We present a measurement of the $CP$-violating weak mixing phase $\phi_s$ using the decay $\bar{ B}^{ 0}_{ s}\to D_{ s}^{ +}D_{ s}^{ -}$ in a data sample corresponding to $3.0$ fb$^{ -1}$ of integrated luminosity collected with the LHCb detector in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. An analysis of the time evolution of the system, which does not constrain $|\lambda|=1$ to allow for the presence of $CP$ violation in decay, yields $\phi_s = 0.02 \pm 0.17$ (stat) $\pm 0.02$ (syst) rad, $|\lambda| = 0.91^{ +0.18}_{ -0.15}$ (stat) $\pm0.02$ (syst). This result is consistent with the Standard Model expectation.
Categories: hep-ex