Differential decay rate of B⁺→ J/ψK⁺ with the LHCb Upgrade I experiment

The normalised decay rate of B⁺ → J/ψ(→ μ⁺μ⁻)K⁺ is measured as a function of the lepton helicity angle using a data sample corresponding to an integrated luminosity of 1.1 fb⁻¹ collected during October 2024 with the upgraded (Upgrade I) LHCb detector. This angular distribution can be parameterised by two coefficients, the forward-backward asymmetry, A_FB, and the flatness parameter, F_H, whose values are constrained by conservation of angular momentum. These coefficients are measured both integrated and differentially across various kinematic and detector-response variables, and the results are found to be in good agreement with expectations. These measurements show that the detector response of the LHCb Upgrade I experiment is understood to the precision required to reliably extract the angular coefficients associated with rare b → sμ⁺μ⁻ and b → dμ⁺μ⁻ transitions, which are particularly sensitive to physics beyond the Standard Model.