engleski

Hypernuclear weak decay puzzle

A general shell model formalism for the nonmesonic weak decay of the hypernuclei has been developed. It involves a partial wave expansion of the emitted nucleon waves, preserves naturally the antisymmetrization between the escaping particles and the residual core, and contains as a particular case the weak lambda-core coupling formalism. The extreme particle-hole model and the quasiparticle Tamm-Dancoff approximation are explicitly worked out. It is shown that the nuclear structure manifests itself basically through the Pauli principle, and a very simple expression is derived for the neutron- and proton-induced decays rates, which does not involve the spectroscopic factors. We use the standard strangeness-changing weak lambdaN -> NN transition potential which comprises the exchange of the complete pseudoscalar and vector meson octets, taking into account some important parity-violating transition operators that are systematically omitted in the literature. The interplay between different mesons in the decay of C(12, lambda) is carefully analyzed. With the commonly used parametrization in the one meson -exchange model(OMEM), the calculated rate gama(NM)= gama(n)+ gama(p) is of the order of the free lambda decay rate and is consistent with experiments. Yet the measurments of gama(n)/gama(p) and of gama(p) are not well accounted for by the theory. It is suggested that, unless additional degrees of freedom are incorporated, the OMEM parameters should be radically modified.

Hypernuclear; Nonmesonic weak decay

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