In the standard mode, rare b → sμμ decays are forbidden at tree level. Instead, they proceed through electroweak penguin and box diagrams, leading to branching fractions of O(10⁻⁷). Over the past decade, measurements performed by several experiments have revealed an intriguing pattern: the observed branching fractions of particles decaying via the specific transition tend to be lower than many of the most precise theoretical predictions. In this talk, we present the measurements of the CP-averaged differential branching fractions as functions of the squared dimuon invariant mass, q² = m²(μμ) of six rare decays, B⁺ → K⁺μμ, B⁺ → K*⁺μμ, B⁰ → K⁰μμ, B⁰ → K*⁰μμ, B⁰s → ϕμμ, and Λ⁰b → Λμμ. The results are derived using pp collisions collected by the CMS detector in 2022–2024 at a center-of-mass energy of 13.6 TeV, corresponding to an integrated luminosity of 172 fb⁻¹. The branching fractions are measured through a simultaneous fit to the six decay modes, normalized to decays containing the J/ψmeson. The results agree with previous measurements and improve their precision. The isospin asymmetries of pseudoscalar B → Kμμand vector B → K*μμdecay modes are also measured as functions of q² with improved precision and are consistent with zero, as expected in the standard model. Ratios of branching fractions for the corresponding ψ(2S) and J/ψ channels are measured and agree with the world-average values.
