Measurement of the $\textit{CP}$-violating phase $\phi_s$ in $B_s^0\rightarrow$J/$\psi\phi$ decays with the ATLAS detector

In the Standard Model of particle physics, \textit{CP} violation arises due to a single complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. Testing the validity of the CKM mechanism as the only source of $\textit{CP}$ violation is one of the major experimental challenges in particle physics today. Precise measurement of the CKM parameters therefore constrains the Standard Model, and may reveal effects beyond the Standard Model. Measurement of the time-dependent decay rates of $B_s^0\rightarrow J/\psi \phi$ provides a theoretically clean method for extracting $\textit{CP}$-violating weak mixing phase $\phi_s$. Standard Model predicts $\phi_s$ to be very small and it is very well constrained, while in many new physics models large $\phi_s$ values are expected. $B_s^0\rightarrow J/\psi \phi$ decay channel is sensitive to the new physics contributions, and already small deviations in a measurement of $\phi_s$ would be hints for the existence of the new particles. In this work, we present a measurement of the $B_s^0\rightarrow J/\psi \phi$ time--dependent $\textit{CP}$ asymmetry parameters using 14.3 fb$^{-1}$ collected by the ATLAS detector at $\sqrt{s}=8$ TeV \textit{pp} collisions at the LHC during 2012. We also present combined results with 4.9 fb$^{-1}$ of data collected at $\sqrt{s}=7$ TeV during 2011. A four-dimensional unbinned maximum likelihood fit, which also includes information of the $B_s^0$ production flavour, is used to extract the parameters from the corresponding measured decay time and angular distributions of the $B_s^0\rightarrow J/\psi(\mu^+\mu^-) \phi(K^+K^-)$ decay. Apart from $\textit{CP}$-violating mixing phase $\phi_s$, several parameters describing the $B_s^0$ meson system are measured. These include the $B_s^0$ lifetime $1/\Gamma_s$, the decay width difference $\Delta\Gamma_s$ between the heavy and the light mass eigenstates, and the transversity amplitudes $|A_0(0)|$ and $|A_\parallel(0)|$. The results are compatible with Standard Model predictions. Within the given uncertainties, the ATLAS results are consistent with the measurements from other LHC experiments. The significance of the deviation from the Standard Model prediction will be clarifed once more data has been accumulated following the LHC upgrade and the statistical precision of the measurements improved.