Large Nongaussianity in Axion Inflation

TL;DR

This work examines axion-driven inflation with a universal axion-gauge coupling $\frac{\alpha}{f} \phi F^{\mu\nu}\tilde{F}_{\mu\nu}$ and shows that gauge-field production can dominate inflaton fluctuations when the decay constant is sub-Planckian, leading to large and observable non-Gaussianity. By computing the inverse-decay contribution to the inflaton perturbations, the authors derive a modified power spectrum and a markedly enhanced equilateral $f_{NL}$, along with tensor-mode implications, and discuss backreaction constraints. The results hold across several UV-mensible models (N-flation, axion monodromy, and related constructions), where $f \lesssim M_p$ is natural, implying that large nongaussianity is a generic feature of minimal axion inflation. These findings place meaningful bounds on the inflaton-gauge coupling and provide distinctive observational signatures for Planck-era CMB and LSS analyses. The work highlights a robust mechanism by which sub-Planckian axion models can produce testable predictions for primordial non-Gaussianity and gravitational waves.

Abstract

The inflationary paradigm has enjoyed phenomenological success, however, a compelling particle physics realization is still lacking. The key obstruction is that the requirement of a suitably flat scalar potential is sensitive to Ultra-Violet (UV) physics. Axions are among the best-motivated inflaton candidates, since the flatness of their potential is naturally protected by a shift symmetry. We re-consider the cosmological perturbations in axion inflation, consistently accounting for the coupling to gauge fields φF \tilde{F}, which is generically present in these models. This coupling leads to production of gauge quanta, which provide a new source of inflaton fluctuations, δφ. For an axion decay constant < 10^{-2} M_p, this effect typically dominates over the standard fluctuations from the vacuum, and saturates the current observational bounds on nongaussianity of the CMB anisotropies. Since sub-Planckian values of the decay constant are typical for concrete realizations that admit a UV completion (such as N-flation and axion monodromy), we conclude that large nongaussianity is easily obtained in very minimal and natural realizations of inflation.

Figures (2)

• Figure 1: Values of parameters leading to the observed COBE normalization of the power spectrum (red line), and reference values for the nongaussianity parameter $f_{NL}^{\mathrm{equil}}=10,266,8000$ along this curve. See the main text for details.
• Figure 2: Observational predictions for the large-field power-law inflation model (\ref{['power_law']}) with $p=1,2$ and assuming $N_e\cong 60$. The spectral index is $n_s=0.975,0.967$ for $p=1,2$. At small $f/\alpha$ the coupling of $\phi$ to $F\tilde{F}$ is stronger and nongaussianity is large. The tensor-to-scalar ratio decreases at strong coupling; however, the decrease is important only at values of $f/\alpha$ which are ruled out by the current bound on $f_{NL}^{\mathrm{equil}}$.