Federica Riti (CERN)

Lepton flavour universality and beyond at CMS

Abstract: Lepton flavour universality (LFU) is an accidental symmetry of the Standard Model (SM), and its validity has been extensively tested across various sectors. In purely leptonic decays and in electroweak boson decays (such as those of the W and Z), LFU is confirmed with high precision. However, in the b-quark sector, this symmetry has been called into question due to several experimental anomalies. In particular, the combination of the R(D) and R(D*) measurements shows a tension of about three standard deviations from the SM predictions. CMS has recently entered this field, performing its first LFU tests in the b-sector with the measurements of R(J/ψ) and R(K). These direct LFU tests are complemented by measurements of other independent and theoretically clean observables in the b-sector that are particularly sensitive to new physics, such as branching ratios and angular observables of rare decays. Among the latter, the P₅′ angular observable in the B → K*μ⁺μ⁻ decay has shown deviations from SM expectations. In this context, CMS has recently published a new angular analysis of the B → K*μ⁺μ⁻ decay, using the full Run 2 dataset to extract a comprehensive set of angular observables. This presentation will explore these topics in detail, focusing on three recent CMS analyses: two testing LFU and one measuring angular observables in rare b-hadron decays.