# Higgs-$R^2$ inflation -- full slow-roll study at tree-level

## TL;DR

This paper performs a comprehensive slow-roll analysis of Higgs inflation with an added $\alpha R^2$ term, revealing conditions under which Higgs-like inflation is possible and characterizing its observational predictions.

## Contribution

It provides the first full slow-roll study of Higgs-$R^2$ inflation at tree-level, identifying viable inflationary solutions and their observational signatures.

## Key findings

- Pure Higgs inflation is impossible with the $R^2$ term present.
- Higgs-like inflation can occur with predictions similar to standard Higgs inflation.
- Tensor-to-scalar ratio ranges from $3.8\times10^{-3}$ to 0.079, consistent with observations.

## Abstract

We consider Higgs inflation with an $\alpha R^2$ term. It adds a new scalar degree of freedom, which leads to a two-field model of inflation. We do a complete slow-roll analysis of the three-dimensional parameter space of the $R^2$ coefficient $\alpha$, the non-minimal coupling $\xi$ and the Higgs self-coupling $\lambda$. We find three classes of inflationary solutions, but only pure $R^2$ and attractor solutions fit observations. We find that pure Higgs inflation is impossible when the $R^2$ term is present regardless of how small $\alpha$ is. However, we can have Higgs-like inflation, where the amplitude of the perturbations does not depend on $\alpha$ and the predictions as a function of e-folds are the same as in Higgs inflation, although the inflationary trajectory is curved in field space. The spectral index is $0.939 < n_R < 0.967$, and constraining it to the observed range, the tensor-to-scalar ratio varies from $3.8\times10^{-3}$ to the maximum allowed by observations, $0.079$. Observational constraints on isocurvature perturbations contribute to these limits, whereas non-Gaussianity is automatically in the range allowed by observations.