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Institute of High Energy Physics, Beijing 100049, People's Republic of China.
G. I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk 630090, Russia.
Phys Rev Lett. 2021 Sep 17;127(12):121802. doi: 10.1103/PhysRevLett.127.121802.
The absolute branching fraction of Λ→pμ^{-}ν[over ¯]{μ} is reported for the first time based on an e^{+}e^{-} annihilation sample of 10×10^{9} J/ψ events collected with the BESIII detector at sqrt[s]=3.097 GeV. The branching fraction is determined to be B(Λ→pμ^{-}ν[over ¯]{μ})=[1.48±0.21(stat)±0.08(syst)]×10^{-4}, which is improved by about 30% in precision over the previous indirect measurements. Combining this result with the world average of B(Λ→pe^{-}ν[over ¯]{e}), we obtain the ratio {[Γ(Λ→pμ^{-}ν[over ¯]{μ})]/[Γ(Λ→pe^{-}ν[over ¯]{e})]} to be 0.178±0.028, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of Λ→pμ^{-}ν[over ¯]{μ} and Λ[over ¯]→p[over ¯]μ^{+}ν_{μ} is also determined, and no evidence for CP violation is found.
基于在北京谱仪III（BESIII）探测器上于质心能量(\sqrt{s}=3.097) GeV收集的(10×10^{9})个(J/\psi)事例的(e^{+}e^{-})湮灭样本，首次报道了(\Lambda→p\mu^{-}\overline{\nu}{\mu})的绝对分支比。测得分支比为(B(\Lambda→p\mu^{-}\overline{\nu}{\mu}) = [1.48±0.21(统计)±0.08(系统)]×10^{-4})，与之前的间接测量相比，精度提高了约30%。将此结果与(B(\Lambda→pe^{-}\overline{\nu}{e}))的世界平均值相结合，我们得到(\frac{\Gamma(\Lambda→p\mu^{-}\overline{\nu}{\mu})}{\Gamma(\Lambda→pe^{-}\overline{\nu}{e})})的比值为(0.178±0.028)，这与假设轻子味普适性的标准模型预测相符。还确定了(\Lambda→p\mu^{-}\overline{\nu}{\mu})和(\overline{\Lambda}→\overline{p}\mu^{+}\nu_{\mu})分支比的不对称性，未发现CP破坏的证据。
