Measurement of the branching fraction ratio $R_K$ at large dilepton invariant mass

TL;DR

This paper measures the ratio of branching fractions of B+ decays to K+ mu+ mu- and K+ e+ e- in high dilepton mass region to test lepton universality, finding results consistent with the Standard Model.

Contribution

First precise test of lepton universality in B+ decays at high dilepton mass using hadron collider data, expanding the scope of previous measurements.

Findings

Measured R_K = 1.08^{+0.11}_{-0.09} (stat)^{+0.04}_{-0.04} (syst)

Result consistent with Standard Model prediction of unity

Most precise test at high dilepton mass in hadron collider environment

Abstract

A test of lepton universality between muons and electrons is performed using $B^+\to K^+\ell^+\ell^-$ decays (where $\ell$ = $e$, $\mu$), in the dilepton invariant-mass-squared region above 14.3 GeV$^2/c^4$. The data used for the measurement consists of beauty meson decays produced in proton-proton collisions, corresponding to an integrated luminosity of 9 $\text{fb}^{-1}$, collected by the LHCb experiment between 2011 and 2018. The ratio of branching fractions for $B^+\to K^+\mu^+\mu^-$ and $B^+\to K^+e^+e^-$ decays is measured to be $R_K = 1.08^{+0.11}_{-0.09}\;(\text{stat})\;^{+0.04}_{-0.04}\;(\text{syst})$, which is consistent with the Standard Model prediction of unity. This constitutes the most precise test of lepton flavour universality using $B^+\to K^+\ell^+\ell^-$ decays with dilepton invariant-mass-squared above the $\psi(2S)$ mass, whilst being the first of its kind at a hadron collider.