  In the Standard Model of particle physics, charged leptons of different
flavour couple to the electroweak force carriers with the same interaction
strength. This property, known as lepton flavour universality, was found to be
consistent with experimental evidence in a wide range of particle decays.
Lepton flavour universality can be tested by comparing branching fractions in
ratios such as $R_K = \mathcal{B}(B^+ \rightarrow K^+ \mu^+
\mu^-)/\mathcal{B}(B^+ \rightarrow K^+ e^+ e^-)$. This observable is measured
using proton-proton collision data recorded with the LHCb detector at CERN's
Large Hadron Collider corresponding to an integrated luminosity of $9
\rm{~fb}^{-1}$. For a dilepton invariant mass range of $q^{2} \in
[1.1,6.0]~\rm{Ge}\kern -0.1em \rm{V}^{2}$, the measured value of
$R_{K}=0.846\,^{+\,0.042}_{-\,0.039}\,^{+\,0.013}_{-\,0.012}$, where the first
uncertainty is statistic and the second systematic, is in tension with the
Standard Model predicted value at the $3.1\sigma$ level raising evidence for
lepton flavour universality violation in $B^+ \rightarrow K^+ \ell^+ \ell^-$
decays.
