Borys KNYSH ; Francois LE DIBERDER

2021
IJCLab Orsay

Abstract: Abstract: The document presents the analysis of the decay of the B+ meson into the K+π+π−γ final state using the early data of the Belle II exper- iment, that is located at the SuperKEKB collider in the KEK laboratory, Japan. The aim of the present work is to pave the way leading to the measurement of the polarization of the photon γ involved in the decay. The Stan- dard Model of particle physics predicts the photon polarization to be overwhelmingly left-handed. A measurement of the polarization away from this prediction would provide a direct indication of Physics beyond the Standard Model. The analysis relies on a detailed Monte-Carlo generator named “GamPola” that was developed by the author for this purpose to include state of the art theoretical understanding of the charged and neutral B decays into the various possible Kππγ final states. The data sample used in the analysis corre- sponds to a luminosity of 62.7 fb−1 that was ac- cumulated by the Belle II experiment until spring 2021. This luminosity being about 50 times smaller than what is needed to perform the mea- surement of the photon polarization, and the Belle II detector being not yet fully understood in this early stage, the aim of the initial analysis is fo- cused on defining a robust procedure to select the K+π+π−γ events. This selection procedure can be used later as a starting point to perform the photon polarization measurement. An overall selection efficiency of 25 % is achieved while maintaining the contribution of background events to a low enough level allow- ing a 10 % statistical precision on the branching ratio measurement. A refined model-independent approach based on phase-space binning leads to a 3 % precision for 1 ab−1. As a preliminary step towards the photon polarization, the measurement of the up-down asymmetry Aud is assessed to reach the percent level, for 1 ab−1. While the analysis is still blind at the present stage, the unblinding of the control samples has been per- formed.

Note: Presented on 07 10 2021
Note: PhD