Measurements of the branching fractions of the semileptonic decays ${\Xi }_c^0\to {\Xi }^{-}{\ell }^{+}{\nu }_{\ell }$ and the asymmetry parameter of ${\Xi }_c^0\to {\Xi }^{-}{\pi }^{+}$

Abstract

Using data samples of 89.5 and 711 fb${}^{-1}$ recorded at energies of $\sqrt{s}=10.52$ and $10.58$ GeV, respectively, with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider, we report measurements of branching fractions of semileptonic decays ${\Xi }_c^0\to {\Xi }^{-}{\ell }^{+}{\nu }_{\ell }$ ($\ell =e$ or $\mu$) and the $CP$-asymmetry parameter of ${\Xi }_c^0\to {\Xi }^{-}{\pi }^{+}$ decay. The branching fractions are measured to be $\mathcal{B}({\Xi }_c^0\to {\Xi }^{-}{e}^{+}{\nu }_e)=(1.31\pm 0.04\pm 0.07\pm 0.38)\%$ and $\mathcal{B}({\Xi }_c^0\to {\Xi }^{-}{\mu }^{+}{\nu }_{\mu })=(1.27\pm 0.06\pm 0.10\pm 0.37)\%$, and the decay parameter ${\alpha }_{\Xi \pi }$ is measured to be $0.63\pm 0.03\pm 0.01$ with much improved precision compared to the current world average. The corresponding ratio $\mathcal{B}({\Xi }_c^0\to {\Xi }^{-}{e}^{+}{\nu }_e)/\mathcal{B}({\Xi }_c^0\to {\Xi }^{-}{\mu }^{+}{\nu }_{\mu })$ is $1.03\pm 0.05\pm 0.07$, which is consistent with the expectation of lepton flavor universality. The first measured asymmetry parameter ${\mathcal{A}}_{CP}=({\alpha }_{{\Xi }^{-}{\pi }^{+}}+{\alpha }_{{\bar{\Xi }}^{+}{\pi }^{-}})/({\alpha }_{{\Xi }^{-}{\pi }^{+}}-{\alpha }_{{\bar{\Xi }}^{+}{\pi }^{-}})=0.024\pm 0.052\pm 0.014$ is found to be consistent with zero. The first and the second uncertainties above are statistical and systematic, respectively, while the third ones arise due to the uncertainty of the ${\Xi }_c^0\to {\Xi }^{-}{\pi }^{+}$ branching fraction.

8 pages, 2 figures, 1 table, accepted for publication as a Letter in Physical Review Letters

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