Asymmetric weak decay of polarized hypernuclei

We measured the asymmetric weak decay of the polarized He-5(Lambda) hypernu clei. The polarization was derived by the observed asymmetry of the mesonic decay pions for the first time. The asymmetry parameter of the nonmesonic decay has been evaluated using the obtained polarization and the asymmetry of the decay protons. The positive asymmetry parameter contradicts theoreti cal prediction based on meson exchange model. The present data indicates th e limitation of the model in Lambda N weak interaction, in particular short range phenomena such the nonmesonic decay of hypernuclei.