Investigating the flavour anomalies through the rare beauty decay $B^0 \to K^{*0} \mu^+ \mu^-$

We measure the $B^0 \to K^{*0} \mu^+ \mu^-$ decay branching fraction as a function of the di-muon invariant mass squared, using the $B^0 \to J/\psi K^{*0}$ decay as normalisation channel. We use proton-proton collision data at $\sqrt{s} = 13$ TeV, collected by the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) during the years 2016, 2017 and 2018. The analysis is validated by comparing the ratio between the branching fractions of the two resonant channels, $B^0 \to J/\psi K^{*0}$ and $B^0 \to \psi(2S) K^{*0}$, with the current world average. Our result is consistent with this value within 0.2 $\sigma$. The $B^0 \to K^{*0} \mu^+\mu^-$ branching fraction results are also consistent with previous LHC Run1 measurements and with the Standard Model (SM) predictions. These preliminary results are the most precise to date. The $B^0 \to K^{*0} \mu^+ \mu^-$ decay is a rare beauty process which can only proceed at loop order in the SM, making it very sensitive to new physics (NP). Tensions with the SM have already been reported in one of the parameters appearing in its decay rate, $P'_5$, belonging to the class of discrepancies in the flavour sector commonly referred to as flavour anomalies. The $B^0 \to K^{*0} \mu^+ \mu^-$ branching fraction can enter in global fits to help constraining NP scenarios and investigating the source of these alluring flavour anomalies.