Cosmological Collider Signatures of Higgs-$R^2$ Inflation

TL;DR

This paper investigates potential cosmological collider signatures in Higgs-$R^2$ inflation, analyzing signals from inflaton interactions with Standard Model particles and isocurvature modes, highlighting their detectability prospects.

Contribution

It provides a detailed analysis of collider signatures in Higgs-$R^2$ inflation, including computations of signal magnitudes and conditions for potential observability.

Findings

Inflaton coupling signals are likely too small for upcoming detection.

Isocurvature mode signals can be significant if the mode is light or comparable to inflation scale.

Higgs-$R^2$ inflation could produce observable non-Gaussianity signals in future 21-cm surveys.

Abstract

We study the cosmological collider signatures in the Higgs-$R^2$ inflation model. We consider two distinct types of signals: one originating from the inflaton coupling to Standard Model fermions and gauge bosons, and another arising from the isocurvature mode interaction with the inflaton. In the former case, we determine that the signal magnitude is likely too small for detection by upcoming probes, primarily due to suppression by both the Planck scale and slow-roll parameters. However, we provide a detailed computation of the signal which could be potentially applicable to various Higgs inflation variants. For the isocurvature mode signals, we observe that the associated couplings remain unsuppressed when the isocurvature mode is relatively light or comparable to the inflationary scale. In this case, we study the Higgs-$R^2$ inflation parameter space that corresponds to the quasi-single-field inflation regime and find that the signal strength could be as large as $|f_{\rm NL}| > 1$, making Higgs-$R^2$ inflation a viable candidate for observation by future 21-cm surveys.