Observation of the $B^0_s \to \eta_c \phi$ decay with the LHCb experiment

The interference between $B^0_s$ meson decay amplitudes to CP final state directly or via mixing gives rise to a measurable CP-violating phase $\phi_s$, which is predicted to be $\phi_s^{SM} = (-0.0370\pm0.0006)~\mathrm{rad}$ in the Standard Model. However, such process may receive contributions from New Physics and change the value of $\phi_s$. At present, the most precise measurement of $\phi_s$ is given by the LHCb experiment and the world average is $\phi_s^{\rm{exp}} = (-0.021\pm 0.032)~\mathrm{rad}$, with uncertainty still dominated by the statistics. In this context, a study of $B^0_s \to \eta_c \phi$ decays is performed using $pp$ collision data corresponding to an integrated luminosity of~3.0\,fb$^{-1}$, collected with the LHCb detector during the Run~1 of the LHC. The observation of the decay $B^0_s \to \eta_c \phi$ is reported, where the $\eta_c$ meson is reconstructed in the $p\bar{p}$, $K^+K^-\pi^+\pi^-$, $\pi^+\pi^-\pi^+\pi^-$ and $K^+K^-K^+K^-$ decay modes and the $\phi(1020)$ in the $K^+ K^-$ decay mode. The decay $B^0_s \to J\psi \phi$ is used as a normalization channel. The measured branching fraction is $\mathcal B (B^0_s \to \eta_c \phi) = \left(5.01 \pm 0.53 \pm 0.27 \pm 0.63 \right) \times 10^{-4}$, where the first uncertainty is statistical, the second systematic and the third uncertainty is due to the limited knowledge of the external branching fractions.