Measurement of the branching fractions of the semileptonic decays B+ → ηlν and B+ → η′lν with signal-side only reconstruction at the Belle experiment
von Uwe Gebauer
Datum der mündl. Prüfung:2021-06-11
Betreuer:Prof. Dr. Ariane Frey
Gutachter:Prof. Dr. Ariane Frey
Gutachter:Prof. Dr. Stan Lai
EnglischThis thesis presents a measurement of the branching fractions of the decays B+ → ηlν and B+ → η'lν. The entire data sample collected by the Belle experiment at the energy of the Υ(4S) resonance is used, which contains in total 772E6 BB pairs. The η meson is reconstructed in two decay channels and the η′ meson in one decay channel. Only the decay products of one of the two B mesons from the BB pair are explicitly reconstructed to achieve a high efficiency. Instead of explicitly reconstructing the remainder of the event, the neutrino in the decay is inferred using conservation laws to allow reconstruction of the B+. Background events are reduced using boosted decision trees. A binned maximum likelihood fit of MC distributions to the data sample is used to extract the signal yield in the end. The entire reconstruction chain keeps the dependence on the transferred momentum q2 as low as possible to reduce effects of the modelling of the decay. The determined branching fractions are Br(B+ → ηlν) = (2.83 ± 0.55 ± 0.34)E−5 and Br(B+ → η′lν) = (2.79 ± 1.29 ± 0.30)E−5 with the statistical uncertainty first and the systematic second.
Keywords: Particle Physics; Belle; High Energy Physics; Semileptonic B Decays; Eta meson