Lucas Martel ; Isabelle Ripp-Baudot ; Giulio Dujany

2023
IPHC Strasbourg, France

Abstract: This thesis describes the first search for the decay of a charged B meson into a charged kaon and a pair of neutrinos using a hadronic tagging method at the Belle II experiment, operating at the asymmetric electron-positron collider SuperKEKB located at KEK, Tsukuba, Japan. The B+ to K+ nu anti-nu decay operates, at the quark level, through a b to s nu anti-nu flavour changing neutral-current transition. This decay has never been observed due to the experimental challenge posed by the undetected pair of neutrinos in its final state. However its branching fraction is predicted with accuracy in the Standard Model of particle physics, thus, a precise measurement of this branching fraction offers a unique opportunity to probe beyond Standard Model contributions. The analysis described therein makes use of the Full Event Interpretation algorithm (FEI), developed by the Belle II collaboration to sequentially reconstruct the most probable decay of the Btag meson accompanying the signal meson Bsig in Upsilon(4S) to Bsig Btag events. The analysis exploits a data sample corresponding to an integrated luminosity of 362 fb^-1 collected at the  Upsilon(4S) resonance mass, completed by a sample of 42 fb^-1 collected 60 MeV below said resonance. Given this dataset, the expected upper limit on the branching fraction of B+ to K+ nu anti-nu is determined to be 2.3x10^-5 at 90% confidence level, using simulated events and data collected in specific control channels. This measurement is expected to be competitive with previous measurements performed by the BaBar and Belle experiments with on-resonance datasets of 421 fb^-1 and 711 fb^-1 respectively. Furthermore, the development of an algorithmic method to improve the Belle II Silicon Vertex Detector (SVD) resolution on position is presented. This method corrects charge sharing effects between silicon strips in the detector, allowing to improve the spatial resolution for specific sensors by 5 to 15%.

Note: Presented on 20 09 2023
Note: PhD