Search for the B+ → π+e+e− decay with the LHCb experiment

Abstract

The B+ → π+e+e− decay is strongly suppressed in the Standard Model, and therefore a highly sensitive probe for new physics. Previous experimental searches have not observed this decay and have only set weak constraints on the upper limit of the branching ratio. This thesis presents the most precise search to date, using data collected by the LHCb detector from 2011-2018 , corresponding to 9 fb−1 of pp collisions. The B+ → π+e+e− signal is observed with a significance of 3.2σ, constituting the first evidence for this decay. The branching ratio is measured to be B(B+ → π+e+e−) = ( 2.44 +0.92−0.83 (stat.) +0.39−0.24 (syst.) ) × 10−8 which is in good agreement with the Standard Model prediction. In addition, this thesis reports the branching ratio in bins of the dielectron invariant mass.