝 spectrum. Based on the 𝑆-matrix principles, we perform a minimum-bias analysis of the underlying reaction amplitude, focusing on the analytic properties that can be related to the microscopic origin of the 𝑃𝑐(4312)+ peak. By exploring several amplitude parametrizations, we find evidence for the attractive effect of the Σ+𝑐¯𝐷0 channel, which is not strong enough, however, to form a bound state.Publicación Low-energy D-wave effects in neutral pion photoproduction(Elsevier, 2009-08-10) Fernández Ramírez, César; Bernstein, Aaron M.; Donnelly, T. W.The contribution of D waves to physical observables for neutral pion photoproduction in the near threshold region is studied. Heavy Baryon Chiral Perturbation Theory to one loop, and up to O(q4), is used to account for the S and P waves, while D waves are added in an almost model-independent way using standard Born terms and vector mesons. It is found that the inclusion of D waves is necessary to extract the E0+ multipole reliably from present and forthcoming data and to assess the low-energy constants of Chiral Perturbation Theory. Arguments are presented demonstrating that F-wave contributions are negligible in the near-threshold region.Publicación Observation of a Narrow Pentaquark State, 𝑃𝑐(4312)+, and of the Two-Peak Structure of the 𝑃𝑐(4450)+(American Physical Society, 2019-06-05) Aaij, R.; Abellán Beteta, C.; Adeva, B.; Adinolfi, M.; Fernández Ramírez, CésarA narrow pentaquark state, 𝑃𝑐(4312)+, decaying to 𝐽/𝜓𝑝, is discovered with a statistical significance of 7.3𝜎 in a data sample of Λ0𝑏→𝐽/𝜓𝑝𝐾− decays, which is an order of magnitude larger than that previously analyzed by the LHCb Collaboration. The 𝑃𝑐(4450)+ pentaquark structure formerly reported by LHCb is confirmed and observed to consist of two narrow overlapping peaks, 𝑃𝑐(4440)+ and 𝑃𝑐(4457)+, where the statistical significance of this two-peak interpretation is 5.4𝜎. The proximity of the Σ+𝑐¯𝐷0 and Σ+𝑐¯𝐷*0 thresholds to the observed narrow peaks suggests that they play an important role in the dynamics of these statesPublicación 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 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 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 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.