Home > Books, Theses & Reports > Theses > Measurement of Lepton Mass Squared Moments of B→X<sub>c</sub>lν Decays with the Belle II Experiment and Determination of the CKM Matrix Element |V<sub>cb</sub>| |

Thesis | BELLE2-PTHESIS-2023-004 |

Maximilian Welsch

2022

Physikalisches Institut
Bonn

**Abstract: **This thesis presents measurements of the first to fourth moments of the lepton mass squared q<sup>2</sup>=(p<sub>l</sub> + p<sub>ν</sub>)<sup>2</sup> distribution of B→X<sub>c</sub>lν decays for l=e,μ and with X<sub>c</sub> denoting a hadronic system containing a charm quark. These results use a sample of e<sup>+</sup>e<sup>-</sup>-collisions at the Υ(4S) resonance corresponding to 62.8 fb<sup>-1</sup> of integrated luminosity and collected by the Belle II experiment in 2019 and 2020. To identify the X<sub>c</sub> system and reconstruct q<sup>2</sup>, 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<sup>+</sup>e<sup>-</sup>→qqcontinuum and BB background events are determined with a binned likelihood fit to the hadronic mass m<sub>X</sub> distribution. To correct the bias in the mean value of the reconstructed q<sup>2</sup> distribution, an event-wised calibration method is applied to the reconstructed q<sup>2</sup> values. The q<sup>2</sup> moments are calculated as a weighted mean using the calibrated q<sup>2</sup> 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 q<sup>2</sup> moments measured as functions of lower q<sup>2</sup> thresholds from q<sup>2</sup> > 1.5 GeV<sup>2</sup> up to q<sup>2</sup> > 8.5 GeV<sup>2</sup>, probing up to 77% of the accessible B→X<sub>c</sub>lν phase space. This is the first measurement of moments in the experimentally challenging range of q<sup>2</sup> ∈ [1.5, 2.5] GeV. In addition, this thesis presents a first determination of |V<sub>cb</sub>| using q<sup>2</sup> moments of inclusive B→X<sub>c</sub>lν 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 q<sup>2</sup> 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<sub>b</sub><sup>4</sup>) in the HQE purely from data and thereby reducing the uncertainty on |V<sub>cb</sub>|. A combined fit to the q<sup>2</sup> moments measured in this work and by the Belle collaboration is used to extract |V<sub>cb</sub>|. The fit results in |V<sub>cb</sub>| = (41.69±0.63)·10<sup>-3</sup>. This result presents an important independent cross-check of, and is consistent with, the previous state-of-the-art inclusive |V<sub>cb</sub>| determinations based on lepton energy and hadronic mass moments.

**Note: **Presented on 23 12 2022 **Note: **PhD

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Record created 2023-03-12, last modified 2023-03-12