In this paper we explore the coupling of a light axion-like particle (ALP) to
top quarks. We use high-energy LHC probes, and examine both the direct probe to
this coupling in associated production of a top-pair with an ALP, and the
indirect probe through loop-induced gluon fusion to an ALP leading to top
pairs. Using the latest LHC Run II data, we provide the best limit on this
coupling. We also compare these limits with those obtained from loop-induced
couplings in diboson final states, finding that the ttˉ+MET channel is
the best current handle on this coupling.Comment: 36 pages, 16 figure