Differential decay rate of $B^{+}\to J\mathrm{/}\psi K^{+}$ with the LHCb Upgrade I experiment

The normalised decay rate of $B^{+}\to J\mathrm{/}\psi (\to {\mu }^{+}{\mu }^{-})K^{+}$ is measured as a function of the lepton helicity angle using a data sample corresponding to an integrated luminosity of $1.1{\text{fb}}^{-1}$ 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 $b\to s{\mu }^{+}{\mu }^{-}$ and $b\to d{\mu }^{+}{\mu }^{-}$ transitions, which are particularly sensitive to physics beyond the Standard Model.