Test of lepton flavour universality with $B_s^0 \rightarrow \phi \ell^+\ell^-$ decays

TL;DR

This paper tests lepton flavour universality in rare $b ightarrow s$ decays using $B_s^0$ meson decays, finding results consistent with the Standard Model across multiple $q^2$ regions.

Contribution

First measurement of lepton flavour universality in $B_s^0$ decays using LHCb data, providing new tests of the Standard Model in rare $b ightarrow s$ transitions.

Findings

Results agree with Standard Model expectations

Branching fraction ratios measured in three $q^2$ regions

No evidence of lepton flavour universality violation

Abstract

Lepton flavour universality in rare $b\rightarrow s$ transitions is tested for the first time using $B_s^0$ meson decays. The measurements are performed using $pp$ collision data collected by the LHCb experiment between 2011 and 2018, corresponding to a total integrated luminosity of 9$\,{\rm fb}^{-1}$. Branching fraction ratios between the $B_s^0 \rightarrow \phi e^+e^-$ and $B_s^0 \rightarrow \phi \mu^+\mu^-$ decays are measured in three regions of dilepton mass squared, $q^2$, with $0.1 < q^2 < 1.1$, $1.1 < q^2 < 6.0$, and $15 < q^2 < 19\,{\rm GeV}^2/c^4$. The results agree with the Standard Model expectation of lepton flavour universality.