Past exam of the particle physics course of the University of Oxford / 2017 / 6 / e / Solution

For a $t\to cZ$ search, the cleanest neutral-boson reconstruction is $Z\to e^{+}{e}^{-}$ or ${\mu }^{+}{\mu }^{-}$: require an isolated opposite-sign same-flavour pair near the $Z$ mass and a jet forming the top mass with that pair. In top-pair events, tagging the other top through its ordinary $bW$ decay provides a bottom jet and, if its $W$ is leptonic, another isolated lepton and missing transverse momentum. Kinematic reconstruction and heavy-flavour information help reject vector-boson-plus-jets and other backgrounds. Hadronic or invisible $Z$ decays have higher rate but less distinctive backgrounds or incomplete mass reconstruction.

For the literal $t\to cZ\gamma$ channel, additionally require a prompt isolated photon and reconstruct the top with $Z+\gamma +$jet. For the alternative $t\to c\gamma$, use an isolated energetic photon and jet near the top mass, again exploiting the other top as a tag. These are search signatures; no present-day observation claim is needed to interpret the historical question.