Measurement of the CP-violating phase $\phi_s$ in $\bar{B}^{0}_{s}\to   D_{s}^{+}D_{s}^{-}$ decays
  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. 
LHCb collaboration: R. Aaij, C. Abell\'an Beteta, B. Adeva, M.   Adinolfi, A. Affolder, Z. Ajaltouni, S. Akar, J. Albrecht, ...
hep-ex