In top-pair events where at least one of the tops decays semileptonically, the identification of the lepton charge allows us to tag not only the top quark charge but also that of the subsequent $b$ quark. In cases where the $b$ also decays semileptonically, the charge of the two leptons can be used to probe $CP$ violation in heavy flavor mixing and decays. This strategy to measure $CP$ violation is independent of those adopted so far in experiments, and can already constrain non standard model sources of $CP$ violation with current and near future LHC data. To demonstrate the potential of this method we construct two $CP$ asymmetries based on same-sign and opposite-sign leptons and estimate their sensitivities. This proposal opens a new window for doing precision measurements of $CP$ violation in $b$ and $c$ quark physics via high $p_T$ processes at ATLAS and CMS.

• Received 8 January 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.232002