Measurement of Lepton Mass Squared Moments in $B\to X_c\ell {\bar{\nu }}_{\ell }$ Decays with the Belle II Experiment

Abstract

We present measurements of the first to fourth moments of the lepton mass squared $q^2$ of \mbox{$B\to X_c\ell {\bar{\nu }}_{\ell }$} decays for $\ell =e,\mu$ and with $X_c$ a hadronic system containing a charm quark. These results use a sample of electron-positron collisions at the ${\rm Y}(4S)$ resonance corresponding to $62.8{\mathrm{fb}}^{-1}$ of integrated luminosity and collected by the Belle II experiment in 2019 and 2020. To identify the $X_c$ system and reconstruct $q^2$, one of the $B$ mesons from an ${\rm Y}(4S)\to B\overline{B}$ decay is fully reconstructed in a hadronic decay mode using a multivariate $B$ tagging algorithm. We report raw and central moments for $q^2>1.5\mathrm{Ge}{\mathrm{V}}^2/{\mathrm{c}}^4$ up to $q^2>8.5\mathrm{Ge}{\mathrm{V}}^2/{\mathrm{c}}^4$, probing up to 77\% of the accessible \mbox{$B\to X_c\ell {\bar{\nu }}_{\ell }$} phase space. This is the first measurement of moments in the experimentally challenging range of $[1.5,2.5]\mathrm{Ge}{\mathrm{V}}^2/{\mathrm{c}}^4$.

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