Unitarization of $R + \alpha R^2$ gravity

TL;DR

This paper applies an improved K-matrix algorithm to unitarize $R+eta R^2$ gravity amplitudes, clarifying the nature of scalar resonances and addressing infrared divergence issues in the Starobisnsky model.

Contribution

It introduces a method to properly unitarize gravity amplitudes with infrared divergences, confirming the physicality of certain resonances and dismissing artifacts.

Findings

Confirmed the existence of a genuine scalar resonance in the model.

Identified the apparent resonance as an artifact caused by infrared regulation.

Analyzed the presence of other dynamical resonances in the amplitudes.

Abstract

We make use of the improved K-matrix algorithm to obtain unitarized amplitudes in $R+\alpha R^2$ gravity (the so-called Starobisnsky model, of cosmological relevance). The procedure is of some complexity because infrared divergences are present and need to be properly regulated. We focus on the behaviour of a bona fide scalar resonance, known to exist in this model, and compare it to an apparent resonance detected in previous studies, thus confirming that the latter seems to be an artifact due to the introduction of the infrared regulator. We analyze the existence of other dynamical resonances and dwell on the amplitudes made unitary by this procedure.