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Asymmetry parameter for nonmesonic hypernuclear decay

We derive general explicit expressions for a shell-model calculation of the intrinsic asymmetry parameter, $a_Λ$, in nonmesonic weak hypernuclear decay. They include the effects of three-body kinematics in the final states of the decay and correctly treat the contribution of transitions originating from single-proton states beyond the s-shell. The expressions are particularized to the cases of $^5_Λ$He and $^{12}_Λ$C and used for the calculation of $a_Λ$ in several one-meson-exchange models. Besides the strictly local approximation usually adopted for the transition potential, we also consider the addition of the first-order nonlocality terms. We find values for $a_Λ$ ranging from -0.62 to -0.24, in qualitative agreement with other theoretical estimates but in contradiction with some recent experimental determinations.