Hypernuclear weak decay studies with FINUDA

Nuclear Physics A, 2009

The FINUDA experiment performed a systematic study of both mesonic and non-mesonic weak Nuclear Physics A 827 (2009) 303c-305c

2011

A direct experimental evidence of the occurrence of the weak reaction $\Lambda np\rightarrow nnp$ in nuclei has been obtained by the FINUDA experiment. Three events have been found that can be attributed to $^{7}_{\Lambda}$Li and $^{9}_{\Lambda}$Be two nucleon-induced non mesonic weak decays. The kinematic analysis of such events is presented here.

Nuclear Physics A, 2010

Physics Letters B, 2014

Previous studies of proton and neutron spectra from Non-Mesonic Weak Decay of eight Λ-Hypernuclei (A = 5÷16) have been revisited. New values of the ratio of the two-nucleon and the one-proton induced decay widths, Γ 2N /Γ p , are obtained from single proton spectra, Γ 2N /Γ p = 0.50 ± 0.24, and from neutron and proton coincidence spectra, Γ 2N /Γ p = 0.36 ± 0.14 stat +0.05sys −0.04sys , in full agreement with previously published ones. With these values, a method is

Nuclear Physics A, 2005

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(Λn → nn)/Γ(Λp → np). In this contribution we discuss a new approach to this problem. We have incorporated a one-meson-exchange model for the ΛN → nN 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, Λnp → nnp, 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. *

Physical Review C, 2005

We have measured the energy spectra of pions and protons emitted in the weak decay of 12 Λ C, 28 Λ Si, and Λ Fe hypernuclei produced via the (π + , K +) reaction. The decay widths of the π − mesonic decay (Λ → pπ −) and the nonmesonic decay (ΛN → N N) were extracted. The present results demonstrate an increase of the mesonic decay width due to a distortion of the pion wave function in nuclear medium for the first time. The ratios of the neutron-to proton-induced nonmesonic decay widths, Γ n (Λn → nn)/Γ p (Λp → np), were evaluated by a direct comparison of the measured proton energy spectra with the calculated ones. No theoretical calculation which has been proposed so far can simultaneously account for both the nonmesonic decay widths and the Γ n /Γ p ratios in the present data.

The European Physical Journal A, 2007

The FINUDA experiment studies the Λ-hypernuclei formation and decay using the reaction (K − stop , π − ) induced by the low-momentum K − from the deacy of the φ-meson produced at the (e + , e − ) collider DAφNE at the Laboratory Nazionali di Frascati of INFN. In this paper we present new data on the proton spectra following the weak decay of 12 Λ C.

Physics Reports, 2002

In this review we discuss the present status of strange nuclear physics, with special attention to the weak decay of Λ-hypernuclei. The models proposed for the evaluation of the Λ decay widths are summarized and their results are compared with the data. The rates Γ NM = Γ n +Γ p (+Γ 2), Γ π 0 and Γ π − are well explained by several calculations. Despite the intensive investigations of the last years, the main open problem remains a sound theoretical interpretation of the large experimental values of the ratio Γ n /Γ p. However, the large uncertainties involved in the experimental determination of the ratio do not allow to reach any definitive conclusion. The Γ n /Γ p puzzle is strongly related to the so-called ∆I = 1/2 rule on the isospin change in the non-mesonic decay, whose possible violation cannot be established at present, again due to the insufficient precision of the data. Although recent works offer a step forward in the solution of the puzzle, further efforts (especially on the experimental side) must be invested in order to understand the detailed dynamics of the non-mesonic decay. Even if, by means of single nucleon spectra measurements, the error bars on Γ n /Γ p have been considerably reduced very recently at KEK (however, with central data compatible with older experiments), a clean extraction of Γ n /Γ p is needed. What is missing at present, but planned for the next future, are measurements of 1) nucleon energy spectra in double coincidence and 2) nucleon angular correlations: such observations allow to disentangle the nucleons produced in one-and two-body induced decays and lead to a direct determination of Γ n /Γ p. Notably, the two-body component of the non-mesonic decay rates has not been measured yet, due to the too low counting rates expected for a coincidence experiment. For the asymmetric non-mesonic decay of polarized hypernuclei the situation is even more puzzling. Indeed, strong inconsistencies appear already among data. A recent experiment obtained a positive intrinsic Λ asymmetry parameter, a Λ , for 5 Λ He. This is in complete disagreement with a previous measurement, which obtained a large and negative a Λ for p-shell hypernuclei, and with theory, which predicts a negative value moderately dependent on nuclear structure effects. Also in this case, improved experiments establishing with certainty the sign and magnitude of a Λ for sand p-shell hypernuclei will provide a guidance for a deeper understanding of hypernuclear dynamics and decay mechanisms.

2015

1996

We analyze the recent data on the non-mesonic decays of light Lambda-hypernuclei up to ^12_Lambda C using the phenomenological model of Block and Dalitz. Fitting the spin-isospin dependent Lambda N -> NN reaction rates to six data points, we predict the remaining data in reasonable consistency. We find that despite the short-range nature of the Lambda N -> NN interaction, the non-mesonic decay of p-shell hypernuclei seems to be strongly induced by the p-shell neutrons. Also, the recent data indicate that the Delta I = 1/2 rule, well proved at the hadronic level, may not be sacred in the nuclear medium and the Delta I = 3/2 interactions seem to be needed to describe the non-mesonic decays of Lambda-hypernuclei.

The non--mesonic weak decay of double--$\Lambda$ hypernuclei is studied within a microscopic diagrammatic approach. Besides the nucleon--induced mechanism, $\Lambda N\to nN$, widely studied in single--$\Lambda$ hypernuclei, additional hyperon--induced mechanisms, $\Lambda \Lambda\to \Lambda n$, $\Lambda \Lambda\to \Sigma^0 n$ and $\Lambda \Lambda\to \Sigma^-p$, are accessible in double--$\Lambda$ hypernuclei and are investigated here. As in previous works on single--$\Lambda$ hypernuclei, we adopt a nuclear matter formalism extended to finite nuclei via the local density approximation and a one--meson exchange weak transition potential (including the ground state pseudoscalar and vector octets mesons) supplemented by correlated and uncorrelated two--pion--exchange contributions. The weak decay rates are evaluated for hypernuclei in the region of the experimentally accessible light hypernuclei $^{10}_{\Lambda\Lambda}$Be and $^{13}_{\Lambda\Lambda}$B. Our predictions are compared with...

We review recent developments concerning the weak decay of Λ hypernuclei. New studies covering the mesonic decay channel as well as recent models for the non-mesonic one are discussed and compared with experimental data. The puzzle of the neutron-to proton-induced decay ratio, Γ n /Γ p , is addressed in connection to the two-nucleon induced decay channel and proposals for more efficient experimental analyses of this ratio are made.