A consistent dynamical model for pion-photoproduction

Pion-photoproduction amplitude is calculated within a consistent isobar model already used to fix the Δ+ + (1232   MeV) parameters in pion-nucleon scattering and bremsstrahlung. This amplitude is expressed in terms of (physical) on-shell quantities and off-shell contributions coming from final state interactions. These latter are isolated in principal value integrals on the non-polar part of the pion-nucleon K-matrix, being the main effect of them to dress the bare form factors present in the γ N → Δ vertex. First, we hide the dressing into effective form factors getting at kγ2 = 0, GM = 2.97 ± 0.08 and GE = 0.055 ± 0.010 in full consistence with recent chiral effective field theory calculations. Then, different models to regularize the mentioned integrals are analyzed. We choose those better describing the non-resonant multipoles, in order to get model independent results for the resonant ones. Finally, we try to predict the bare form factors getting GM0 = 1.69 ± 0.02 and GE0 = 0.028 ± 0.008, which are consistent with recent lattice quark calculations and other more sophisticated dynamical models.Facultad de Ciencias Exacta

CC and NC pion production

The disappearance searching experiments nu_mu ---> nu_x use charged current quasielastic (CCQE) reaction to detect an arriving neutrino and reconstruct its energy, while the CC1 pi^+ production can mimic the CCQE signal process. In nu_mu --->nu_e appearance experiments, the NC1 pi^0 production process can lead to a fake e^- event by the impossibility for the detector of distinguish an arriving electron or a photon. Here we present a consistent model, from the point of view of the construction of the elemental amplitude, for the mentioned pion production background processes including bounding, smearing and final state interaction (FSI) effects in a single fashion. Results are comparable with more evolved approaches based on Monte Carlo simulations.Fil: Mariano, Alejandro Edgardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Barbero, César Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: López Castro, Gabriel. Instituto Politécnico Nacional. Centro de Investigación y de Estudios Avanzados. Departamento de Física; Méxic

Determination of the Δ++ magnetic dipole moment

We study the elastic and radiative π+p scattering within a full dynamical model which incorporates the finite width effects of the Δ++. The scattering amplitudes are invariant under contact transformations of the spin 3/2 field and gauge-invariance is fulfilled for the radiative case. The pole parameters of the Δ++ obtained from the elastic cross section are mΔ = (1211.2±0.4) MeV and ΓΔ = (88.2±0.4) MeV. From a fit to the most sensitive observables in radiative π+p scattering, we obtain μΔ = (6.14±0.51)e/2mp for the magnetic dipole moment of the Δ++.Facultad de Ciencias Exacta

Single photon production νlN→νlNγ in neutrino-nucleon scattering

The quasielastic charged current (CCQE) νen→e⎯pνen→e−p scattering is the dominant mechanism to detect appearance of a νe in an almost νμ flux at the 1 GeV scale. Actual experiments show a precision below 1% and between less known background contributions, but necessary to constraint the event excess, we have the radiative corrections. A consistent model recently developed for the simultaneous description of elastic and radiative πN scattering, pion-photoproduction and single pion production processes, both for charged and neutral current neutrino-nucleon scattering, is extended for the evaluation of the radiative νlN→νlNγνlN→νlNγ cross section. Our results are similar to a previous (but inconsistent) theoretical evaluation in the low energy region, and show an increment in the upper region where the Δ resonance becomes relevant.Facultad de Ciencias Exacta

2p2h effects on the weak pion production cross section

The νln → l−p QE reaction on the A-target is used as a signal event or/and to reconstruct the neutrino energy, using two-body kinematics. Competition of another processes could lead to misidentification of the arriving neutrinos, being important the fake events coming from the CC1π background. A precise knowledge of cross sections is a prerequisite in order to make simulations in event generators to substract the fake ones from the QE countings, and in this contribution we analyze the different nuclear effects on the CC1π channel. Our calculations also can be extended for the NC case.Instituto de Física La Plat

A consistent dynamical model for pion-photoproduction

Pion-photoproduction amplitude is calculated within a consistent isobar model already used to fix the Δ+ + (1232   MeV) parameters in pion-nucleon scattering and bremsstrahlung. This amplitude is expressed in terms of (physical) on-shell quantities and off-shell contributions coming from final state interactions. These latter are isolated in principal value integrals on the non-polar part of the pion-nucleon K-matrix, being the main effect of them to dress the bare form factors present in the γ N → Δ vertex. First, we hide the dressing into effective form factors getting at kγ2 = 0, GM = 2.97 ± 0.08 and GE = 0.055 ± 0.010 in full consistence with recent chiral effective field theory calculations. Then, different models to regularize the mentioned integrals are analyzed. We choose those better describing the non-resonant multipoles, in order to get model independent results for the resonant ones. Finally, we try to predict the bare form factors getting GM0 = 1.69 ± 0.02 and GE0 = 0.028 ± 0.008, which are consistent with recent lattice quark calculations and other more sophisticated dynamical models.Facultad de Ciencias Exacta

Efecto de excitaciones; de tres partículas - tres agujeros sobre la resonancia Gamow - Teller: regla de suma

Es bien conocido el efecto de reducción de intensidad, presente en las resonancias Gamow - Teller, producidas por las reacciones de tipo (p, n). Uno de los mecanismos de origen nuclear propuestos y más estudiados es la tranferencia de intensidad a regiones de mayor energía, mediante la inclusión de excitaciones de dos partículas-dos agujeros (2p-2h) en los estados nucleares. Dado que actuando sobre un estado fundamental correlacionado el operador de Gamow-Teller (στ) puede producir también excitaciones de tres partículas-tres agujeros (ep-3h), será relevante la inclusión de las mismas para el cierre de la regla de suma corrrespondiente. En el presente trabajo se realiza el cálculo de la regla de suma del operador (στ) en espacios que contienen configuraciones de lp-lh, 2p-2h, 3p-3h. A través de los resultados obtenidos se evidencia que con un estado fundamental normalizado, no basta incluir sólo excitaciones de 2p-2h para satisfacer la regla de suma sino también los de 3p-3h. Como conclusión todos los posibles grados de libertad nucleares deben ser tenidos en cuenta.Facultad de Ciencias Exacta

A formalism for weak interactions in nuclei

The neutrino-nucleus cross-section and the muon capture rate are discussed within a simple formalism which facilitates the nuclear structure calculations. The corresponding formulae only depend on four types of nuclear matrix elements, which are currently used in the nuclear beta decay. We have also considered the non-locality effects arising from the velocity-dependent terms in the hadronic current. We show that for both observables in 12C the higher order relativistic corrections are of the order of ∼ 4% only, and therefore do not play a significant role.Facultad de Ciencias Exacta

Scattering of GeV electrons in the framework of the relativistic Hartree approximation

The high momentum transfer electron-nucleus scattering cross section is evaluated within the plane wave impulse approximation (PWIA) supplemented by the relativistic Hartree approximation (RHA). Binding effects on the struck nucleon are introduced through the scalar and vector meson-exchange fields within the framework of quantum hadrodynamics. This model naturally satisfies the current conservation, with the off-shell nucleons behaving as being on the mass shell but having an effective mass. The nucleon inelastic response is included via different parameterizations of the structure function measured at SLAC, while the smearing of the Fermi surface is introduced through a momentum distribution obtained from a perturbative nuclear matter calculation. Recent CEBAF data on inclusive scattering of 4.05 GeV electrons on 56Fe are well reproduced for all measured geometries by the first time. Scaling effects are analyzed as well, and the scaling properties of the nuclear response in terms of the y variable associated to the PWIA within the RHA framework are discussed. The theoretical scaling function obtained in our approach also significantly improves previous PWIA calculations within the multi-GeV electron scattering regime, and describes properly the CEBAF scaling data.Facultad de Ciencias Exacta

Scattering of GeV electrons in the framework of the relativistic Hartree approximation

The high momentum transfer electron-nucleus scattering cross section is evaluated within the plane wave impulse approximation (PWIA) supplemented by the relativistic Hartree approximation (RHA). Binding effects on the struck nucleon are introduced through the scalar and vector meson-exchange fields within the framework of quantum hadrodynamics. This model naturally satisfies the current conservation, with the off-shell nucleons behaving as being on the mass shell but having an effective mass. The nucleon inelastic response is included via different parameterizations of the structure function measured at SLAC, while the smearing of the Fermi surface is introduced through a momentum distribution obtained from a perturbative nuclear matter calculation. Recent CEBAF data on inclusive scattering of 4.05 GeV electrons on 56Fe are well reproduced for all measured geometries by the first time. Scaling effects are analyzed as well, and the scaling properties of the nuclear response in terms of the y variable associated to the PWIA within the RHA framework are discussed. The theoretical scaling function obtained in our approach also significantly improves previous PWIA calculations within the multi-GeV electron scattering regime, and describes properly the CEBAF scaling data.Facultad de Ciencias Exacta