Nonmesonic Weak Decay Of Hypernuclei Research Articles

Λ N → NN EFT potentials and hypertriton non-mesonic weak decay

The potential for the Λ N → NN weak transition, the main responsible for the non-mesonic weak decay of hypernuclei, has been developed within the framework of effective field theory (EFT) up to next-to-leading order (NLO). The leading order (LO) and NLO contributions have been calculated in both momentum and coordinate space, and have been organised into the different operators which mediate the N → NN transition. We compare the ranges of the one-meson and two-pion exchanges for each operator. The non-mesonic weak decay of the hypertriton has been computed within the plane-wave approximation using the LO weak potential and modern strong EFT NN potentials. Formally, two methods to calculate the final state interactions among the decay products are presented. We briefly comment on the calculation of the mesonic weak decay.

Non-mesonic weak decay of hypernuclei with effective field theory

The ΛN → NN interaction, the main responsible for the decay of heavy hypernuclei, is described within the effective field theory (EFT) framework. The EFT is developed up to (q⃗2), where q⃗ is the transferred momentum between the interacting baryons. Numerical values for the leading order low-energy constants are obtained by two means. First, from a fit to the hypernuclear decay observables, and second, by comparing the EFT to the one-meson-exchange model describing the same interaction. All the two-pion exchange diagrams entering at the next-to-leading in the EFT have been calculated.

One-loop contributions in the effective field theory for theΛN→NNtransition

We consider the $\ensuremath{\Lambda}N\ensuremath{\rightarrow}NN$ weak transition, responsible for a large fraction of the nonmesonic weak decay of hypernuclei. We follow the previously derived effective field theory and compute the next-to-leading one-loop corrections. Explicit expressions for all diagrams are provided, which result in contributions to all relevant partial waves.

The Contribution of the Three-Body Process in the Nonmesonic Weak Decay of theΛ12CHypernucleus.

The serious inconsistency problem between the values of experimental and theoretical n/ p ratio of the nonmesonic weak decay ofhypernuclei has been resolved recently. We have shown that the reason behind the problem was the quenching of the nucleon yields which in turn was due to the contribution of the three-body process. We have measured that the branching ratio of the three-body process in nonmesonic weak decay, 0.29±0.13, is so large that the absolute values of n and p must be determined taking account of the contribution of the 3-body process. In this paper, the recent studies toward the finding of the 3-body process via the exclusive coincidence experiments are presented.

Mesonic and Non-Mesonic Weak Decay of Hypernuclei with FINUDA

The FINUDA experiment performed a systematic study of both mesonic and non-mesonic weak decay of p- shell Λ-hypernuclei. Recent results on the mesonic decay rates and the non-mesonic decay ratios are illustrated and briefly discussed.

Publisher's Note: Kinetic energy sum spectra in nonmesonic weak decay of hypernuclei [Phys. Rev. C 78, 044312 (2008)

Publisher's Note: Kinetic energy sum spectra in nonmesonic weak decay of hypernuclei [Phys. Rev. C 78, 044312 (2008)

Kinetic energy sum spectra in nonmesonic weak decay of hypernuclei

We evaluate the coincidence spectra in the nonmesonic weak decay (NMWD) $\ensuremath{\Lambda}N\ensuremath{\rightarrow}\mathit{nN}$ of \ensuremath{\Lambda} hypernuclei ${}_{\ensuremath{\Lambda}}^{4}\mathrm{He}$, ${}_{\ensuremath{\Lambda}}^{5}\mathrm{He}$, ${}_{\ensuremath{\Lambda}}^{12}\mathrm{C}$, ${}_{\ensuremath{\Lambda}}^{16}\mathrm{O}$, and ${}_{\ensuremath{\Lambda}}^{28}\mathrm{Si}$, as a function of the sum of kinetic energies ${E}_{\mathit{nN}}={E}_{n}+{E}_{N}$ for $N=n,p$. The strangeness-changing transition potential is described by the one-meson-exchange model, with commonly used parametrization. Two versions of the independent-particle shell model (IPSM) are employed to account for the nuclear structure of the final residual nuclei. They are as follows: (a) IPSM-a, where no correlation, except for the Pauli principle, is taken into account and (b) IPSM-b, where the highly excited hole states are considered to be quasistationary and are described by Breit-Wigner distributions, whose widths are estimated from the experimental data. All np and nn spectra exhibit a series of peaks in the energy interval 110 MeV $<{E}_{\mathit{nN}}<170$ MeV, one for each occupied shell-model state. Within the IPSM-a, and because of the recoil effect, each peak covers an energy interval proportional to ${A}^{\ensuremath{-}1}$ , going from $\ensuremath{\cong}4$ MeV for ${}_{\ensuremath{\Lambda}}^{28}\mathrm{Si}$ to $\ensuremath{\cong}40$ MeV for ${}_{\ensuremath{\Lambda}}^{4}\mathrm{He}$. Such a description could be pretty fair for the light ${}_{\ensuremath{\Lambda}}^{4}\mathrm{He}$ and ${}_{\ensuremath{\Lambda}}^{5}\mathrm{He}$ hypernuclei. For the remaining, heavier, hypernuclei it is very important, however, to consider as well the spreading in strength of the deep-hole states and bring into play the IPSM-b approach. Notwithstanding the nuclear model that is employed the results depend only very weakly on the details of the dynamics involved in the decay process proper. We propose that the IPSM is the appropriate lowest-order approximation for the theoretical calculations of the of kinetic energy sum spectra in the NMWD. It is in comparison to this picture that one should appraise the effects of the final-state interactions and of the two-nucleon-induced decay mode.

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 L-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-oddcores. 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, Gn and Gp, which does not involve the spectroscopic factors. For the strangeness-changing weak L N ®N N transition potential we use the One-Meson-Exchange Model (OMEM), which comprises the exchange of the complete pseudoscalar and vector meson octets (p, h,K, r, w, K*).We evaluate 3H, 4H, 4LHe, 5LHe, 11B, 12LC, 16LO, 17LO, and 28Si hypernuclei, with commonly used parametrization for the OMEM, and compare the results with the available experimental information. The calculated rates GNM = Gn + Gp are consistent with the data, but the measurements of Gn/p = Gn/Gp 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.

Nonmesonic weak decay of hypernuclei

Nonmesonic weak decay of hypernuclei

Four-baryon pointΛN→NNinteraction for the nonmesonic weak decay of the hypernucleiΛ4H,Λ4He,Λ5He,andΛ12C

In an effort to resolve the theoretical difficulties in explaining the total and partial decay rates and the ratio ${\ensuremath{\Gamma}}_{n}/{\ensuremath{\Gamma}}_{p},$ a four-baryon point $\ensuremath{\Lambda}\stackrel{\ensuremath{\rightarrow}}{N}\mathrm{NN}$ interaction is suggested as a new model for the short-range part of the nonmesonic weak decay of hypernuclei in addition to the traditional one-pion-exchange model for the long-range part. It is shown that with any value of the axial vector coupling constant ${g}_{A}$ in the range $\ensuremath{-}0.1<~{g}_{A}<~0.3$ for the isoscalar weak current of nucleons, i.e., $p(n)\ensuremath{\rightarrow}p(n)$ transitions, the partial and the total nonmesonic decay rates ${\ensuremath{\Gamma}}_{n},$ ${\ensuremath{\Gamma}}_{p},$ and ${\ensuremath{\Gamma}}_{\mathrm{nm}}={\ensuremath{\Gamma}}_{n}+{\ensuremath{\Gamma}}_{p}$ and the ratios ${\ensuremath{\Gamma}}_{n}/{\ensuremath{\Gamma}}_{p}$ of ${}_{\ensuremath{\Lambda}}^{4}\mathrm{He},$ ${}_{\ensuremath{\Lambda}}^{5}\mathrm{He},$ and ${}_{\ensuremath{\Lambda}}^{12}\mathrm{C}$ are excellently fitted in the experimental data.

Violation of the ΔI=1/2 rule in the nonmesonic weak decay of hypernuclei

Violations of the ΔI=1/2 rule are investigated in the nonmesonic weak hypernuclear decay using a weak ΛN→NN transition potential based on meson exchange. While the weak ΔI=3/2 couplings of baryons to pseudoscalar mesons are known to be very small, the analogous couplings of baryons to vector mesons, evaluated using the factorization approximation, are found to produce potentially significant changes in the predictions for hypernuclear decay observables in the case of Λ 12 C .

Proton asymmetry in the weak decay of polarized hypernuclei

Polarization observables are evaluated for the nonmesonic weak decay of hypernuclei in a relativistic nuclear model The asymmetries in the decay of Λ 11 B and Λ 12 C are in good agreement with preliminary experimental data from KEK. The hypernuclear dependence of the intrinsic Λ asymmetry parameter a Λ is discussed. The resulting spin polarization of the outgoing proton is also calculated. In contrast to the nonmesonic decay rate, the polarization observables are not very sensitive to short-range correlations, whereas the effect of interactions in the final state is substantial.

Asymmetries in the weak decay of polarized hypernuclei

Polarization observables and decay rates are evaluated for the nonmesonic weak decay of hypernuclei in a relativistic nuclear model. The single-particle bound and scattering states are obtained by solving the Dirac equation with large potentials. The total decay rates are very sensitive to ΔN short-range correlations in the initial state. The experimental total and partial decay rates of Λ 5He, Λ 11B and Λ 12C cannot be reproduced with only pion- and kaon-exchange in the process ΛN→NN. However, the proton asymmetries in the decay of with preliminary experimental data from KEK. The polarization observables are found to be sensitive to the final-state interaction of the ejected nucleons.