Hierarchical Axiverse

Abstract

String compactifications generically produce ${\cal O}(100)$ light axion-like particles, yet low-energy mixing has not previously been exploited as a structuring constraint on their spectrum. We show that the requirement of well-defined QCD-induced mass mixing forces the axion masses into a hierarchically spaced ordering, while stochastic mixing organizes the decay constants into a hierarchically split distribution -- two populations separated by an essentially empty window. Both patterns are insensitive to ultraviolet input and satisfy the relevant Swampland constraints within standard large-volume compactifications. We term this emergent pattern the hierarchical axiverse.