Search for New Physics in the $B^0_s \to J/\psi\phi$ decay channel at LHCb

In the $B^0_s \to J/\psi\phi$ channel, the phase $\phi_s$ is predicted to be significantly small in the Standard Model. This observable is an excellent probe for New Physics because the $B^0_s \to \overline{B}^0_s$ mixing takes place via a loop diagram where new particles is expected to intervene. These two reasons makes the $\phi_s$ parameter an excellent probe for New Physics processes. We developed a simplified selection for the 2010 data. It avoids any bias on the proper time distribution in order to reduce systematic uncertainty. In addition, we control the tagging performance for $B^0_s \to J/\psi\phi$ events using the similar $B^0_d \to J/\psi K^{*0}$ and $B^+ \to J/\psi K^+$ channels. With the 2010 data, we obtain 570 signal events in 36 pb$^{−1}$ of integrated luminosity, a tagging power of (2.2 ± 0.4) % and a proper time resolution of 50 fs. We investigated an alternative selection which maximizes the $\phi_s$ sensitivity using a proper time biasing cuts. We proposed a data-driven method to correct the proper time acceptance. We designed a fitting program to determine the $\phi_s$ phase. With the 2010 data, It is found to be: $\phi_s$ = [−2.7,−0.5] rad at 68 % of confidence. This result is compatible with the Standard Model prediction.