Test of lepton flavour universality in $b\to s\ell^+\ell^-$ decays

The Standard Model of particle physics predicts that charged leptons have the same electroweak interaction strength. This symmetry, called lepton flavour universality, was found to hold in a wide range of particle decays. One observable that is sensitive to lepton flavour universality is the ratio of branching fractions $R_K=\mathcal B(B^+\to K^+\mu^+\mu^-)$ $/$ $\mathcal B(B^+\to K^+e^+e^-)$. This quantity is measured using $9$ fb$^{-1}$ of proton-proton collision data recorded by the LHCb experiment at CERN's Large Hadron Collider. For the dilepton invariant mass squared range $q^2\in(1.1$ $\text{Ge}\kern -0.1em \text V^2,$ $6.0$ $\text{Ge}\kern -0.1em \text V^2)$, the result is $R_K=0.846^{+0.042}_{-0.039}{}^{+0.013}_{-0.012}$, where the first uncertainty is statistical and the second systematic. The measured value is in tension with the Standard Model prediction at the level of $3.1$ $\sigma$, thus providing evidence for the violation of lepton flavour universality in $B^+\to K^+\ell^+\ell^-$ decays.