Non-mesonic Weak Decay Of Λ-hypernuclei Research Articles

Relativistic model for the nonmesonic weak decay of single-lambda hypernuclei

Having in mind its future extension for theoretical investigations related to charmed nuclei, we develop a relativistic formalism for the nonmesonic weak decay (NMWD) of single-Λ hypernuclei in the framework of the independent-particle shell model and with the dynamics represented by the one-meson-exchange model. Numerical results for the one-nucleon-induced transition rates of are presented and compared with those obtained in the analogous nonrelativistic calculation. There is satisfactory agreement between the two approaches, and the only noteworthy difference is that the ratio is appreciably higher and closer to the experimental value in the relativistic calculation. The ability of describing existing data, including the most recent ones, on NMWD of Λ-hypernuclei, warrants application of the formalism to evaluate similar decay processes in charmed nuclei.

Determination of non-mesonic weak decay widths of 5ΛHe and 11ΛB Hypernuclei

The recent determination of the partial decay widths for the one-proton and the two-nucleon induced Non-Mesonic Weak Decay of Λ-Hypernuclei in the A=5–16 range permitted to reconstruct the full pattern of decay widths for 5ΛHe and 11ΛB. A consistency check on 12ΛC decay widths confirms the validity of the adopted method.

On the effect of the [formula omitted] in hypernuclear non-mesonic weak decay: A microscopic approach

The non-mesonic weak decay of Λ-hypernuclei is studied within a microscopic diagrammatic approach which includes, for the first time, the effect or the Δ-baryon resonance. We adopt a nuclear matter formalism extended to finite nuclei via the local density approximation, a one-meson exchange weak transition potential, a Bonn nucleon–nucleon strong potential and a ΔN→NN strong potential based on the Landau–Migdal theory. Ground state correlations and final state interactions (FSI), at second order in the baryon–baryon strong interaction, are introduced on the same footing for all the isospin channels of one- and two-nucleon induced decays. Weak decay rates and single and double-coincidence nucleon spectra are predicted for CΛ12 and compared with recent KEK and FINUDA data. The Δ(1232) introduces new FSI-induced decay mechanisms which lead to an improvement when comparing the obtained nucleon spectra with data, while it turns out to have a negligible effect on the decay rates. Discrepancies with experiment remain only for emission spectra involving protons, but are mostly restricted to double-nucleon correlations in the non-back-to-back kinematics.

Microscopic approach to nucleon spectra in hypernuclear non-mesonic weak decay

A consistent microscopic diagrammatic approach is applied for the first time to the calculation of the nucleon emission spectra in the non-mesonic weak decay of Λ-hypernuclei. We adopt a nuclear matter formalism extended to finite nuclei via the local density approximation, a one-meson exchange weak transition potential and a Bonn nucleon–nucleon strong potential. Ground state correlations and final state interactions, at second order in the nucleon–nucleon interaction, are introduced on the same footing for all the isospin channels of one- and two-nucleon induced decays. Single and double-coincidence nucleon spectra are predicted for 12ΛC and compared with recent KEK and FINUDA data. The key role played by quantum interference terms allows us to improve the predictions obtained with intranuclear cascade codes. Discrepancies with data remain for proton emission.

Exchange terms in the two-nucleon induced non-mesonic weak decay of Λ-hypernuclei

The contribution of Pauli exchange terms to the two-nucleon induced non-mesonic weak decay of C Λ 12 hypernuclei, Λ N N → n N N ( N = n or p), is studied within a nuclear matter formalism implemented in a local density approximation. We have adopted a weak transition potential including the exchange of the complete octets of pseudoscalar and vector mesons as well as a residual strong interaction modeled on the Bonn potential. Among the exchange contributions, only the dominant ones have been evaluated microscopically from the corresponding Goldstone diagrams; a Landau–Migdal model has been adopted for the remaining exchange terms. The introduction of exchange terms turns out to reduce the two-nucleon induced non-mesonic rate by 18% and, jointly with an increase in the one-nucleon induced rate by the same magnitude, reveals to be significant for an accurate determination of the full set of hypernuclear non-mesonic decay widths in theoretical and experimental analyses.

Two-pion-exchange in the non-mesonic weak decay of Λ-hypernuclei

The non-mesonic weak decay of Λ-hypernuclei is studied within a one-meson-exchange potential supplemented by a chirally motivated two-pion-exchange mechanism. The effects of final state interactions on the outgoing nucleons are also taken into account. Particular attention is payed to the asymmetry of the protons emitted by polarized hypernuclei. The one-meson-exchange model describes the non-mesonic rates and the neutron-to-proton ratio satisfactorily but predicts a too large and negative asymmetry parameter. The two-pion exchange mechanism modifies the strength and sign of some decay amplitudes. As a consequence, while the rates change moderately, the asymmetry parameter is strongly affected, acquiring values that lie well within the experimental observations.

Quantum interference terms in non-mesonic weak decay of Λ-hypernuclei within an RPA formalism

Single and double coincidence nucleon spectra in the Λ-hypernuclei weak decay are evaluated and discussed using a microscopic formalism. Nuclear matter is employed together with the local density approximation which allows us to analyze the 12 Λ C hypernucleus non-mesonic weak decay. Final state interactions (FSI) are included via the first order (in the nuclear residual interaction) terms to the RPA, where the strong residual interaction is modelled by a Bonn potential. At this level of approximation, these FSI are pure quantum interference terms between the primary decay ( Λ N → N N ) and ( Λ N → N N → N N ) , where the strong interaction is responsible for the last piece in the second reaction. Also the Pauli exchange contributions are explicitly evaluated. We show that the inclusion of Pauli exchange terms is important. A comparison with data is made. We conclude that the limitations in phase space in the RPA make this approximation inadequate to reproduce the nucleon spectra. This fact does not allow us to draw a definite conclusion about the importance of the interference terms.

Non-mesonic weak decay of Λ-hypernuclei: a new determination of the [formula omitted] ratio

Theoretical descriptions of the non-mesonic weak decay of Λ-hypernuclei are unable to reproduce the experimental values of the ratio Γ n / Γ p ≡ Γ ( Λ n → n n ) / Γ ( Λ p → n p ) . In this contribution we discuss a new approach to this problem. We have incorporated a one-meson-exchange model for the Λ N → n N transition in finite nuclei in an intranuclear cascade code for the calculation of double-coincidence nucleon distributions corresponding to the non-mesonic decay of 5 Λ He and 12 Λ C. The two-nucleon induced decay mechanism, Λ n p → n n p , has been taken into account within a local density approximation scheme using a one-pion-exchange model supplemented by short range correlations. A weak decay model independent analysis of preliminary KEK coincidence data for 5 Λ He allows us to extract Γ n / Γ p = 0.39 ± 0.11 when the two-nucleon induced channel is neglected (i.e., Γ 2 = 0 ) and Γ n / Γ p = 0.26 ± 0.11 when Γ 2 / Γ 1 = 0.2 .

Is Nuclear Structure Relevant to Non-Mesonic Hyper-nuclear Weak Decay?

This work studies the relevance of nuclear structure in the nonmesonic weak decay of Λ-hypernuclei, with the mechanism of ΛN → NN transition being restricted to one pion exchange (OPE) only. As an application, for the hypernucleus Λ12C a comparison between the L-S coupling and the j-j coupling gives an estimate of the range of nuclear structure effects. A considerable dependence is found of the total decay rate and the ratio Гn/qGp on the single particle properties of nuclear models. The possible contribution from the ΔI = 3/2 channel is investigated in a phenomenological manner.

The ΔI=1/2 rule in non-mesonic weak decay of Λ-hypernuclei

By employing recent data on non-mesonic decay of s-shell Λ-hypernuclei we study, within the phenomenological model of Block and Dalitz, the validity of the ΔI=1/2 rule in the ΛN→ NN process. Due to the low experimental precision, a possible violation of this rule can be neither proved nor excluded at present with sufficient accuracy: a pure ΔI=1/2 transition amplitude is excluded at 40% confidence level.