Nonmesonic weak decay of double-Λ hypernuclei: A microscopic approach

The nonmesonic weak decay of double-Lambda hypernuclei is studied within a microscopic diagrammatic approach. In addition to the nucleon-induced mechanism, Lambda N -> nN and Lambda NN -> nNN, widely studied in single-Lambda hypernuclei, additional hyperon-induced mechanisms, Lambda Lambda -> Lambda n, Lambda Lambda -> Sigma(0)(n), and Lambda Lambda -> Sigma(-)(p), are accessible in double-Lambda hypernuclei and are investigated here. As in previous works on single-Lambda hypernuclei, we adopt a nuclear matter formalism extended to finite nuclei via the local density approximation and a one-meson exchange weak transition potential (including the ground-state pseudoscalar and vector octets mesons) supplemented by correlated and uncorrelated two-pion-exchange contributions. The weak decay rates are evaluated for hypernuclei in the region of the experimentally accessible light hypernuclei Be-10(Lambda Lambda) and B-13(Lambda Lambda). Our predictions are compared with a few previous evaluations. The rate for the Lambda Lambda -> Lambda n decay is dominated by K-, K*-, and eta-exchange and turns out to be about 2.5% of the free Lambda decay rate, Gamma(free)(Lambda) , while the total rate for the Lambda Lambda -> Sigma(0)(n) and Lambda Lambda -> Sigma(-)(p) decays, dominated by pi-exchange, amounts to about 0.25% of Gamma(free)(Lambda). The experimental measurement of these decays would be essential for the beginning of a systematic study of the nonmesonic decay of strangeness -2 hypernuclei. This field of research could also shed light on the possible existence and nature of the H dibaryon.