Axion Mixing in the String Axiverse

Abstract: The string axiverse presents a fascinating and complex landscape for axion physics. In this work, we study axion mass mixing within the type IIB string axiverse, focusing specifically on the LARGE volume scenario (LVS) axiverse. The LVS axiverse necessitates at least two axions to ensure the presence of both a QCD axion candidate and at least one additional axion-like particle (ALP). This study presents the first systematic exploration of axion mass mixing in scenarios involving more than two axions. The 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 simultaneously 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. Our work contributes to the understanding of axion physics in string theory.