Mesonic and Non-Mesonic Weak Decay of Hypernuclei with FINUDA

Proceedings of The IX International Conference on …, 2007

The physics of the weak decay of hypernuclei is briefly reviewed from a theoretical point of view. Special regard is devoted to the recent progress concerning the determination of the non-mesonic decay widths and the asymmetry parameters. While convincing evidence has been achieved for a solution of the long-standing puzzle on the ratio Γn/Γp, the discrepancies between theory and experiment on the decay asymmetries clearly highlight the exigence of dedicating further efforts in exploring new aspects of the dynamics underlying the non-mesonic weak decay.

2015

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...

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.

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.

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.

Physics Letters B, 2009

The FINUDA experiment performed a systematic study of the charged mesonic weak decay channel of p-shell Λ-hypernuclei. Negatively charged pion spectra from mesonic decay were measured with magnetic analysis for the first time for 7 Λ Li, 9 Λ Be, 11 Λ B and 15 Λ N. The shape of the π − spectra was interpreted through a comparison with pion distorted wave calculations that take into account the structure of both hypernucleus and daughter nucleus. Branching ratios Γ π − /Γtot were derived from the measured spectra and converted to π − decay rates Γ π − by means of known or extrapolated total decay widths Γtot of p-shell Λ-hypernuclei. Based on these measurements, the spin-parity assignment 1/2 + for 7 Λ Li and 5/2 + for 11 Λ B ground-state are confirmed and a spin-parity 3/2 + for 15 Λ N ground-state is assigned for the first time.

Nuclear Physics A, 2010

A theoretical review is presented of hypernuclear weak decay, with special emphasis on the nonmesonic modes. We focus on recent progress which lead to the solution of long-standing puzzles on the Γ n /Γ p ratio and on the asymmetry parameter. Perspectives for the future are also indicated.

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.

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.

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. *

Resum v vii per a la desintegració hipernuclear no mesònicaés notablement curta, havent-hi només uns pocs observables de desintegració d'hipernuclis lleugers mesurats amb bona precisió. Aquesta escassetat de dades ens ha portat a considerar només la contribució a primer ordre en l'amplitud de la desintegració ⇤N ! NN, que com hem dit conté només dos paràmetres independents, els quals sóm capaços de determinar mitjançant un procés de minimització. Hem dut a terme un ajust als ritmes de desintegració totals, als parcials, ⇤p ! np o ⇤n ! nn, i a l'asimetria dels protons emergents, la qual es pot relacionar amb la interferència entre las parts de violació i conservació de paritat de l'amplitud feble. Això ens ha permès extreure els valors de les dues constants de baixa energia que apareixen al primer ordre de la teoria. Malauradament, les constants de baixa energia trobades no són completament independents del model usat en la descripció de la interacció forta. Per tal d'obtenir un coneixement més complet del problema hem comparat les interaccions de contacte que van acompanyades de les constants de baixa energia amb els potencials corresponents a l'intercanvi d'un mesó pesat. Això s'ha fet de la següent manera, primer expandim els potencials d'intercanvi d'un mesó en potències de q, i després igualem la teoria efectiva amb el model d'intercanvi d'un mesó, ordre a ordre. D'aquesta manera som capaços d'estudiar la possible contribució d'un mesó escalar-isoescalar en models d'intercanvi mesònic, tal i com s'havia suggerit en treballs previs. Un cop finalitzat aquest càlcul ens hem centrat en la desintegració feble de l'hipertritó. Aquestés un càlcul consistent, on tots els ingredients, siguin forts o febles, deriven d'una teoria efectiva. En aquest cas, hem presentat el resultat de la desintegració de l'hipertritó cap a estats finals de tres nucleons o d'un deuteró i un neutró. Finalment, mostrem, com a exemple, una comparació teòrica entre les contribucions dels diagrames d'intercanvi de dos pions, que entren a segon ordre en la teoria, i les corresponents a diagrames d'intercanvi d'un mesó que entren a ordre zero. S'ha trobat que la contribució de l'intercanvi de dos pionsés comparable en tamany a les contribucions d'intercanvi d'un pió i d'un kaó, i com a conseqüència s'observen importants interferències tant constructives com destructives entre totes dues contribucions. Al final de la tesi presentem les possibles extensions i aplicacions del formalisme desenvolupat dins del camp de la física hipernuclear.