Nonmesonic decays and lifetimes of hypernuclei

The nonmesonic decay rates and the lifetimes of hypernuclei of mass A=4-209 are extensively calculated based on the 1π, correlated-2π, and 1ω exchange potentials. Two types of new hyperon-nucleon potentials have been constructed in which the two pions are correlated and coupled to {ρ} and/or {σ} in the exchange process. The roles of these potentials and 1ω exchange potential are discussed. The theoretical decay rates are consistent with the existing data for s-shell and p-shell hypernuclei within the present model. The calculated decay rate increases gradually up to A60 and then tends to be almost constant for A60. The A-dependent behavior of the hypernuclear lifetimes tends to be constant over the mass region $A>30$ of hypernuclei in our model. It is most remarkable that the cooperative effect of the correlated-2π and 1ω exchanges enhances the neutron-stimulated decay rate Γₙ (proton-stimulated one Γₚ) by 400--450 % (20--30 %) with respect to the standard 1π-exchange estimate for $A>5.$ As a result the Γₙ/Γₚ ratios for light-to-heavy hypernuclei are calculated to be 0.4--0.5, which values are several times larger than the 1π-exchange estimate. Although the experimental ratios seem still about two times larger than these theoretical values, it has revealed that the representative 2π-exchange and 1ω exchange mechanisms give rise to a clear improvement on the Γₙ/Γₚ ratios.