Freeze-in Axion-like Dark Matter
  We present an interesting Higgs portal model where an axion-like particle
(ALP) couples to the Standard Model sector only via the Higgs field. The ALP
becomes stable due to CP invariance and turns out to be a natural candidate for
freeze-in dark matter because its properties are controlled by the perturbative
ALP shift symmetry. The portal coupling can be generated non-perturbatively by
a hidden confining gauge sector, or radiatively by new leptons charged under
the ALP shift symmetry. Such UV completions generally involve a CP violating
phase, which makes the ALP unstable and decay through mixing with the Higgs
boson, but can be sufficiently suppressed in a natural way by invoking
additional symmetries.
