Probing the standard model of particle physics with rare B decays

Abstract

This thesis discusses two analyses of rareBmeson decays that are sensitive to effectsbeyond the Standard Model of particles physics. Both measurements are based on theLHCb Run 1 data sample of proton-proton collisions corresponding to an integratedluminosity of3 fb−1recorded at centre-of-mass energies of 7 and 8 TeV.The first measurement is a search for light scalar particles inB0(s)→μ+μ−μ+μ−decays. Additionally to the Standard Model contributions without intermediateresonances, a specific supersymmetric model in which the decays proceed via lightscalar and pseudoscalar particles is studied. No signal candidates are observed suchthat upper limits on the branching fractions of theB0andB0sdecays of the order of10−9 and10−10 are set at95 %confidence level. These yield an improvement of aboutone order of magnitude compared to the previous most stringent limits.The second measurement is a test of lepton flavour universality with the ratio ofbranching fractions of the decaysB+→K+π+π−μ+μ−andB+→K+π+π−e+e−. Theanalysis is still in progress. Hence, no final result can be presented. The ratio of signalyields obtained from the LHCb Run 1 data sample is determined with a statisticaluncertainty of17 %, which can be reduced to8 %by extending the measurement tothe Run 2 data sample.