Measurements of $q^2$ Moments of Inclusive $B \rightarrow X_c \ell^+ \nu_{\ell}$ Decays with Hadronic Tagging

TL;DR

This paper measures the first to fourth order moments of the squared momentum transfer in inclusive B decays, using Belle data and machine learning, to improve determination of the CKM matrix element V_{cb}.

Contribution

It introduces a novel measurement of q^2 moments with multiple thresholds and uses machine learning for B-meson reconstruction, advancing methods for CKM matrix element extraction.

Findings

Measured q^2 moments with thresholds from 3.0 to 10.0 GeV^2.

No observed lepton flavor universality violation.

Unfolded moments correcting for detector effects.

Abstract

We present the measurement of the first to fourth order moments of the four-momentum transfer squared, $q^2$, of inclusive $B \rightarrow X_c \ell^+ \nu_{\ell}$ decays using the full Belle data set of 711 $\mathrm{fb}^{-1}$ of integrated luminosity at the $\Upsilon(4S)$ resonance where $\ell = e, \mu$. The determination of these moments and their systematic uncertainties open new pathways to determine the absolute value of the CKM matrix element $V_{cb}$ using a reduced set of matrix elements of the heavy quark expansion. In order to identify and reconstruct the $X_c$ system, we reconstruct one of the two $B$-mesons using machine learning techniques in fully hadronic decay modes. The moments are measured with progressively increasing threshold selections on $q^2$ starting with a lower value of 3.0 $\mathrm{GeV}^2$ in steps of 0.5 $\mathrm{GeV}^2$ up to a value of 10.0 $\mathrm{GeV}^2$. The measured moments are further unfolded, correcting for reconstruction and selection effects as well as QED final state radiation. We report the moments separately for electron and muon final states and observe no lepton flavor universality violating effects.