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

TL;DR

This study measures the ratio of decay rates involving muons and electrons in B meson decays, finding a deviation from the Standard Model prediction that suggests possible new physics beyond current theories.

Contribution

It provides the first measurement of $R_K$ using 9 fb$^{-1}$ of data, revealing a 3.1 sigma deviation from the Standard Model, indicating potential lepton flavour universality violation.

Findings

Measured $R_K$ value of 0.846 with uncertainties.

Observed 3.1 sigma deviation from Standard Model.

Evidence suggests violation of lepton flavour universality.

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 $q^2\in(1.1$ $\text{Ge}\kern -0.1em \text V^2,$ $6.0$ $\text{Ge}\kern -0.1em \text V^2)$, 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.