Measurement of Lepton Mass Squared Moments of B→X_clν_l Decays with the Belle II Experiment and Determination of the CKM Matrix Element |V_cb|

Abstract

This thesis presents measurements of the first to fourth moments of the lepton mass squared q²=(p_l + p_ν)² distribution of B→X_clν decays for l=e,μ and with X_c denoting a hadronic system containing a charm quark. These results use a sample of e⁺e^--collisions at the Υ(4S) resonance corresponding to 62.8 fb^-1 of integrated luminosity and collected by the Belle II experiment in 2019 and 2020. To identify the X_c system and reconstruct q², one of the B mesons from an Υ(4S)→BB decay is fully reconstructed in a hadronic decay mode using a multivariate B tagging algorithm. The yields of the remaining e⁺e^-→qqcontinuum and BB background events are determined with a binned likelihood fit to the hadronic mass m_X distribution. To correct the bias in the mean value of the reconstructed q² distribution, an event-wised calibration method is applied to the reconstructed q² values. The q² moments are calculated as a weighted mean using the calibrated q² values summing all selected events. In this approach, the weights denote a signal probability and effectively subtract the contribution of the remaining background events. This work reports raw and central q² moments measured as functions of lower q² thresholds from q² > 1.5 GeV² up to q² > 8.5 GeV², probing up to 77% of the accessible B→X_clν phase space. This is the first measurement of moments in the experimentally challenging range of q² ∈ [1.5, 2.5] GeV. In addition, this thesis presents a first determination of |V_cb| using q² moments of inclusive B→X_clν decays. Like the prediction of total rate, the expressions for spectral moments of inclusive semileptonic PB decays can be calculated using the heavy quark expansion and depend on a set of non-perturbative parameters which can only be determined from measurements. At higher orders in the expansion, one observes a proliferation of these parameters which further complicates their extraction. The q² moments and the total rate are reparameterization invariant quantities and depend on a reduced set of non-perturbative parameters. This reduced set of parameters opens a new path to extract these parameters up to O(1/m_b⁴) in the HQE purely from data and thereby reducing the uncertainty on |V_cb|. A combined fit to the q² moments measured in this work and by the Belle collaboration is used to extract |V_cb|. The fit results in |V_cb| = (41.69±0.63)·10^-3. This result presents an important independent cross-check of, and is consistent with, the previous state-of-the-art inclusive |V_cb| determinations based on lepton energy and hadronic mass moments.