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

TL;DR

This paper reports precise measurements of the angular distribution in the decay $B^+ o J/ K^+$ using upgraded LHCb data, testing the detector's ability to probe rare transitions sensitive to new physics beyond the Standard Model.

Contribution

First measurement of the decay's angular coefficients using the upgraded LHCb detector, demonstrating detector response understanding for rare decay analyses.

Findings

Measured angular coefficients $A_{FB}$ and $F_{H}$ are consistent with Standard Model expectations.

Validated the detector response for future studies of rare $b o s \u03bc^+bc^-$ and $b o d \u03bc^+bc^-$ transitions.

Confirmed the upgraded detector's capability to analyze angular distributions in rare decay processes.

Abstract

The normalised decay rate of $B^+ \to J/\psi(\to \mu^+\mu^-) K^+$ 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.