# Search for lepton-universality violation in $B^+\to K^+\ell^+\ell^-$ decays

## TL;DR

This paper reports a precise measurement of the ratio of branching fractions in B+ decays to K+ and dileptons, testing lepton universality and finding a slight deviation from the Standard Model.

## Contribution

First precise measurement of R_K in B+ decays over multiple energies, providing important tests of lepton universality.

## Key findings

- R_K measured as 0.846 with uncertainties
- Measurement compatible with Standard Model within 2.5 sigma
- Most precise R_K measurement to date

## Abstract

A measurement of the ratio of branching fractions of the decays $B^+\to K^+\mu^+\mu^-$ and $B^+\to K^+e^+e^-$ is presented. The proton-proton collision data used correspond to an integrated luminosity of $5.0\,$fb$^{-1}$ recorded with the LHCb experiment at centre-of-mass energies of $7$, $8$ and $13\,$TeV. For the dilepton mass-squared range $1.1 < q^2 < 6.0\,$GeV$^2\!/c^4$ the ratio of branching fractions is measured to be $R_K = {0.846\,^{+\,0.060}_{-\,0.054}\,^{+\,0.016}_{-\,0.014}}$, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $R_K$ to date and is compatible with the Standard Model at the level of 2.5 standard deviations.