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Astroparticle Physics
Abstract:
We study the adiabatic conversion between the QCD axion and axion-like particle (ALP) at level crossing which occurs when their masses become close to each other in the early universe. This is similar to the Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations. After clarifying 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. As a consequence, the ALP-photon coupling is enhanced by a few orders of magnitude, which is advantageous for the on-going and future axion search experiments using the axion-photon coupling.
