Nonmesonic decay of the Λ hyperon in heavy hypernuclei

The nonmesonic decay of Λ hyperons has been investigated by the observation of delayed fission of heavy hypernuclei produced in proton–U collisions at T_p &equal; 1.9 GeV. The lifetime of heavy hypernuclei with masses A ≈ 220 of the present work, τ_Λ &equal; (138 ± 6 (stat) ± 17 (syst)) ps, is the most accurate result obtained in proton and antiproton induced collisions on a U target so far. This result together with the values of the lifetime measured previously by the COSY-13 collaboration, for p+Bi (161 ± 7(stat) ± 14(syst)) ps and p+Au (130 ± 13(stat) ± 15(syst)) ps, gives an average value for the lifetime of heavy hypernuclei with masses A > 180: τ_Λ &equal; (145 ± 11) ps. This value was obtained by averaging the above lifetimes with weights taken as reciprocals of the sum of squares of statistical and systematic errors. By comparing the lifetimes of light and heavy hypernuclei we found—on the confidence level of 0.98—a violation of the phenomenological ΔI &equal; 1/2 rule known from the weak mesonic decays of strange particles.