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

Abstract

The normalised decay rate of $B^{+}\to J/\ddot{I}ˆ(\to \hat{I}{¼}^{+}\hat{I}{¼}^{-})K^{+}$ 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 $b\to s\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and $b\to d\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ transitions, which are particularly sensitive to physics beyond the Standard Model.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/5256/ (LHCb public pages)