Axion Mixing in the String Axiverse

Abstract

String axiverse provides a fascinating and complex landscape for axion physics. The requirement in type IIB string axiverse models necessitates at least two axions to ensure the presence of both a QCD axion candidate and an additional axion-like particle (ALP). In this work, we study axion mass mixing and adopt a bottom-up perspective to investigate what conditions an axion model must satisfy in order to exhibit maximal mixing -- the scenario where the degree of effective mixing is maximized. We find that maximal mixing occurs when the masses of all ALPs are smaller than the zero-temperature mass of the QCD axion, with no two ALP masses being equal, and when the decay constants of all ALPs are uniformly either smaller or larger than the decay constant of the QCD axion. Additionally, the transfer of axion energy density ultimately takes place only between the two axions with the closest masses. These findings provide critical insights into axion dynamics not only within type IIB string axiverse models but also in broader multi-axion mixing frameworks. The potential cosmological implications of axion mass mixing are also addressed at the end.