Persona: Fernández Ramírez, César
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Fernández Ramírez
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Publicación Dispersive analysis of 𝜔/𝜙→3𝜋,𝜋𝛾*(American Physical Society, 2015-05-27) Danilkin, J. V.; Fernández Ramírez, César; Guo, P.; Mathieu, V.; Schott, D.; Shi, M.; Szczepaniak, P.The decays 𝜔/𝜙 →3𝜋 are considered in the dispersive framework that is based on the isobar decomposition and subenergy unitarity. The inelastic contributions are parametrized by the power series in a suitably chosen conformal variable that properly accounts for the analytic properties of the amplitude. The Dalitz plot distributions and integrated decay widths are presented. Our results indicate that the final-state interactions may be sizable. As a further application of the formalism we also compute the electromagnetic transition form factors of 𝜔/𝜙 →𝜋0𝛾*.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 Accurate Test of Chiral Dynamics in the → 𝛾 𝑝 →𝜋0𝑝 Reaction(American Physical Society, 2013-08-07) Hornidge, D; Aguar Bartolomé, P.; J. R. M. Annand,; Fernández Ramírez, César; et alA precision measurement of the differential cross sections 𝑑𝜎/𝑑𝛺 and the linearly polarized photon asymmetry 𝛴 ≡(𝑑𝜎⊥ −𝑑𝜎∥)/(𝑑𝜎⊥ +𝑑𝜎∥) for the →𝛾𝑝 →𝜋0𝑝 reaction in the near-threshold region has been performed with a tagged photon beam and almost 4𝜋 detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multiphoton detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the 𝑆- and all three 𝑃-wave amplitudes for the first time and provide the most stringent test to date of the predictions of chiral perturbation theory and its energy region of agreement with experiment.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 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 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 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, César; et al. (LHCb Collaboration)A 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 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 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.