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 Low-energy D-wave effects in neutral pion photoproduction(Elsevier, 2009-08-10) Fernández Ramírez, César; Bernstein, A. 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 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 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.