Measurement of the differential branching fraction and A CP of the rare decay B + → π + µ + µ −

Abstract

This thesis presents an analysis of the rare decay B+ → π+μ+μ−. The differential branching fraction is measured in bins of the dimuon invariant mass squared, q2. The branching fraction ratio with the decay B+ → K+μ+μ− is measured in the same bins. World-leading precision is achieved, and the first binned measurement is made of the B+ → π+μ+μ− CP asymmetry. The total B+ → π+μ+μ− branching fraction is obtained from the measured differential branching fraction in each q2 bin: B(B+ → π+μ+μ−) =(︁ 1.985+0.122 −0.112 (stat) ± 0.131 (syst))︁ × 10−8. The total ACP is found to be ACP (B− → π−μ+μ−) = −0.034+0.061 −0.060 (stat) ± 0.015 (syst). The results are obtained using 9 fb−1 of data collected by the LHCb detector at CERN, at centre- of-mass energies of 7, 8 and 13 TeV. A comparison is made to Standard Model predictions, with the results showing good agreement with the expectation, within 2.0σ. Fits to the results for different Wilson coefficients are carried out, showing consistency with the Standard Model within less than 0.5σ.