Exclusive Measurement of the Nonmesonic Weak Decay of ^{5}_ΛHe Hypernucleus

Abstract

We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of ^{5}_ΛHe formed via the ^{6}Li(π^+,K^+) reaction. The energies of 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 decay of Λp --> n p and Λn --> n n, respectively. The ratio of the nucleon pair numbers was N_{nn}/N_{np}=0.45 \pm 0.11(stat)\pm 0.03(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 Γ(Λp --> np)/Γ(Λn --> nn). The ratio is consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture.

Submitted to Phys. Rev. lett., 4 pages, 3 figures

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• Weak Decay of $\Lambda$ in Nuclei : Quarks vs Mesons
• Neutron and proton energy spectra from the non-mesonic weak decays of 5_Lambda-He and 12_Lambda-C
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• pi^0 decay branching ratios of 5_Lambda-He and 12_Lambda-C hypernuclei

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