Field redefinitions, perturbative unitarity and Higgs inflation

TL;DR

This paper investigates the energy scale at which perturbation theory fails in Higgs inflation models, introducing a gauge-independent method to estimate the cutoff and comparing metric and Palatini formulations.

Contribution

It presents a novel, amplitude-calculation-free approach to determine the cutoff scale, confirming higher cutoffs during inflation and extending analysis to Palatini gravity.

Findings

Cutoff is higher during inflation than in vacuum.

Method is gauge-independent and does not require amplitude calculations.

Results are consistent with previous studies.

Abstract

For inflation driven by the Higgs field coupled non-minimally to gravity, we study the cutoff energy scale above which perturbation theory breaks down. Employing the metric formulation, we first give an overview of known results and then provide a new way to calculate a lower bound on the cutoff. Our approach neither relies on a gauge choice nor does it require any calculation of amplitudes. Instead, it exploits the fact that the S-matrix is invariant under field redefinitions. In agreement with previous findings, we demonstrate that the cutoff is significantly higher during inflation than in vacuum, which ensures the robustness of semi-classical predictions. Along the way, we generalize our findings to the Palatini formulation and comment on a useful parametrization of the Higgs doublet in both scenarios.