First test of lepton flavor universality in the charmed baryon decays ${\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\ell }^{+}{\nu }_{\ell }$ using data of the Belle experiment

We present the first observation of the ${\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\mu }^{+}{\nu }_{\mu }$ decay and present measurements of the branching fraction ratios of the ${\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\ell }^{+}{\nu }_{\ell }$ decays compared to the reference mode ${\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\pi }^{+}$, ($\ell =e$ or $\mu$). This analysis is based on $89.5{\mathrm{fb}}^{-1}$, $711{\mathrm{fb}}^{-1}$, and $121.1{\mathrm{fb}}^{-1}$ data samples collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider at the center-of-mass energies of 10.52 GeV, 10.58 GeV, and 10.86 GeV, respectively. The ${\mathrm{\Omega }}_c^0$ signal yields are extracted by fitting $M_{\mathrm{\Omega }\ell }$ and $M_{\mathrm{\Omega }\pi }$ spectra. The branching fraction ratios $\mathcal{B}({\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{e}^{+}{\nu }_e)/\mathcal{B}({\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\pi }^{+})$ and $\mathcal{B}({\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\mu }^{+}{\nu }_{\mu })/\mathcal{B}({\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\pi }^{+})$ are measured to be $1.98\pm 0.13(\text{stat})\pm 0.08(\text{syst})$ and $1.94\pm 0.18(\text{stat})\pm 0.10(\text{syst})$, respectively. The ratio of $\mathcal{B}({\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{e}^{+}{\nu }_e)/\mathcal{B}({\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\mu }^{+}{\nu }_{\mu })$ is measured to be $1.02\pm 0.10(\text{stat})\pm 0.02(\text{syst})$, which is consistent with the expectation of lepton flavor universality.

Figure 1

The fits to the (a) $M_{\mathrm{\Omega }\pi }$, (b) $M_{\mathrm{\Omega }e}$, and (c) $M_{\mathrm{\Omega }\mu }$ distributions for the selected candidates from data. The dots with error bars represent the data, the solid lines are the best fits, and the dashed lines are the fitted total backgrounds. The blank areas between the red dashed lines and shaded histograms are from backgrounds with mischosen ${\ell }^{+}$. The “$\mu -\pi$ misID” in plot (c) means the background component from ${\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\pi }^{+}+\text{hadrons}$ decays. The other fit components are illustrated by the legends.