Filippo Dattola ; Alexander Glazov ; Kerstin Tackmann

2022
DESY Hamburg

Abstract: This thesis presents two searches for the rare B → K(∗)νν ̄ decays at the Belle II experiment, operating at the SuperKEKB asymmetric-energy electron-positron collider, at the KEK laboratories in Tsukuba, Japan. The B → K(∗)νν ̄ decays are based on the b → sνν ̄ flavour-changing neutral-current transition. This process is forbidden at the Born level in the Standard Model (SM) and can only occur at higher orders in SM perturbation theory, at the cost of large suppression. Nevertheless, the B → K(∗)νν ̄ decays are predicted with high theoretical accuracy. Therefore, precise measurements of their branching fractions offer a unique opportunity to test the SM and its extensions. Searches for the B → K(∗)νν ̄ decays are affected by major experimental limitations related to the presence of two neutrinos in the final state and are only possible at B factories. Here, millions of B meson decays, produced at the Υ(4S) resonance, can be recorded in kinematically-constrained and low-background con- ditions. The searches presented in this thesis are performed with a novel measurement approach based on the implementation of an inclusive tagging method. In the inclusive tagging, both the characteristic features of the signal decay and the inclusive properties of the accompanying B meson in the Υ(4S) → BB ̄ event are exploited to suppress backgrounds. Such a technique provides higher signal efficiency and improved sensitivity compared to the other existing tagging methods, in which the second B meson is explicitly recon- structed in hadronic or semileptonic decays. The first data analysis presented in this thesis is a search for the B+ → K+νν ̄ decay with the inclusive tag- ging performed in 2021. This search uses a data sample corresponding to an integrated luminosity of 63 fb−1 collected by the Belle II detector at the Υ(4S) resonance and an off-resonance sample of 9 fb−1 collected at an energy 60 MeV below the resonance. No statistically significant signal is observed and an upper limit of 4.1 × 10−5 on the B+ → K+νν ̄ branching fraction is set at a 90% confidence level. This measurement is competitive with the results of previous searches obtained using significantly larger data samples. In the second analysis of the thesis, which is not yet finalised, the inclusive tagging is used to search for the B0 → K∗0νν ̄ decay. The analysis is optimised for larger data samples corresponding to on-resonance and off-resonance integrated luminosities of 189 fb−1 and 18 fb−1 , respectively. Using simulated events and collision data reconstructed in dedicated control channels, the measurement sets an expected upper limit of 4.4 × 10−5 on the B0 → K∗0νν ̄ branching fraction at a 90% confidence level. The expected limit is in the ballpark of the previous results, despite the smaller integrated luminosity.

Note: Presented on 26 09 2022
Note: PhD