Latest LHCb measurements of $CP$ violation in the interference between mixing and decay are presented based on $pp$ collision data collected during LHC Run I, corresponding to an integrated luminosity of $3.0, {rm fb}^{-1}$. Approximately $27, 000$ $B_s^0to J/psi pi^+pi^-$ signal events are used to make what is at the moment the most precise single measurement of the $CP$-violating phase in $bto cbar{c}s$ transitions, $phi_s^{cbar{c}s}=0.070pm0.068{rm (stat.)}pm0.008{rm (syst.)}$ rad. The most accurate measurement of the $CP$-violating phase in $bto sbar{s}s$ transitions, $phi_s^{sbar{s}s}$, is found from approximately $4, 000$ $B_s^0to phiphi$ signal events to be $phi_s^{sbar{s}s}=-0.17pm{rm (stat.)}0.15pm0.03{rm (syst.)}$ rad.
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