Unitarizing Higgs Inflation

TL;DR

The paper addresses the unitarity problem of Higgs inflation caused by a large non-minimal coupling to gravity, proposing a minimal UV completion with a real scalar $\sigma$ that unitarizes the theory up to $M_P$. Through a linear-sigma-model construction, they show that integrating out $\sigma$ below the cutoff recovers the original Higgs inflation, while the full theory remains well-behaved at and above the cutoff, with inflation driven along a flat direction involving both fields. The slow-roll predictions (e.g., $n_s \approx 0.966$, $r \approx 2.4\times10^{-3}$ for $N\approx60$) match those of Higgs inflation, but the Hubble rate and reheating can depend on $\sigma$-sector couplings, and the inflaton becomes a mixture of $\sigma$ and Higgs. Overall, the work provides a calculable, UV-complete framework that preserves Higgs-inflation phenomenology while resolving the unitarity concerns up to the Planck scale, with a COBE normalization fixing a high-scale vev $\Lambda \sim 10^{16}$ GeV.

Abstract

We consider a simple extension of the Standard Model Higgs inflation with one new real scalar field which preserves unitarity up to the Planck scale. The new scalar field (called sigma) completes in the ultraviolet the theory of Higgs inflation by linearizing the Higgs kinetic term in the Einstein frame, just as the non-linear sigma model is unitarized into its linear version. The unitarity cutoff of the effective theory, obtained by integrating out the sigma field, varies with the background value of the Higgs field. In our setup, both the Higgs field and the sigma field participate in the inflationary dynamics, following the flat direction of the potential. We obtain the same slow-roll parameters and spectral index as in the original Higgs inflation but we find that the Hubble rate during inflation depends not only on the Higgs self-coupling, but also on the unknown couplings of the sigma field.