Axion Free-kick Misalignment Mechanism

Abstract: We propose an alternative scenario for the axion misalignment mechanism based on the nontrivial interplay between the axion and a light dilaton in the early universe. Dark matter abundance is still sourced by the initial misalignment of the axion field, whose motion along the potential kicks the dilaton field away from its minimum, and dilaton starts to oscillate later with a delayed onset time for oscillation and a relatively large misalignment value due to the kick; eventually the dilaton dominates over the axion in their energy densities, and the dilaton is identified as dark matter. The kick effect due to axion motion is the most significant if the initial field value of dilaton is near its minimum; therefore, we call this scenario axion free-kick misalignment mechanism, where axion plays the role similar to a football player. Dark matter abundance can be obtained with a lower axion decay constant compared to the conventional misalignment mechanism.