  Axion-like particles (ALPs) and heavy neutral leptons (HNLs) are both
well-motivated extensions of the Standard Model. As ALPs couple to on-shell
fermions proportionally to their masses, processes involving both types of
particles may give rise, for TeV HNLs, to relevant phenomenology at colliders.
In this work, we point out a particularly clean process, whose final state
consists of four jets and two charged leptons, and we estimate its current and
future sensitivity at the LHC. For on-shell HNLs, i) there is no dependence on
the overall scale of the mixing between HNLs and the active neutrinos, making
this process sensitive down to the type-I Seesaw line; ii) the signal strength
of the process is sizable only as far as the HNL masses are below a few TeVs,
contrary to what the proportionality to masses of the ALP couplings may
suggest. Although ALPs and HNLs have been mainly studied independently in the
literature, considering their interplay may lead to joint limits that are much
stronger than those on the individual particles taken separately. This concise
study paves the way for similar searches at colliders and other experiments.
