Ultraboosted Zt and γt production at the HL-LHC and FCC-hh

Searches for anomalous Zt and gamma t production provide an excellent probe of flavour-changing top interactions when the energies considered are very large. In this note we estimate the sensitivity to these interactions at the high-luminosity phase of the LHC and a future 100 TeV pp collider (FCC-hh). For the LHC, the expected limits on t -> uZ/u gamma branching ratios from Zt and gamma t production will reach the 10(-5) level, one order of magnitude better than the existing projections for t -> uZ from t (t) over bar t production. For the FCC-hh, the limits on t -> uZ/u gamma could reach an impressive sensitivity at the 10(-6) level, with limits on t -> cZ/c gamma at the 10(-5) level.