First measurement of the $CP$-violating phase in $B^0_s\to J/\psi(\to e^+e^-)\phi$ decays

TL;DR

This paper reports the first measurement of the $CP$-violating phase in $B^0_s o J/eta o e^+e^-\, ext{and}\, ext{phi}$ decays using LHCb data, confirming Standard Model predictions.

Contribution

It introduces a novel analysis of $B^0_s o J/eta o e^+e^-\, ext{and}\, ext{phi}$ decays, measuring $CP$ violation with an $e^+e^-$ final state for the first time.

Findings

Measured $CP$-violating phase $\, ext{phi}_s=0.00\, ext{rad}$ with uncertainties

Determined $B^0_s$ lifetime and decay width differences consistent with previous results

Results align with Standard Model predictions.

Abstract

A flavour-tagged time-dependent angular analysis of $B^0_s\to J/\psi\phi$ decays is presented where the $J/\psi$ meson is reconstructed through its decay to an $e^+e^-$ pair. The analysis uses a sample of $pp$ collision data recorded with the LHCb experiment at centre-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb$^{-1}$. The $CP$-violating phase and lifetime parameters of the $B^0_s$ system are measured to be $\phi_s=0.00\pm0.28\pm0.07$ rad, $\Delta\Gamma_s=0.115\pm0.045\pm0.011$ ps$^{-1}$ and $\Gamma_s=0.608\pm0.018\pm0.012$ ps$^{-1}$ where the first uncertainty is statistical and the second systematic. This is the first time that $CP$-violating parameters are measured in the $B^0_s\to J/\psi\phi$ decay with an $e^+e^-$ pair in the final state. The results are consistent with previous measurements in other channels and with the Standard Model predictions.