Test of lepton universality using $B^{+}\rightarrow K^{+}\ell^{+}\ell^{-}$ decays

TL;DR

This paper reports a precise measurement of the ratio of branching fractions for B+ decays to muons and electrons, testing lepton universality and finding results compatible with the Standard Model within 2.6 standard deviations.

Contribution

It provides the most precise measurement to date of the lepton universality ratio in B+ decays, using LHCb data at 7 and 8 TeV.

Findings

Measured ratio of branching fractions: 0.745^{+0.090}_{-0.074} (stat) ± 0.036 (syst)

Result compatible with Standard Model within 2.6 standard deviations

First measurement with this level of precision for this decay channel

Abstract

A measurement of the ratio of the branching fractions of the $B^{+}\rightarrow K^{+}\mu^{+}\mu^{-}$ and $B^{+}\rightarrow K^{+}e^{+}e^{-}$ decays is presented using proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$, recorded with the LHCb experiment at center-of-mass energies of 7 and 8 $\mathrm{\,Te\kern -0.1em V}$. The value of the ratio of branching fractions for the dilepton invariant mass squared range $1<q^{2}<6\mathrm{\,Ge\kern -0.1em V^2\!/}c^4$ is measured to be $0.745^{+0.090}_{-0.074}\mathrm{\,(stat)}\,\pm0.036\mathrm{\,(syst)}$. This value is the most precise measurement of the ratio of branching fractions to date and is compatible with the SM prediction within $2.6$ standard deviations.