Measurement of Vcb in inclusive B decays at Belle II

The Standard model (SM) of particle physics describes the known elementary particles, the quarks and the leptons, and the forces that act between them. While collider experiments up to this date have confirmed the SM, it is also known from astronomy that SM particles make up only about 5% of the energy content of the universe. It is thus the prime objective of modern particle physics to search for physics beyond the SM. A promising avenue is to look for minute deviations from the SM expectation for various observables in particle physics. This approach is hampered however by the limited knowledge of some fundamental SM parameters: The magnitude of the Cabibbo-Kobayashi-Maskawa matrix element Vcb governs the transition rate of b- into c-quarks and is affected by a 3-sigma discrepancy between different experimental methods of accessing this quantity (so-called inclusive and exclusive determinations). The main scientific goal of this project is to revisit the inclusive determination of |Vcb| based on new, more precise calculations of the semileptonic decay width. The experimental data for this analysis is being collected by the Belle II experiment, located at the electron-positron collider SuperKEKB in Japan, that started operation with the full experimental setup in the year 2019. The capabilities of the new collider and the detector enable measurements with high precision and will provide a data sample 50- times larger than any previous experiment. The |Vcb| parameter will be determined from collisions that produce a pair of so-called B-mesons. In particular we reconstruct B meson decays into a hadronic system with charm, a lepton (either an electron or a muon) and the associated neutrino. Here, the experimental challenges are the (undetectable) neutrino in the final state and the combinatorial background from the second B-meson in the event. This results in high background levels that have to be carefully studied, decoupled from the signal and discriminated against without biasing the distributions of signal events. The result of the measurement will be the invariant mass of the lepton-neutrino system and its moments that are needed to determine parameters in the theoretical expression of the semileptonic decay width. Preliminary studies indicate that even the current Belle II data sample (of only 0.2% of the final one) allows to determine the parameter |Vcb| with insignificant statistical uncertainty, as the number of the signal events already exceeds one million. The main focus will therefore be the control of systematics uncertainties. The study of the latter will be a significant part of this project: they consist of the precision of tracking system, the lepton identification efficiency and the modelling of the residual background.