Welsch, Maximilian: Measurement of Lepton Mass Squared Moments of B→Xcl Decays with the Belle II Experiment and Determination of the CKM Matrix Element |Vcb|. - Bonn, 2023. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-70107
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-70107,
author = {{Maximilian Welsch}},
title = {Measurement of Lepton Mass Squared Moments of B→Xcl Decays with the Belle II Experiment and Determination of the CKM Matrix Element |Vcb|},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2023,
month = mar,

note = {This thesis presents measurements of the first to fourth moments of the lepton mass squared q2=(pl + pν)2 distribution of B→Xclν decays for l=e,μ and with Xc 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 Xc 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 mX 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→Xclν 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 |Vcb| using q2 moments of inclusive B→Xclν 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/mb4) in the HQE purely from data and thereby reducing the uncertainty on |Vcb|. A combined fit to the q2 moments measured in this work and by the Belle collaboration is used to extract |Vcb|. The fit results in |Vcb| = (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 |Vcb| determinations based on lepton energy and hadronic mass moments.},
url = {https://hdl.handle.net/20.500.11811/10670}

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