Persona: Fernández Ramírez, César
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Fernández Ramírez
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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 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 Upper energy limit of heavy baryon chiral perturbation theory in neutral pion photoproduction(Elsevier, 2013-07-23) Fernández Ramírez, César; Bernstein, Aaron M.With the availability of the new neutral pion photoproduction from the proton data from the A2 and CB-TAPS Collaborations at Mainz it is mandatory to revisit Heavy Baryon Chiral Perturbation Theory (HBChPT) and address the extraction of the partial waves as well as other issues such as the value of the low-energy constants, the energy range where the calculation provides a good agreement with the data and the impact of unitarity. We find that, within the current experimental status, HBChPT with the fitted LECs gives a good agreement with the existing neutral pion photoproduction data up to ∼170 MeV and that imposing unitarity does not improve this picture. Above this energy the data call for further improvement in the theory such as the explicit inclusion of the (1232). We also find that data and multipoles can be well described up to ∼185 MeV with Taylor expansions in the partial waves up to first order in pion energy.Publicación Regge phenomenology of the 𝑁* and Δ* poles(American Physical Society, 2019-01-13) Silva castro, J.A.; Fernández Ramírez, César; Albaladejo, M.; Danilkin, I.V.; Jackura, A.; Mathieu, V.; Nys, J.; Pilloni, A.; Szczepaniak, A. P.We use Regge phenomenology to study the structure of the poles of the 𝑁* and Δ* spectrum. We employ the available pole extractions from partial wave analysis of meson scattering and photoproduction data. We assess the importance of the imaginary part of the poles (widths) to obtain a consistent determination of the parameters of the Regge trajectory. We compare the several pole extractions and show how Regge phenomenology can be used to gain insight into the internal structure of baryons. We find that the majority of the states in the parent Regge trajectories are compatible with a mostly compact three-quark state picture.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, 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 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 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 al.A 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.