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.