Measurement of the $CP$-violating phase $\phi_\mathrm{s}$ in the B$^0_\mathrm{s}$ $\to$ J$/\psi\, \phi$(1020) $\to$ $\mu^+\mu^-$K$^+$K$^-$ channel in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Understand

The $CP$-violating weak phase $\phi_\mathrm{s}$ and the decay width difference $\Delta\Gamma_\mathrm{s}$ between the light and heavy B$^0_\mathrm{s}$ mass eigenstates are measured with the CMS detector at the LHC in a sample of 48 500 reconstructed B$^0_\mathrm{s}$ $\to$ J$/\psi\, \phi$(1020) $\to$ $\mu^+\mu^-$K$^+$K$^-$ events.

• The measurement is based on a data sample corresponding to an integrated luminosity of 96.4 fb$^{-1}$, collected in proton-proton collisions at $\sqrt{s} =$ 13 TeV in 2017-2018.
• To extract the values of $\phi_\mathrm{s}$ and $\Delta\Gamma_\mathrm{s}$, a time-dependent and flavor-tagged angular analysis of the $\mu^+\mu^-$K$^+$K$^-$ final state is performed.
• The analysis employs a dedicated tagging trigger and a novel opposite-side muon flavor tagger based on machine learning techniques.