Freeze-in Axion-like Dark Matter

Understand

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.