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

Decays mediated via $b\to s\ell^+\ell^-$ transitions are suppressed in the Standard Model and are thus sensitive probes of New Physics. Over the last decade, these processes have shown tensions with Standard Model predictions, most prominently in the differential decay rates and angular observables of $B^0\to K^{*0}\mu^+\mu^-$ and $B^0_s\to\phi\mu^+\mu^-$ decays.Lepton flavour universality (LFU) tests provide particularly clean tests of the Standard Model, since these are almost free of hadronic uncertainties. Although recent results are consistent with LFU, new and improved measurements remain well motivated and provide strong constraints on physics beyond the Standard Model.This seminar presents the first test of LFU in $B_s^0 \to\phi\ell^+\ell^-$ decays, where $\ell = e,\mu$. The analysis covers three distinct regions of the dilepton invariant mass squared ($q^2$), including the first measurement at high $q^2$ above the $\psi(2S)$ resonance, constituting the most precise test of LFU in $B_s^0$ decays.

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