Phenomenology of a Pseudo-Scalar Inflaton: Naturally Large Nongaussianity

TL;DR

The paper shows that a pseudo-scalar inflaton coupled to a gauge field via φ F~F can produce copious gauge-field quanta during inflation, which in turn source inflaton fluctuations through inverse decay. This mechanism yields a highly non-Gaussian curvature perturbation with an equilateral-like bispectrum, while maintaining standard large-field predictions for n_s and r in many regimes. The non-Gaussian signal grows exponentially with the coupling α/f, allowing current COBE/WMAP bounds to tightly constrain the coupling. The work further surveys a range of axion-inflation models (Natural Inflation, N-flation, axion monodromy, etc.) and discusses how inverse-decay NG provides a powerful diagnostic for these scenarios and their UV completions.

Abstract

Many controlled realizations of chaotic inflation employ pseudo-scalar axions. Pseudo-scalars φare naturally coupled to gauge fields through c φF \tilde{F}. In the presence of this coupling, gauge field quanta are copiously produced by the rolling inflaton. The produced gauge quanta, in turn, source inflaton fluctuations via inverse decay. These new cosmological perturbations add incoherently with the "vacuum" perturbations, and are highly nongaussian. This provides a natural mechanism to generate large nongaussianity in single or multi field slow-roll inflation. The resulting phenomenological signatures are highly distinctive: large nongaussianity of (nearly) equilateral shape, in addition to detectably large values of both the scalar spectral tilt and tensor-to-scalar ratio (both being typical of large field inflation). The WMAP bound on nongaussianity implies that the coupling, c, of the pseudo-scalar inflaton to any gauge field must be smaller than about 10^{2} M_p^{-1}.

Figures (7)

• Figure 1: Exact evaluation and large argument approximation of the function $f_2$.
• Figure 2: Exact evaluation and large argument approximation of the function $f_3\left( \xi ;\, 1 ,\, 1 \right)$.
• Figure 3: Exact evaluation and large argument approximation of the function $f_{h,L} \left( \xi \right)$.
• Figure 4: 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 5: Value of $f_{\rm NL}^{\mathrm{equil}} \left( \xi \right)$ due to the perturbations $\zeta$ sourced by the gauge quanta. Also shown is the $95 \%$ CL upper bound from WMAP 7 wmap7.