Enhanced photon coupling of ALP dark matter adiabatically converted from the QCD axion

Understand

We revisit the adiabatic conversion between the QCD axion and axion-like particle (ALP) at level crossing, which can occur in the early universe as a result of the existence of a hypothetical mass mixing.

• This is similar to the Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations.
• After refining the conditions for the adiabatic conversion to occur, we focus on a scenario where the ALP produced by the adiabatic conversion of the QCD axion explains the observed dark matter abundance.
• Interestingly, we find that the ALP decay constant can be much smaller than the ordinary case in which the ALP is produced by the realignment mechanism.