Natural Inflation and Low Energy Supersymmetry

TL;DR

Kappl, Nilles, and Winkler show that natural (axionic) inflation at high scales can coexist with low-energy supersymmetry by employing a two-axion alignment mechanism drawn from string moduli. They analyze both supergravity embeddings and string-inspired constructions, addressing moduli stabilization and the saxion problem, and present a concrete model where near-alignment yields an effectively trans-Planckian axion decay constant. The resulting inflationary dynamics produce observable tensor modes with predictions for n_s and r compatible with current data, while heavy-modulus effects can further tune the spectrum. This work provides a UV-complete pathway for TeV-scale SUSY to be compatible with high-scale inflation, with testable cosmological signatures.

Abstract

Natural (axionic) inflation provides a well-motivated and predictive scheme for the description of the early universe. It leads to sizeable primordial tensor modes and thus a high mass scale of the inflationary potential. Naively this seems to be at odds with low (TeV) scale supersymmetry, especially when embedded in superstring theory. We show that low scale supersymmetry is compatible with natural (high scale) inflation. The mechanism requires the presence of two axions that are provided through the moduli of string theory.

Figures (4)

• Figure 1: Comparison of the scalar potentials \ref{['eq:coscosh']} and \ref{['eq:modpot']} arising from a quadratic Kähler potential (left panel) and from a logarithmic Kähler potential (right panel). In the logarithmic case, the saxion $\chi$ is only protected by a small barrier from runaway.
• Figure 2: Potential \ref{['eq:axsax']} in axion and saxion direction. The inflationary trajectory (last 60 e-folds) is indicated by the gray line. Observe that we have stretched the scale of $\chi$ compared to $\varphi$. The valley is in fact much narrower than it appears in the figure.
• Figure 3: Axion (left) and saxion (right) displacement as a function of time. Also shown is the corresponding number of e-folds. During inflation, the saxion is frozen at a non-vanishing field value by a large Hubble mass term.
• Figure 4: Combined Planck, BICEP2/Keck Array, BAO constraints on the spectral index and the tensor-to-scalar ratio Ade:2015lrj. Predictions of natural inflation with and without including flattening effects (see text) are also shown. The upper and lower ends of the bands refer to 50 and 60 e-folds respectively.