This paper sets the world's tightest limits on these hypothetical particles (call "vector-like quarks") that some theorist believe may be the key to understanding the properties of the Higgs boson and the unification of the fundamental forces.
"Combination of the Searches for Pair-Produced Vectorlike Partners of the Third-Generation Quarks at √s=13 TeV with the ATLAS Detector", M. Aaboud et al. (ATLAS Collaboration), Phys. Rev. Lett. 121, 211801 (20 November 2018)