  The asymmetry parameter alpha_p^NM for a proton exclusively emitted in the
Lambda p -> np process was, for the first time, measured in the non-mesonic
weak decay of a polarized 5_La,bda_He hypernucleus by selecting the
proton-neutron pairs emitted in the back-to-back kinematics. The highly
polarized 5_Lambda_He was abundantly produced with the (pi+,K+) reaction at
1.05GeV/c in the scattering angular range of +-15$ degrees. The obtained value
alpha_p^NM=0.31+-0.22, as well as that for inclusive protons,
alpha_p^NM=0.11+-0.08+-0.04, largely contradicts recent theoretical values of
around -0.6, although these calculations well reproduce the branching ratios of
non-mesonic weak decay.
