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 Pauli Principle, and a very simple expression is derived for the neutron and proton induced decays rates, $\Gamma_n$ and $\Gamma_p$, which does not involve the spectroscopic factors. We use the standard strangeness-changing weak $\Lambda N\to NN$ transition potential which comprises the exchange of the complete pseudoscalar and vector meson octets ($\pi,\eta,K,\rho,\omega,K^*$), 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 ${^{12}_\Lambda C}$ is carefully analyzed. With the commonly used parametrization in the One-Meson-Exchange Model (OMEM), the calculated rate $\Gamma_{NM}= \Gamma_n+\Gamma_p$ is of the order of the free $\Lambda$ decay rate $\Gamma^0(\Gamma_{NM}^{\rm th}\cong \Gamma^0)$ and is consistent with experiments. Yet, the measurements of $\Gamma_{n/p}=\Gamma_n/\Gamma_p$ and of $\Gamma_p$ are not well accounted for by the theory ($\Gamma_{n/p}^{\rm th}\lsim 0.42; \Gamma_p^{\rm th}\gsim 0.60 \Gamma^0$). It is suggested that, unless additional degrees of freedom are incorporated, the OMEM parameters should be radically modified.