Search for pair-production of vector-like $T$ quarks decaying into a top quark and a spin-0 particle in the diphoton final state in proton proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for pair-produced vector-like T quarks (VLQs) is presented. Each VLQ is assumed to decay into a top quark and an exotic (pseudo)scalar S. The search targets events with at least one resonant $S \to γγ$ decay and large summed transverse momentum and missing transverse momentum from other final-state particles. The analysis uses data from $pp$ collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector from 2015 to 2018 and corresponding to an integrated luminosity of 140 fb$^{-1}$. This is the first VLQ search performed in this final state. No significant excess over the Standard Model background is observed. The analysis is model-independent to a large extent but targets cascade decays into final states with electroweak bosons or $t\bar{t}$. Results are provided in the form of upper limits on the fiducial cross-section times branching ratio as a function of the masses of the VLQ and the spin-0 particle. The fiducial volume is defined by kinematic requirements on the photons from the spin-0 decay.