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

Abstract

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 $q^2\in (1.1$ $\text{Ge}{\text{V}}^2,$ $6.0$ $\text{Ge}{\text{V}}^2)$, the result is $R_K=0.846_{-0.039}^{+0.042}{}_{-0.012}^{+0.013}$, 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.

Comments: 6 pages, 4 figures. Contribution to the Electroweak session of the 55th Rencontres de Moriond, 21-27 March 2021