Persona: Fernández Ramírez, César
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Fernández Ramírez
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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 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 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 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.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 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 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, A. 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 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.