Measurements of <em>q</em>² moments of inclusive <em>B</em> → <em>X_c</em>ℓν decays toward an alternative determination of |<em>V_cb</em>|

Abstract

This thesis presents the first measurement of the first to the fourth moments of the four-momentum transfer squared <em>q</em>² spectrum for inclusive <em>B</em> → <em>X_c</em>ℓν decays, where ℓ = <em>e</em>, μ. The determination of these moments and their systematic uncertainties are crucial experimental inputs for a novel, alternative method to determine the magnitude of the CKM matrix element <em>V_cb</em> using a reduced set of hadronic matrix elements in the heavy quark expansion. The analysis investigates the complete ϒ<em>(4S)</em> data set collected by the Belle detector, corresponding to 711 fb^-1 of integrated luminosity and a total of (772 ± 10) × 10⁶ <em>B</em> meson pairs. To identify and reconstruct the kinematic variables of inclusive semileptonic <em>B</em> meson decays, machine learning techniques are employed to fully reconstruct one of the <em>B</em> mesons in purely hadronic decay modes. Consequently, this approach allows for the kinematics of the remaining <em>B</em> meson to be inferred using conservation laws leading to the explicit reconstruction of the hadronic <em>X_c</em> system and, in turn, the <em>q</em>² spectrum. The moments are measured with progressively increasing threshold selections on <em>q</em>²: from <em>q</em>² > 3.0 GeV² to <em>q</em>² > 10.0 GeV² while incrementing the threshold in 0.5 GeV² steps. To ensure an unbiased result, the measured moments are unfolded and corrected for reconstruction and selection effects. The measured moments are reported separately for electron and muon final states, allowing for a test of lepton flavour universality. No deviation from the expectation of unity is observed. In addition, the presented measurement is combined with theoretical predictions of the ⟨<em>qⁿ</em>⟩ moments and the semileptonic decay rate up to order 1/<em>m_b<sup style='position: relative; left: -.5em;'>4</sup></em> to perform a simultaneous fit and extract |<em>V_cb</em>| in a data-driven manner. A value of |<em>V_cb</em>| × 10³ = 41.7 ± 1.2 is obtained, which is consistent with previous determinations within experimental uncertainty.