Nonmesonic weak decay spectra of light hypernuclei

The nonmesonic weak decay spectra of light hypernuclei have been evaluated in a systematic way. As theoretical framework we adopt the independent particle shell model with three different one-meson-exchange transition potentials. Good agreement with data is obtained for proton and neutron kinetic energy spectra of 4ΛHe, and 5ΛHe, when the recoil effect is considered. The coincidence spectra of proton-neutron pairs are also accounted for quite reasonably, but it was not possible to reproduce the data for the neutron–neutron pair spectra. It is suggested that the π + K meson-exchange model with soft monopole form factors could be a good starting point for describing the dynamics responsible for the decays of these two hypernuclei. The 4ΛH spectra are also presented.Instituto de Física La Plat

Nuclear structure in nonmesonic weak decay of hypernuclei

A general shell model formalism for the nonmesonic weak decay of the hypernuclei has been developed. It involves a partial wave expansion of the emitted nucleon waves, preserves naturally the antisymmetrization between the escaping particles and the residual core, and contains as a particular case the weak Λ-core coupling formalism. The hypernuclei are grouped having in view their A-1 cores, that is in those with even-even, even-odd and odd-odd cores. It is shown that in all three cases the nuclear structure manifests itself basically through Pauli Principle, and very simple expressions are derived for the neutron and proton induced decays rates Γn and Γp, which does not involve the spectroscopic factors. For the strangeness-Changing weak ΛN → NN transition potential we use the One-Meson-Exchange Model (OMEM), which comprises the exchange of the complete pseudoscalar and vector meson octets (π,η, K, ρ, ω, K*). We evaluate 3H, 4H 4ΛHe, 5ΛHe,11B, 12ΛC, 16ΛO, 17ΛO, and 28ΛSi hypernuclei, with commonly used parametrization for the OMEM, and compare the results with the available experimental information, The calculated rates ΓNM = Γn + Γp are consistent with the data, but the measurements of Γn/p = Γn/Γp are not well accounted for by the theory. It is suggested that, unless additional degrees of freedom are incorporated, the OMEM parameters should be radically modified.Facultad de Ciencias Exacta

Simple analytical expression for vector hypernuclear asymmetry in nonmesonic decay of 5Λ He and 12Λ C

We present general explicit expressions for a shell-model calculation of the vector hypernuclear parameter in nonmesonic weak decay. We use a widely accepted effective coupling Hamiltonian involving the exchange of the complete pseudoscalar and vector meson octets (π, η, K, ρ, ω, K*). In contrast to the approximated formula widely used in the literature, we correctly treat the contribution of transitions originated from single-proton states beyond the s-shell. Exact and simple analytical expressions are obtained for the particular cases of 5ΛHe and 12ΛC, within the one-pion-exchange model. Numerical computations of the asymmetry parameter, aΛ, are presented. Our results show a qualitative agreement with other theoretical estimates but also a contradiction with recent experimental determinations. Our simple analytical formulas provide a guide in searching the origin of such discrepancies, and they will be useful for helping to solve the hypernuclear weak decay puzzle.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

Simple analytical expression for vector hypernuclear asymmetry in nonmesonic decay of 5Λ He and 12Λ C

We present general explicit expressions for a shell-model calculation of the vector hypernuclear parameter in nonmesonic weak decay. We use a widely accepted effective coupling Hamiltonian involving the exchange of the complete pseudoscalar and vector meson octets (π, η, K, ρ, ω, K*). In contrast to the approximated formula widely used in the literature, we correctly treat the contribution of transitions originated from single-proton states beyond the s-shell. Exact and simple analytical expressions are obtained for the particular cases of 5ΛHe and 12ΛC, within the one-pion-exchange model. Numerical computations of the asymmetry parameter, aΛ, are presented. Our results show a qualitative agreement with other theoretical estimates but also a contradiction with recent experimental determinations. Our simple analytical formulas provide a guide in searching the origin of such discrepancies, and they will be useful for helping to solve the hypernuclear weak decay puzzle.Facultad de Ciencias Exacta

Nuclear structure in nonmesonic weak decay of hypernuclei

A general shell model formalism for the nonmesonic weak decay of the hypernuclei has been developed. It involves a partial wave expansion of the emitted nucleon waves, preserves naturally the antisymmetrization between the escaping particles and the residual core, and contains as a particular case the weak Λ-core coupling formalism. The hypernuclei are grouped having in view their A-1 cores, that is in those with even-even, even-odd and odd-odd cores. It is shown that in all three cases the nuclear structure manifests itself basically through Pauli Principle, and very simple expressions are derived for the neutron and proton induced decays rates Γn and Γp, which does not involve the spectroscopic factors. For the strangeness-Changing weak ΛN → NN transition potential we use the One-Meson-Exchange Model (OMEM), which comprises the exchange of the complete pseudoscalar and vector meson octets (π,η, K, ρ, ω, K*). We evaluate 3H, 4H 4ΛHe, 5ΛHe,11B, 12ΛC, 16ΛO, 17ΛO, and 28ΛSi hypernuclei, with commonly used parametrization for the OMEM, and compare the results with the available experimental information, The calculated rates ΓNM = Γn + Γp are consistent with the data, but the measurements of Γn/p = Γn/Γp are not well accounted for by the theory. It is suggested that, unless additional degrees of freedom are incorporated, the OMEM parameters should be radically modified.Facultad de Ciencias Exacta

Quasiparticle-rotor model description of carbon isotopes

In this work we perform quasiparticle-rotor coupling model calculations within the usual BCS and the projected BCS for the carbon isotopes 15C, 17C and 19C using 13C as the building block. Owing to the pairing correlation, we find that 13C as well as the cores of the other isotopes, namely 14C, 16C and 18C acquire strong and varied deformations. The deformation parameter is large and negative for 12C, very small (or zero) for 14C and large and positive for 16C and 18C. This finding casts a doubt about the purity of the supposed simple one-neutron halo nature of 19C.Instituto de Física La PlataFacultad de Ciencias Astronómicas y Geofísica

One-nucleon-induced nonmesonic hypernuclear decay in laboratory coordinates

We present a formalism for the computation of one-nucleon-induced nonmesonic weak hypernuclear decay rates in laboratory coordinates, within an independent-particle shell model framework, with a view to its generalization to the case of two-nucleon-induced transitions.Fil: Galeão, A. P.. Universidade Estadual Paulista Julio de Mesquita Filho. Instituto de Física Teórica; BrasilFil: Barbero, Cesar Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Física La Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Exactas; ArgentinaFil: de Conti, C.. Universidade Estadual Paulista Julio de Mesquita Filho. Campus Experimental de Itapeva; BrasilFil: Krmpotic, Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Física La Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentin

Quasiparticle-rotor model description of carbon isotopes

In this work we perform quasiparticle-rotor coupling model calculations within the usual BCS and the projected BCS for the carbon isotopes 15C, 17C and 19C using 13C as the building block. Owing to the pairing correlation, we find that 13C as well as the cores of the other isotopes, namely 14C, 16C and 18C acquire strong and varied deformations. The deformation parameter is large and negative for 12C, very small (or zero) for 14C and large and positive for 16C and 18C. This finding casts a doubt about the purity of the supposed simple one-neutron halo nature of 19C.Instituto de Física La PlataFacultad de Ciencias Astronómicas y Geofísica

Neutrino and antineutrino cross sections in 12C

We extend our formalism of weak interaction processes, obtaining new expressions for the transition rates, which greatly facilitate numerical calculations, of neutrino (ν) and antineutrino (ν)-nucleus reactions and the muon capture. We have done a thorough study of exclusive (ground state) properties of 12B and 12N within the Projected Quasiparticle Random Phase Approximation (PQRPA). Good agreement with experimental data is achieved in this way. The inclusive ν/ν-nucleus reactions 12C(ν, e-)12N and 12C(ν, e+)12B are calculated within both the PQRPA, and the Relativistic QRPA (RQRPA). It is found that the magnitudes of the resulting cross-sections: i) are close to the sum-rule limit at low energy, but significantly smaller than this limit at high energies both for ν and ν, ii) steadily increase when the size of the configuration space is augmented, and particulary for ν/ν energies > 200 MeV, and iii) converge for sufficiently large configuration spaces and final state angular momenta.Facultad de Ciencias Exacta