The CMS collaboration at CERN has made its first differential measurement of the rare Bs → φμ+μ– decay, a process sensitive to physics beyond the Standard Model. This decay involves a bottom quark transforming into a strange quark via a flavor-changing neutral current, which in the Standard Model only occurs through higher-order loop diagrams and is heavily suppressed. As such, it serves as a powerful probe for detecting new particles or forces.

Using 138 fb⁻¹ of proton-proton collision data collected at 13 TeV, CMS measured the decay's branching fraction as a function of q2, the dimuon invariant mass squared. In the low-q2 region (1.1 to 6 GeV²), the result shows a 4.2σ deficit compared to Standard Model predictions based on Flavio calculations. This deviation strengthens earlier hints seen by LHCb, which reported a 3.6σ tension using older data.

The angular observables FL and A6, extracted alongside the branching fraction, align with theoretical expectations, suggesting the anomaly lies specifically in the decay rate. Systematic uncertainties are reduced by normalizing to the Bs → J/ψφ channel, which shares the same final state topology. However, current limits are statistical, and upcoming Run 3 data will improve precision.

If confirmed, this deficit could indicate new physics such as leptoquarks or Z′ bosons influencing the decay loop. The beauty quark sector continues to show persistent anomalies, making this a key area for discovery in the coming years.