Unitarity bounds on low scale quantum gravity

TL;DR

This paper investigates the unitarity constraints on low scale quantum gravity models, revealing that their first detectable signals at colliders are linked to unitarity restoration mechanisms rather than gravitational phenomena.

Contribution

It provides a detailed analysis of unitarity bounds in low scale quantum gravity models and explores the implications for collider signals and inflation with non-minimally coupled scalars.

Findings

Models with low scale quantum gravity face unitarity issues below the strong gravity scale.

The first collider signals are related to unitarity restoration, not gravity itself.

A scalar with large non-minimal coupling can lower the effective Planck scale to TeV energies.

Abstract

We study the unitarity of models with low scale quantum gravity both in four dimensions and in models with a large extra-dimensional volume. We find that models with low scale quantum gravity have problems with unitarity below the scale at which gravity becomes strong. An important consequence of our work is that their first signal at the Large Hadron Collider would not be of a gravitational nature such as graviton emission or small black holes, but rather linked to the mechanism which fixes the unitarity problem. We also study models with scalar fields with non minimal couplings to the Ricci scalar. We consider the strength of gravity in these models and study the consequences for inflation models with non-minimally coupled scalar fields. We show that a single scalar field with a large non-minimal coupling can lower the Planck mass in the TeV region. In that model, it is possible to lower the scale at which gravity becomes strong down to 14 TeV without violating unitarity below that scale.