Thesis BELLE2-PTHESIS-2023-004

Measurement of Lepton Mass Squared Moments of B&#8594;X<sub>c</sub>l&#957; Decays with the Belle II Experiment and Determination of the CKM Matrix Element |V<sub>cb</sub>|

Maximilian Welsch

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>&#957;</sub>)<sup>2</sup> distribution of B&#8594;X<sub>c</sub>l&#957; decays for l=e,&#956; 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 &#933;(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 &#933;(4S)&#8594;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>&#8594;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&#8594;X<sub>c</sub>l&#957; phase space. This is the first measurement of moments in the experimentally challenging range of q<sup>2</sup> &#8712; [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&#8594;X<sub>c</sub>l&#957; 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&#177;0.63)&#183;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

The record appears in these collections:
Books, Theses & Reports > Theses > PhD Theses

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

Download fulltext

Rate this document:

Rate this document:
(Not yet reviewed)