Search for flavour-changing neutral $tqH$ interactions with $H$ → γγ in $pp$ collisions at $ \sqrt{s} $ = 13 TeV using the ATLAS detector
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 $\sqrt{s}$ = 13 TeV by the ATLAS experiment at the Large Hadron Collider. Both the decay process t → qH in $t\bar{t}$ production 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 → $b\bar{b}$ 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 ).