Weak decays of 4He-Lambda

TL;DR

This study measured the lifetime and decay rates of the 4He-Lambda hypernucleus, providing detailed insights into its decay mechanisms and testing theoretical models of hypernuclear decay processes.

Contribution

First direct measurement of 4He-Lambda lifetime and decay rates, including mesonic and non-mesonic channels, with analysis of decay mechanisms and implications for hypernuclear physics.

Findings

Lifetime of 245 +/- 24 ps for 4He-Lambda

Mesonic decay rates: Gamma_pi-/Gamma_tot = 0.270 +/- 0.024, Gamma_pi0/Gamma_tot = 0.564 +/- 0.036

Proton-stimulated decay rate: Gamma_p/Gamma_tot = 0.169 +/- 0.019

Abstract

We measured the lifetime and the mesonic and non-mesonic decay rates of the 4He-Lambda hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K- beam on a liquid 4He target by the reaction 4He(K-,pi-)4He-Lambda. The 4He-Lambda lifetime was directly measured using protons from Lambda p -> n p non-mesonic decay (also referred to as proton-stimulated decay) and was found to have a value of tau = 245 +/- 24 ps. The mesonic decay rates were determined from the observed numbers of pi-'s and pi0's as Gamma_pi-/Gamma_tot = 0.270 +/- 0.024 and Gamma_pi0/Gamma_tot = 0.564 +/- 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Gamma_p/Gamma_tot = 0.169 +/- 0.019 and Gamma_n/Gamma_tot <= 0.032 (95% CL), respectively. The effects of final-state interactions and possible 3-body Lambda N N decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the non-mesonic branching fractions. The implications of the results of this analysis were considered for the empirical Delta I = 1/2 rule and the decay rates of the 4H-Lambda hypernucleus.