Search for a vector-like quark T’ → tH via the diphoton decay mode of the Higgs boson in proton-proton collisions at √s = 13 TeV

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 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 κ _T = 0.25 and a relative decay width Γ/M _T′ < 5%. [Figure not available: see fulltext.].