Abstract
We study implications of perturbative unitarity for quasi-single field inflation with the inflaton and one massive scalar. Analyzing high energy scattering, we show that non-Gaussianities with |fNL| ≳ 1 cannot be realized without turning on interactions which violate unitarity at a high energy scale. Then, we provide a relation between fNL and the scale of new physics that is required for UV completion. In particular we find that for the Hubble scale H ≳ × 109 GeV, Planck suppressed operators can easily generate too large non-Gaussanities and so it is hard to realize successful quasi-single field inflation without introducing a mechanism to suppress quantum gravity corrections. Also we generalize the analysis to the regime where the isocurvature mode is heavy and the inflationary dynamics is captured by the inflaton effective theory. Requiring perturbative unitarity of the two-scalar UV models with the inflaton and one heavy scalar, we clarify the parameter space of the P(X, ϕ) model which is UV completable by a single heavy scalar.
Highlights
The Higgs trilinear coupling is no more free parameter once the Higgs vev and the Higgs mass are specified
If we focus on two-derivative interactions, perturbative unitarity requires flatness of the field space in the UV complete theory, which in turn specifies the scale of new physics to be the curvature scale of the SMEFT field space [24]
If the radial direction has a mass near the Hubble scale, the dynamics of primordial fluctuations is governed by the inflaton fluctuations and the isocurvature modes with the Hubble scale mass
Summary
We review quasi-single field inflation [25] with an emphasis on typical energy scales of the model and their relations to primordial non-Gaussianities. After summarizing phenomenological aspects of the model, we demonstrate that in the two-field quasi-single field inflation, the nonlinearity parameter fNL is too small to observe if its main source is renormalizable interactions. This motivates us to study implications of perturbative unitarity
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