Measurement of the $\textbf{\emph{{CP}}}$-violating phase $\boldsymbol{\phi_s}$ in $\boldsymbol{B_s^0\rightarrow J/\psi\phi}$ decays in ATLAS at 13~TeV

A measurement of the $B_s^0\rightarrow J/\psi\phi$ decay parameters using 80.5 $\mathrm{fb}^{-1}$ of integrated luminosity collected with the ATLAS detector from 13~TeV proton–proton collisions at the LHC is presented. The measured parameters include the $CP$-violating phase $\phi_s$, the width difference $\Delta\Gamma_s$ between the $B_s^0$ meson mass eigenstates and the average decay width $\Gamma_s$. The values measured for the physical parameters are combined with those from 19.2~$\mathrm{fb}^{-1}$ of 7 TeV and 8 TeV data, leading to the following: \\ \begin{center} \begin{equation*} \begin{matrix} \phi_s &= &-0.087 &\pm& 0.036~(\mathrm{stat.})&\pm &0.019~(\mathrm{syst.})~\mathrm{rad}\\ \Delta\Gamma_s &= &0.0641&\pm&0.0043~(\mathrm{stat.})&\pm &0.0024~(\mathrm{syst.})~\mathrm{ps}^{-1}\\ \Gamma_s &= &0.6697&\pm&0.0014~(\mathrm{stat.})&\pm &0.0015~(\mathrm{syst.})~\mathrm{ps}^{-1} \end{matrix} \end{equation*} \end{center} Results for $\phi_s$ and $\Delta\Gamma_s$ are also presented as 68\% confidence level contours in the $\phi_s-\Delta\Gamma_s$ plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. $\phi_s$ and $\Delta\Gamma_s$ measurements are in agreement with the Standard Model predictions.