The UV Sensitivity of Axion Monodromy Inflation

Abstract

We revisit axion monodromy inflation in the context of UV-inspired models and point out that its cosmological observables are sensitive to heavy fields with masses far above the Hubble scale, such as the moduli of flux compactifications. By studying a string-inspired two-field extension of axion monodromy with a small turning rate, we reveal that the oscillatory modulation of the axion potential leads to continuous excitation of heavy fields during inflation when the modulation frequency exceeds the field masses. This finding challenges the conventional single-field description, heavy moduli cannot be simply integrated out. Using a full bootstrap analysis, we demonstrate that this mechanism produces cosmological collider signals that bypass the usual Boltzmann suppression for heavy masses. Specifically, we identify detectably large signatures of heavy moduli in the primordial bispectrum, offering a promising avenue for probing high-energy physics through cosmological observations.

Figures (3)

• Figure 1: A sketch of the axion monodromy potential with a heavy modulus field. The orange curve corresponds to the background trajectory of the inflaton with oscillations driven by the axion periodic modulation (the wiggles have been enhanced for visibility).
• Figure 2: The Feynman diagrams with leading resonance contributions to the $\zeta$ power spectrum and bispectrum. The purple dots denote vertices with oscillating couplings.
• Figure 3: Oscillatory pattern in the squeezed bispectrum for $\alpha = 30$ and $\mu=10$ (which has $E_1^P=0.016$ and $E_1^B=0.128$). The shape has both the scale-invariant collider signals perpendicular to the $k_3/k_1={\rm const.}$ lines and scale-dependent oscillations along the $k_3/k_1={\rm const.}$ lines, which is significantly distinct from standard resonant/collider templates.