New results on mesonic weak decay of p-shell Λ-hypernuclei

2010

New spectra from the FINUDA experiment of the Non-Mesonic Weak Decay (NMWD) proton kinetic energy for 9 Λ Be, 11 Λ B, 12 Λ C, 13 Λ C, 15 Λ N and 16 Λ O are presented and discussed along with the published data on 5 Λ He and 7 Λ Li. Exploiting the large mass number range and the low energy threshold (15 MeV) for the proton detection of FINUDA, an evaluation of both Final State Interactions (FSI) and the two-nucleon induced NMWD contributions to the decay process has been done. Based on this evaluation, a linear dependence of FSI on the hypernuclear mass number A is found and for the two-nucleon stimulated decay rate the experimental value of Γ2/Γp=0.43±0.25 is determined for the first time. A value for the two-nucleon stimulated decay rate to the total decay rate Γ2/ΓNMWD=0.24±0.10 is also extracted.

Nuclear Physics A, 2010

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.

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

Physics Letters B, 2011

Nuclear Physics A, 2012

The FINUDA experiment has recently completed an extensive study of the weak decay of p-shell Λhypernuclei 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 ΛN N → nN N decay channel: its weight has been evaluated to be Γ 2 /Γ NM = 0.21 ± 0.07 stat +0.03 sys −0.02 sys. Finally, a direct experimental evidence of the occurrence of the weak reaction Λnp → nnp in nuclei is presented for the first time. Three events have been found which can be attributed to 7 Λ Li and 9 Λ Be two-nucleon induced non-mesonic weak decay; the kinematical analysis is discussed here.

The decay of $\Lambda$-hypernuclei without pion emission, known as Non Mesonic Weak Decay (NMWD), gives an effective tool to investigate $\Delta$S=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 $\Gamma_{2N}$/$\Gamma_{NMWD}$=0.24$\pm$${0.03_{stat}}^{+0.03_{sys}}_{{-{0.02_{sys}}}}$ has been deduced, with an error reduced by a factor more than two compared with the previous assessment.

Physics Letters B, 2007

The non-mesonic weak decay of Λ-hypernuclei is studied within a one-mesonexchange 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. In view of the severe discrepancies between theoretical expectations and experimental data, 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 uncorrelated and correlated two-pion mechanisms change the rates moderately, thus maintaining the agreement with experiment. The modification in the strength and sign of some decay amplitudes becomes crucial and produces asymmetry parameters which lie well within the experimental observations.

Physical Review Letters, 2005

The non-mesonic weak decay of polarized Λ-hypernuclei is studied for the first time by taking into account, with a Monte Carlo intranuclear cascade code, the nucleon final state interactions. A one-meson-exchange model is employed to describe the ΛN → nN processes in a finite nucleus framework. The relationship between the intrinsic Λ asymmetry parameter aΛ and the asymmetry a M Λ accessible in experiments is discussed. A strong dependence of a M Λ on nucleon final state interactions and detection threshold is obtained. Our results for a M Λ are consistent with 11 Λ B and 12 Λ C data but disagree with observations in 5 Λ He.

EPJ Web of Conferences, 2016

Λ + c hypernuclei are expected to have binding energies and other properties similar to those of strange hypernuclei in view of the similarity between the quark structures of the strange and charmed hyperons, namely Λ(uds) and Λ + c (udc). One striking difference however occurs in their mesonic decays, as there is almost no Pauli blocking in the nucleonic decay of a charm hypernucleus because the final-state nucleons leave the nucleus at high energies. The nuclear medium nevertheless affects the mesonic decays of charm hypernucleus because the nuclear mean fields modify the masses of the charm hyperon. In the present communication we present results of a first investigation of the effects of finite baryon density on different weak mesonic decay channels of the Λ + c baryon. We found a non-negligible reduction of the decay widths as compared to their vacuum values.