First evidence for mixing-induced $CP$ violation in B$^0_\mathrm{s}$ $\to$ J/$\psi\,\phi$(1020) decays in pp collisions at $\sqrt{s} = $ 13 TeV

Abstract

A novel machine-learning-based flavor-tagging algorithm combining same-side and opposite-side tagging is used to obtain the equivalent of 27$\,$000 tagged B$^0_\mathrm{s}$ $\to$ J/$\psi\, \phi$(1020) decays from pp collisions at $\sqrt{s}$ $=$ 13 TeV, collected by the CMS experiment and corresponding to an integrated luminosity of 96.5 fb$^{-1}$. A time- and flavor-dependent angular analysis of the $\mu^+\mu^-$K$^+$K$^-$ final state, consistent with a $\phi$(1020) $\to$ K$^+$K$^-$ decay, is used to measure parameters of the $\mathrm{B}^0_\mathrm{s}-\overline{\mathrm{B}}^0_\mathrm{s}$ system. The weak phase is measured to be $\phi_\mathrm{s}$ = $-$73 $\pm$ 22 (stat) $\pm$ 10 (syst) mrad, which, combined with the $\sqrt{s}$ $=$ 8 TeV CMS result, gives $\phi_\mathrm{s}$ = $-$75 $\pm$ 23 mrad. This value differs from zero by 3.2 standard deviations, providing the first evidence for mixing-induced $CP$ violation in B$^0_\mathrm{s}$ $\to$ J/$\psi\,\phi$(1020) decays. All measured physics parameters are found to agree with standard model predictions where available.