000003316 001__ 3316
000003316 005__ 20221104092259.0
000003316 037__ $$aBELLE2-PTHESIS-2022-011
000003316 041__ $$aeng
000003316 088__ $$adoi:10.3204/PUBDB-2022-06373
000003316 100__ $$aFilippo Dattola
000003316 245__ $$aSearches for the rare 
000003316 260__ $$aHamburg$$bDESY$$c2022
000003316 300__ $$a208
000003316 500__ $$aPresented on 26 09 2022
000003316 502__ $$aPhD$$bHamburg, University of Hamburg$$c2022
000003316 520__ $$aThis 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.
000003316 700__ $$aAlexander Glazov$$edir.
000003316 700__ $$aKerstin Tackmann $$edir.
000003316 8560_ $$ffilippo.dattola@desy.de
000003316 8564_ $$uhttps://docs.belle2.org/record/3316/files/BELLE2-PTHESIS-2022-011.pdf
000003316 980__ $$aTHESIS