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

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 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.

Neutron-rich hypernuclei: evidence for6ΛH and search for9ΛHe

“…If for BR(π − ) the value of 0.323±0.062 is assumed, which is the experimental one for the mesonic weak decay of 5 Λ He [18], an upper limit is obtained for R of (2.3 + 1.9)/0.323 · 10 −6 /K − stop = 1.3 · 10 −5 /K − stop (at 90% C.L. ), improving by over an order of magnitude the previous upper limit set in KEK experiment [8].…”

Experimental Studies on the Weak Decay of Λ-Hypernuclei

“…Sometimes the outcome of the observation of the hypernuclear weak decay permitted to achieve results to some extent beyond the initial scope, thus opening important discovery opportunities. It was the case of the first assignment of the spin-parity for the ground state of the hypernucleus 15 Λ N [12], where in the end the analysis of the kinetic energy spectrum of the charged pion emitted in its mesonic decay process turned out to be an indirect spectroscopic investigation tool. Another example is the discovery of the extremely neutron-rich hypernucleus 6 Λ H [13]: the detection in coincidence of the two charged pions from the strangeness-exchange production reaction and, respectively, from its subsequent mesonic decay process played a fundamental role in developing a robust analysis strategy, which led to such an important achievement.…”

“…More precisely, α/(2 + α) is the fraction of protons undergoing one, or more, FSI. In order to calculate α for all the Λ-hypernuclei under study, published Γ p /Γ Λ values for 5 Λ He and 12 Λ C were used; moreover a linear scaling law with A was assumed for the FSI contribution and, consequently, for α. The ratio Γ TOT /Γ Λ was instead evaluated by substituting in the Γ TOT expression (6) the experimental values existing in literature.…”

“…(7) when available (first column); in the case of multiple measurements, the weighted average was evaluated, while for the remaining Λ-hypernuclei, for which no determination exists, the parametrization Γ TOT /Γ Λ (A) = (0.990 ± 0.094) + (0.018 ± 0.010) · A, proposed in Ref. [12], was used. The obtained results are displayed as a function of the mass number A in Fig.…”

High precision mass measurements of hypernuclei - achievements and perspectives

“…More recently decay pion spectroscopy has been successfully performed at KEK [33] and by the FINUDA Collaboration at the DAΦNE e + e − collider in electronic experiments. Nevertheless, the momentum resolution and hence the absolute mass determination was limited in these mesurements [34].…”