Measurement of the Branching Ratio $B \to D^{**} \ell \nu_{\ell}$ using the hadronic Full Event Interpretation with Belle II Data

by Noreen Rauls
Doctoral thesis
Date of Examination:2024-05-27
Date of issue:2024-07-17
Advisor:Prof. Dr. Ariane Frey
Referee:Prof. Dr. Ariane Frey
Referee:Prof. Dr. Thibaud Humair
Persistent Address: http://dx.doi.org/10.53846/goediss-10602

 

 

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Abstract

English

The Belle II detector is situated at the asymmetric $e^{-}e^{+}$ collider SuperKEKB at KEK in Tsukuba in Japan, which operates with an energy slightly above the mass of the Y(4S) resonance. As a result, pairs of B mesons are predominantly produced. Due to the clean experimental environment, various measurements to test the Standard Model and precisely determine its parameters can be performed. This is e.g. the element $V_{\mathrm{cb}}$ of the CKM matrix among others, where one of the leading systematic uncertainties arises due to decays of the nature $B \to D^{**} \ell \nu_{\ell}$ . This thesis presents the first measurement of the branching ratio $B \to D^{**} \ell \nu_{\ell}$ using the data set collected with the Belle II detector between 2019 and 2022 and corresponds to about $364\;\mathrm{fb^{-1}}$ of integrated luminosity at the Y(4S) resonance. To reconstruct the "other" B meson, a multivariate analysis tool called "Full Event Interpretation" is used, which reconstructs a variety of different hadronic decay modes. For the reconstruction of the signal side B meson, a light charged lepton is combined with a $D^{**}$ meson that is reconstructed from various $D \pi$ and $D^{*} \pi$ combinations. In contrast to previously performed analyses, this analysis also takes the $\pi^0$ into account in the reconstruction of the $D^{**}$ meson. A selection of events is applied to obtain a sample enriched in $B \to D^{**} \ell \nu_{\ell}$ decays and a binned maximum likelihood fit is performed to determine the branching ratio for the different $B \to D^{**} \ell \nu_{\ell}$ decay channels. To account for effects in the simulation that are currently not well understood, the branching ratio is normalised to the branching ratio of the much more frequent decay $B \to D^{*} \ell \nu_{\ell}$ .
Keywords: Particle Physics; Belle II; Semileptonic B Decays
 
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