Measurement of the Branching Fraction of the Decay B -> D l nu using the Data of the Belle II Experiment

Eight years after the conclusion of the Belle experiment, the upgraded Belle II detector started operation in 2018. Belle II is a particle collider experiment located at the KEK Laboratory in Tsukuba, Japan. By colliding electrons and positrons at a center-of-mass energy of 10.58 GeV, equivalent to approximately two times the B meson rest mass, large amounts of B mesons are produced. The goal of this analysis is to measure the branching ratio of the semileptonic decays B− → Deνe and B− → Dμνμ, using data of the 2019 and 2020 physics runs. The decay is reconstructed by combining final state particles originating in the decay from detector data. The predominant B → D∗+l−ν l (l = e, μ) background is suppressed by employing a deep neural network and other multivariate analysis techniques. A dataset of 34.6 fb−1, corresponding to approximately 39 million B± mesons is analyzed to yield a B− → D0l−ν l (l = e, μ) branching fraction of (2.173 ± 0.060(stat.) ± 0.163(sys.))%. This is in agreement with measurements of previous experiments such as Belle and BaBar as well as with the world average (2.35± 0.03± 0.09)%. The measured ratio of electron to muon branching ratiosR (e/μ) = Br(B →Deνe) Br(B−→D0μ−νμ) = 1.026 ± 0.055(stat.) ± 0.074(sys.) is also in agreement with the lepton universality expectation R (e/μ) = 1. This thesis provides a first look at measuring the B− → D0l−ν l branching fraction at Belle II, laying grounds for a comprehensive study of these modes with the further goal of extracting the CKM parameter |Vcb| via this decay mode.