  A search for the electroweak production of a vector-like quark T$'$, decaying
to a top quark and a Higgs boson is presented. The search is based on a sample
of proton-proton collision events recorded at the LHC at $\sqrt{s}$ = 13 TeV,
corresponding to an integrated luminosity of 138 fb$^{-1}$. This is the first
T$'$ search that exploits the Higgs boson decay to a pair of photons. For
narrow isospin singlet T$'$ states with masses up to 1.1 TeV, the excellent
diphoton invariant mass resolution of 1-2% results in an increased sensitivity
compared to previous searches based on the same production mechanism. The
electroweak production of a T$'$ quark with mass up to 960 GeV is excluded at
95% confidence level, assuming a coupling strength $\kappa_\mathrm{T}$ = 0.25
and a relative decay width $\Gamma/M_{\mathrm{T}'}$ $\lt$ 5%.

High Energy Physics - Experiment