Nonmesonic Weak Decay Research Articles

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

Abstract A consistent microscopic model is applied 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 interaction, 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. Double-coincidence nucleon spectra are predicted for 12 Λ C and compared with recent KEK data. Discrepancies with data remain for proton emission and for both neutron–proton and neutron–neutron emission in the non-back-to-back kinematics region. Motivated by a recent triple-coincidence measurement by FINUDA, we estimate the three-body induced decay, where we have found, Γ 3 / Γ 2 ≅ 0.23 . Our microscopic model also predicts values for the asymmetry parameter in 12 Λ C, consistent with the asymmetry measured at KEK.

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.

Three-Body ΛNN → nNN Nonmesonic Weak Decay Process of Λ Hypernuclei

The fundamental motivation to study the non-mesonic weak decay (NMWD) of Λ hypernuclei is that it provides the unique channel for the information of the baryon–baryon weak interaction in SU3f group. The relative strength of the two main channels of NMWD whose decay processes are the Λp → np and Λn → nn has been the long standing puzzle during last several decades. The puzzling status has been settled down recently with the Γn / Γp value converging to \({\sim 0.5}\) . The large experimental values of Γn / Γp of the previous measurements turned out to be due to the surprisingly large contribution of the competing three-body NMWD, ΛNN → nNN process. This paper discusses about the first measurement of the branching ratio of the three-body NMWD process and the renormalized intra-nuclear cascade method adopted to disentangle the contribution of the three-body process out of those due to final state interaction. And the prospects of the more accurate measurement of the three-body process also are presented.

Microscopic approach to the proton asymmetry in the nonmesonic weak decay ofΛhypernuclei

The non--mesonic weak decay of polarized $\Lambda$-hypernuclei is studied with a microscopic diagrammatic formalism in which one- and two-nucleon induced decay mechanisms, $\vec{\Lambda} N \to NN$ and $\vec{\Lambda} NN \to NNN$, are considered together with (and on the same ground of) nucleon final state interactions. We adopt a nuclear matter formalism extended to finite nuclei via the local density approximation. Our approach adopts different one-meson-exchange weak transition potentials, while the strong interaction effects are accounted for by a Bonn nucleon-nucleon interaction. We also consider the two-pion-exchange effect in the weak transition potential. Both the two-nucleon induced decay mechanism and the final state interactions reduce the magnitude of the asymmetry. The quantum interference terms considered in the present microscopic approach give rise to an opposite behavior of the asymmetry with increasing energy cuts to that observed in models describing the nucleon final state interactions semi-classically via the intranuclear cascade code. Our results for the asymmetry parameter in $^{12}_{\Lambda}$C obtained with different potential models are consistent with the asymmetry measured at KEK.

Hypernuclear weak decay studies with FINUDA

The FINUDA experiment has recently completed an extensive study of the weak decay of p -shell Λ -hypernuclei including both mesonic and non-mesonic modes. Charged mesonic decay rates have been determined based on the analysis of π − kinetic energy spectra, never measured before. The study of proton spectra from non-mesonic weak decay for p -shell hypernuclei, both single and in coincidence with a neutron, has triggered the investigation of the two-nucleon induced Λ N N → n N N decay channel: its weight has been evaluated to be Γ 2 / Γ N M = 0.21 ± 0.07 stat − 0.02 sys + 0.03 sys . Finally, a direct experimental evidence of the occurrence of the weak reaction Λ n p → n n p in nuclei is presented for the first time. Three events have been found which can be attributed to Li Λ 7 and Be Λ 9 two-nucleon induced non-mesonic weak decay; the kinematical analysis is discussed here.

Study of the two–nucleon induced Non Mesonic Weak Decay with FINUDA

The latest results from the FINUDA experiment on the Non-Mesonic Weak Decay (NMWD) channels of p-shell Λ-hypernuclei are presented and discussed. Spectra of protons from NMWD were obtained for 5ΛHe, 7ΛLi, 9ΛBe, 11ΛB, 12ΛC, 13ΛC, 15ΛN and 16ΛO. An estimation of the contributions of both Final State Interactions (FSI) and two-nucleon induced (2N) decay processes was performed, following a model independent approach. The previously published results are confirmed by a new analysis of the triple coincidence (π−, p, n) events.

Many-body cascade calculation of final state interactions in C nonmesonic weak decay

We study the nonmesonic weak decay (NMWD) ΛN → nN of hypernucleus induced by one nucleon N = n, p. The whole process is described by two coupled models: one for the primary NMWD of Λ hyperon in the nuclear environment, and the other taking into account the final state interactions (FSI) of the two outgoing nucleons within the residual nucleus. The NMWD dynamics is represented by the one-meson-exchange potential with usual parametrization, while the independent-particle shell model is used as the nuclear structure framework. The FSI are accounted for by a time-dependent multicollisional Monte Carlo cascade scheme, implemented within the CRISP code (Collaboration Rio-São Paulo), which describes in a phenomenological way both the nucleon–nucleon scattering inside the nucleus and the escaping of nucleons from the nuclear surface. An evaporation phase is included to cool down the cascade residual nucleus as well. We analyze the single-nucleon kinetic energy spectra, and the two-nucleon coincidence spectra as a function of both: (i) the sum of the kinetic energies and (ii) the opening angle. The theoretical results are compared with recent data from KEK and FINUDA experiments, obtaining fairly good agreement for inclusive and exclusive kinetic energy spectra. Further theoretical improvements are required to explain angular distributions.

The three-body nonmesonic weak decay process of 12ΛC hypernucleus and its exclusive measurement at J-PARC (E18)

In spite of the recent crucial progresses in the studies of nonmesonic weak decay (NMWD) of Λ hypernuclei, its consistent understanding is yet to be achieved. In the experimental progresses, the long standing Γn/Γp problem has finally been solved recently. And for the first time, two-nucleon induced NMWD channel has been identified experimentally and its decay width, 0.27±0.13, measured but only with a 2σ confidence level. On the other hand, the decay asymmetry remains to be understood yet and the accurate measurement of the decay width of each channel is much awaited for the consistent understanding of NMWD. In E18 of J-PARC, we are going to measure Γi of NMWD including Γ2N with 10 % statistical uncertainties. Main concerns in E18 experiment such as the handling of the high beam intensity, the neutron background contamination in the neutron coincidence yields and the control of the trigger rates with the beam intensity as high as 107π+/spill are discussed and it is shown that they can be handled properly.

Nonmesonic Hyperon Weak Decay Spectra in 12ΛC

We study the nonmesonic weak decay (NMWD) ΛN → nN of the 12ΛC hypernucleus induced by the nucleon N = n,p with transition rate ΓN. The nuclear process is described by the interplay of two models; one describing the NMWD of hyperon Λ in the nuclear environment, and the other taking into account the Final State Interaction (FSI) of the two outgoing nucleons with the residual nucleus. The first one is done in the framework of the Independent-Particle Shell-Model (IPSM), with the decay dynamics represented by the exchange of π+η+K+ρ+ω+ K*mesons with usual parametrization. For the second one is used a time dependent multicollisional intranuclear cascade schema (implemented in the CRISP code - Collaboration Rio-São Paulo). The results obtained for inclusive and exclusive kinetic energy spectra, and the angular correlation are compared with recent data from KEK and FINUDA experiments. The calculated ratio (Γn/Γp)FSI, between the numbers of emitted back-to-back nn and np pairs, is in good agreement with the experimental data.

Neutron–proton coincidences from Non-Mesonic Weak Decay of p-shell Λ-hypernuclei and determination of the two-nucleon induced process

The decay of Λ-hypernuclei without π emission, known as Non-Mesonic Weak Decay (NMWD), gives an effective tool to investigate ΔS=1 four-baryon interactions. It was theoretically suggested that the two-nucleon induced mechanism could play a substantial role in reproducing the observed NMWD decay rates and nucleon spectra, but at present no direct evidence of such a mechanism has been obtained. The FINUDA experiment, exploiting the possibility to detect both charged and neutral particles coming from the hypernucleus decay, has allowed us to deduce the relative weight of the two nucleon induced decay rate to the total NMWD rate. The value of Γ2N/ΓNMWD=0.21±0.07stat−0.02sys+0.03sys has been deduced, well consistent with the previous determinations.

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.

Role of the isovector nucleon-nucleus interaction potential in nonmesonic hypernuclear decay

We develop a theoretical finite nucleus framework for the evaluation of the coincidence number of pairs ${N}_{\mathit{nN}}$ and the asymmetry parameter ${a}_{\ensuremath{\Lambda}}$ in nonmesonic hypernuclear decay $\ensuremath{\Lambda}N\ensuremath{\rightarrow}\mathit{nN}$, and introduce the effect of final-state interactions. The primary nonmesonic weak hyperon-nucleon decay process is described through the independent-particle shell model. We work within the $2p1h$ subspace of final states, assuming a simple approach for finite nuclei based on the eikonal approximation. An optical potential including the nucleon-nucleus isoscalar and isovector interactions is introduced to take into account the minimum nuclear-medium effects. The calculated values for ${}_{\ensuremath{\Lambda}}^{5}\mathrm{He}$ and ${}_{\ensuremath{\Lambda}}^{12}\mathrm{C}$ show that our treatment of final-state interactions, even those considered as restricted to a subspace of states, brings the independent-particle model results closer to the experimental data.

RECENT PROGRESSES IN THE STUDY OF NONMESONIC WEAK DECAY OF Λ HYPERNUCLEI

Fundamental interest on Non Mesonic Weak Decay (NMWD) is that it provides practically the only means to study the elementary ΔS = 1 baryon-baryon weak interaction process, Λ N → NN (Λ p → np and Λ n → nn ), since it is difficult to be realized in the free space. Thanks to the recent developments in experimental and theoretical studies, the relative strength of two channels, the Γn/Γp ratios whose experimental and theoretical values have showed a long standing discrepancy have been converged to ~ 0.5 and the discrepancy problem finally has been solved.1–3 However, despite the progresses, the decay interaction of NMWD is not yet well established and there remain the important issues such as the discrepancy of asymmetry parameter between experimental and theoretical values, confirmation of 3-body NMWD process, and whether the I = 1/2 rule for ΔS = 1 decay would hold or not in NMWD. Especially the recent experimental indication of the 3-body decay process Λ NN → NNN whose contribution was predicted to be significant in the theoretical calculations seems to be strong and showed a surprisingly large contribution in the quenching of singles and coincidence nucleon yields in NMWD. Recent results of the asymmetry parameter αnm of NMWD showed small values for both s -shell [Formula: see text] and p -shell [Formula: see text] and consistent with each other. However, the discrepancy of the theoretical values from those of experimental ones remains to be understood. It now becomes one of the urgent issues in NMWD study to determine the contribution of the 3-body NMWD channel experimentally, in order to understand the discrepancy of the asymmetry papameter, and to test the ΔI = 1/2 rule for NMWD which are the main purposes of J-PARC 50 GeV PS experiment, E18 and E22.

NEW RESULTS ON NONMESONIC WEAK DECAY OF Λ HYPERNUCLEI WITH FINUDA

The FINUDA experiment has performed a systematic study of the NonMesonic Weak Decay (NMWD) of Λ hypernuclei analizing all the data collected from 2003 up to 2007. The results of a measurement of the spectra of protons coming from the NMWD of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] are the subject of this paper.

MESON EXCHANGE MODEL WITH THE AXIAL VECTOR A1-MESON EXCHANGE AND HYPERNUCLEAR NONMESONIC DECAY ASYMMETRIES

The axial vector a1-meson exchange is introduced in the nonmesonic weak decay model. The a1-exchange is found to give remarkable modifications of the parity-conserving decay potentials in 1,3S → 1,3S and 3S1 → 3D1 channels at short range. Calculated asymmetry parameters αΛ for s- and p-shell hypernuclei in the 'π + 2π/ρ + 2π/σ + ω + K + ρπ/a1 + σπ/a1'-exchange model are obtained to be positive and small, which are consistent with the available data.

Delayed clusters accompanying nonmesonic weak decay of the Λ-hypernuclei: a clue to nonleptonic processes

The nonmesonic decay of Λ-hypernuclei provides access to the nonleptonic weak decay process ΛN → NN, which is achievable only through the observation of hypernuclear ground-state decays. We continue the discussion of some specific cases which make it possible to detect a few exclusive transitions, namely, the stripping of nucleon from the ground state results in a resonance state decaying via emission of two clusters. Delayed clusters accompanying weak decay of light hypernuclei give a unique information on spin dependence of the weak decay matrix elements.

Relationships between nonmesonic weak decays in different hypernuclei

Using as a tool the s-wave approximation (sWA), this work demonstrates that the nonmesonic weak decay transition rates $\Gamma_{n}$ and $\Gamma_{p}$ can be expressed in all hypernuclei up to $^{29}_\Lambda $Si (and very likely in heavier ones too) in the same way as in the s-shell hypernuclei, i.e. as a linear combination of only three elementary transition rates. This finding leads to the analytic prediction that, independently of the transition mechanism, all hypernuclei that are on the stability line (N = Z), i.e. $^5_\Lambda $He, $^7_\Lambda $Li, $^9_\Lambda $Be, $^{11}_\Lambda$B, $^{13}_\Lambda $C, $^{17}_\Lambda $O, $^{29}_\Lambda $Si, etc should roughly have the same ratio $\Gamma_{n}/\Gamma_{p}$, the magnitude of which rapidly increases when one approaches the neutron drip-line (N >> Z), and opposite happens when one goes toward the proton drip-line (N << Z).

Asymmetry in the Nonmesonic Weak Decay of a Polarized Hypernucleus ${}^{12}_{Lambda} m{C}$ based on the Four-Baryon Point Interaction Model

Asymmetry in the Nonmesonic Weak Decay of a Polarized Hypernucleus ${}^{12}_{Lambda} m{C}$ based on the Four-Baryon Point Interaction Model

Final state interactions effects on kinetic energy sum spectra in nonmesonic weak decay

We analyze the effect of final state interactions (FSI) on coincidence spectra in nonmesonic hypernuclear weak decay, ΛN → nN, as a function of the kinetic energy sum, EnN = En+ E N, both for np and nn events. Adopting a formalism recently developed, the effects of FSI originated from the interaction between the outgoing nucleons and those in the residual core are included analytically in a very simple way within the eikonal approximation through the modification of the emerging particles momenta. Numerical results are shown for 5ΛHe and 12ΛC hypernucleus. We found that coincidence spectra are only slightly modified and the disagreement between theory and experimental data still persists, mainly for nn events in 5ΛHe where enough statistics exists. We conclude that admixtures of excitations in the final state produced by FSI need to be added to our approach in order to improve the agreement with data.

Role of ground-state correlations in hypernuclear nonmesonic weak decay

The contribution of ground state correlations (GSC) to the non--mesonic weak decay of $^{12}_\Lambda$C and other medium to heavy hypernuclei is studied within a nuclear matter formalism implemented in a local density approximation. We adopt 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. Leading GSC contributions, at first order in the residual strong interaction, are introduced on the same footing for all isospin channels of one-- and two--nucleon induced decays. Together with fermion antisymmetrization, GSC turn out to be important for an accurate determination of the decay widths. Besides opening the two--nucleon stimulated decay channels, for $^{12}_\Lambda$C GSC are responsible for 14% of the rate $\Gamma_1$ while increasing the $\Gamma_{n}/\Gamma_{p}$ ratio by 4%. Our final results for $^{12}_\Lambda$C are: $\Gamma_{\rm NM}=0.98$, $\Gamma_{n}/\Gamma_{p}=0.34$ and $\Gamma_2/\Gamma_{\rm NM}=0.26$. The saturation property of $\Gamma_{\rm NM}$ with increasing hypernuclear mass number is clearly observed. The agreement with data of our predictions for $\Gamma_{\rm NM}$, $\Gamma_n/\Gamma_p$ and $\Gamma_2$ is rather good.