We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay of ${}_{\Lambda }{}^5\mathrm{He}$ formed via the ${}^6\mathrm{Li}({\pi }^{+},K^{+})$ reaction. The energies of the two nucleons and the pair number distributions in the opening angle between them were measured. In both $np$ and $nn$ pairs, we observed a clean back-to-back correlation coming from the two-body weak reactions of $\Lambda p\to np$ and $\Lambda n\to nn$, respectively. The ratio of the nucleon pair numbers was $N_{nn}/N_{np}=0.45\pm 0.11(\mathrm{stat})\pm 0.03(\mathrm{syst})$ in the kinematic region of $\cos {\theta }_{NN}<-0.8$. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of $\Gamma (\Lambda p\to np)/\Gamma (\Lambda n\to nn)$. The ratio is consistent with recent theoretical calculations based on the heavy meson and/or direct-quark exchange picture.

• Received 11 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.062301