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We study the cosmological collider signatures in the Higgs-$R^2$ inflationmodel. We consider two distinct types of signals: one originating from theinflaton coupling to Standard Model fermions and gauge bosons, and anotherarising from the isocurvature mode interaction with the inflaton. In the formercase, we determine that the signal magnitude is likely too small for detectionby upcoming probes, primarily due to suppression by both the Planck scale andslow-roll parameters. However, we provide a detailed computation of the signalwhich could be potentially applicable to various Higgs inflation variants. Forthe isocurvature mode signals, we observe that the associated couplings remainunsuppressed when the isocurvature mode is relatively light or comparable tothe inflationary scale. In this case, we study the Higgs-$R^2$ inflationparameter space that corresponds to the quasi-single-field inflation regime andfind that the signal strength could be as large as $|f_{\rm NL}| > 1$, makingHiggs-$R^2$ inflation a viable candidate for observation by future 21-cmsurveys.

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