  Light pseudoscalars interacting pre-dominantly with Standard Model gauge
bosons (so-called axion-like particles or ALPs) occur frequently in extensions
of the Standard Model. In this work we review and update existing constraints
on ALPs in the keV to GeV mass region from colliders, beam dump experiments and
astrophysics. We furthermore provide a detailed calculation of the expected
sensitivity of Belle II, which can search for visibly and invisibly decaying
ALPs, as well as long-lived ALPs. The Belle II sensitivity is found to be
substantially better than previously estimated, covering wide ranges of
relevant parameter space. In particular, Belle II can explore an interesting
class of dark matter models, in which ALPs mediate the interactions between the
Standard Model and dark matter. In these models, the relic abundance can be set
via resonant freeze-out, leading to a highly predictive scenario consistent
with all existing constraints but testable with single-photon searches at Belle
II in the near future.
