Measurements of the branching fractions of the semileptonic decays $\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}{\ell }^{+}\hat{I}{½}_{\ell }$ and the asymmetry parameter of $\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}\ddot{I}{€}^{+}$

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 $\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}{\ell }^{+}\hat{I}{½}_{\ell }$ ($\ell =e$ or $\hat{I}¼$) and the $CP$-asymmetry parameter of $\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}\ddot{I}{€}^{+}$ decay. The branching fractions are measured to be $\mathcal{B}(\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}{e}^{+}\hat{I}{½}_e)=(1.31\pm 0.04\pm 0.07\pm 0.38)\%$ and $\mathcal{B}(\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}\hat{I}{¼}^{+}\hat{I}{½}_{\hat{I}}¼)=(1.27\pm 0.06\pm 0.10\pm 0.37)\%$, and the decay parameter $\hat{I}{\pm }_{\hat{I}ž\ddot{I}€}$ 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 $\mathcal{B}(\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}{e}^{+}\hat{I}{½}_e)/\mathcal{B}(\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}\hat{I}{¼}^{+}\hat{I}{½}_{\hat{I}}¼)$ is $1.03\pm 0.05\pm 0.07$, which is consistent with the expectation of lepton flavor universality. The first measured asymmetry parameter ${\mathcal{A}}_{CP}=(\hat{I}{\pm }_{\hat{I}{ž}^{-}\ddot{I}{€}^{+}}+\hat{I}{\pm }_{\bar{\hat{I}}{ž}^{+}\ddot{I}{€}^{-}})/(\hat{I}{\pm }_{\hat{I}{ž}^{-}\ddot{I}{€}^{+}}-\hat{I}{\pm }_{\bar{\hat{I}}{ž}^{+}\ddot{I}{€}^{-}})=0.024\pm 0.052\pm 0.014$ 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 $\hat{I}{ž}_c^0\to \hat{I}{ž}^{-}\ddot{I}{€}^{+}$ branching fraction.

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