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

M Agnello,A Feliciello,D Nakajima,A Andronenkov,A Zenoni,P Gianotti,S Marcello,R Wheadon,G Bonomi,N Grion,M Sekimoto,D Calvo,A Fontana,T Nagae,E Botta,A Toyoda,M Bregant,G Beer,P Genova,V Paticchio,E Pace,O Hartmann,L Busso,M Palomba,T Maruta,A Gal,B Dalena,F De Mori,R Rui,G Simonetti,H Outa,O Morra,S Bufalino,T Bressani,N Mirfakhrai,M Bertani,S Piano,V Lucherini,P Montagna,A Panzarasa,A Filippi,L Benussi,P Camerini,A Pantaleo,V Lenti,F Pompili,E M Fiore ,A Sánchez Lorente ,B H Kang ,Hiroyuki Fujioka ,G D’erasmo ,H Bhang ,F Fabbri

doi:10.1016/j.physletb.2009.09.061

Abstract

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 7ΛLi, 9ΛBe, 11ΛB and 15ΛN. 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 Γπ−/Γtot were derived from the measured spectra and converted to π− decay rates Γπ− 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 7ΛLi and 5/2+ for 11ΛB ground-state are confirmed and a spin-parity 3/2+ for 15ΛN ground-state is assigned for the first time.

Highlights

A Λ-hypernucleus in its ground-state decays to non-strange nuclear systems through the mesonic (MWD) and non-mesonic (NMWD) weak decay mechanisms
For a Λhypernucleus, the total decay width Γtot(AΛ Z) is given by the sum of the mesonic decay width (Γm) and the non-mesonic decay width (Γnm), where the first term can be further expressed as the sum of the decay widths for the emission of negative (Γπ− ) and neutral (Γπ0 ) pions: Γtot(AΛ Z) = Γπ− + Γπ0 + Γnm, (3)
In the present work we report on new measurements by the FINUDA experiment of MWD of hypernuclei in the p-shell, comparing the measured π− spectra and decay rates with the calculations by one of the authors [18] that update the calculations by Motoba et al [11,12,13]

Summary

Introduction

A Λ-hypernucleus in its ground-state decays to non-strange nuclear systems through the mesonic (MWD) and non-mesonic (NMWD) weak decay mechanisms. For low-energy pions, the pion-nucleus potential has been studied so far through π-nucleus scattering experiments [16] and measurements of X-rays from pionic atoms [17]; the study of MWD in which a pion is created by the decay of a Λ hyperon deep inside the nucleus offers important opportunities to investigate in-medium pions and to discriminate between different off-shell extrapolations inherent in potential models. For this reason MWD continues to be an interesting item of hypernuclear physics, and precise and systematic determinations of Γπ− and Γπ0 are very welcome.

Experimental and analysis techniques

MWD decay ratios and total decay rates

Conclusions

Findings

Aknowledgements