decay rates of p-shell hypernuclei revisited

Abstract

Explicit expressions for the parity-violating s-wave and the parity-conserving p-wave contributions to π − weak-decay rates of Λ hypernuclei in the 1p shell are given in the weak-coupling limit, to update previous shell-model calculations and to compare with recent π − spectra and total decay rates measured by the FINUDA Collaboration for 7 Λ Li, 9 Λ Be, 11 Λ B and 15 Λ N. A useful sum rule for the summed strength of Λ1s → p1p hypernuclear π − weak decays is derived. Fair agreement between experiment and calculations is reached, using the primary s-wave amplitude and Cohen-Kurath nuclear wavefunctions. The role of the p-wave amplitude is studied in detail for 15 Λ N and found to be secondary. Previous assignments of ground-state spin-parity values J π ( 7 Λ Lig.s.) = 1 2 + and J π ( 11 Λ Bg.s.) = 5 2 + are confirmed, and a new assignment J π ( 15 Λ Ng.s.) = 3 2 + is made, based on the substantial suppression calculated here for the 15 Λ N( 1 2 + ) → π − 15 Og.s. weak decay rate. Key words: mesonic weak decays of hypernuclei, ground-state spins of hypernuclei PACS: 13.30.Eg, 21.80.+a Email address: avragal@vms.huji.ac.il (Avraham Gal).

Shell-model calculations for p-shell hypernuclei

“…T plays a particularly important role in the ground-state doublet (p (6), S N fits the increase in the excitation energy of the excited-state doublet over the spacing of the p-hole states in 15 O.…”

Hypernuclear weak decay studies with FINUDA

“…[20,21] (full (green) squares) and Ref. [22] [7], compared with previous measurements (full (black) triangles) [16][17][18][19]14] and with theoretical calculations [20,21] (full (green) squares) and [22] (full (blue) stars). See Ref.…”

“…[22] these calculations were revisited, following the same approach, and a new sum rule was introduced to encapsulate the suppressive effect of the Pauli principle on the total and partial π − decay rates. In Fig.…”

“…total decay rates for the other set of ground state spin-parity calculated in Refs. [20][21][22] 15 Λ N, for which the calculated values reported in Ref. [21] do not allow to fix the ground state spin-parity.…”

“…[21,20,22], allowing to determine the spin-parity configuration of the hypernucleus ground state. In Fig.…”