Abstract
A search for flavour-changing neutral interactions involving the top quark, the Higgs boson and an up-type quark q (q=c,u) is presented. The proton-proton collision data set used, with an integrated luminosity of 139 fb−1, was collected at s√=13TeV by the ATLAS experiment at the Large Hadron Collider. Both the decay process t→qH and the production process pp→tH, with the Higgs boson decaying into two photons, are investigated. No significant excess is observed and upper limits are set on the t→cH and the t→uH branching ratios of 4.3×10−4 and 3.8×10−4, respectively, at the 95% confidence level, while the expected limits in the absence of signal are 4.7×10−4 and 3.9×10−4. Combining this search with ATLAS searches in the H→τ+τ− and H→bb¯ final states yields observed (expected) upper limits on the t→cH branching ratio of 5.8×10−4 (3.0×10−4) at the 95% confidence level. The corresponding observed (expected) upper limit on the t→uH branching ratio is 4.0×10−4 (2.4×10−4)