String loops and gravitational positivity bounds: imprint of light particles at high energies

TL;DR

This paper investigates how light particles affect gravitational positivity bounds in effective field theories, revealing a natural cancellation mechanism at high energies that explains potential violations.

Contribution

It introduces a string-inspired model demonstrating how high-energy loop contributions cancel low-energy enhancements, clarifying positivity violations in gravitational theories.

Findings

High-energy loops can cancel low-energy negative contributions.

Infrared sensitivity persists even at high energies.

A natural cancellation mechanism explains positivity violations.

Abstract

We study loop corrections to positivity bounds on effective field theories in the context of $2\to 2$ scattering in gravitational theories, in the presence of light particles. It has been observed that certain negative contributions at low energies are enhanced by inverse powers of a small mass $m$ and are nontrivial to cancel against other low-energy contributions. These originate from near the forward limit of diagrams involving graviton exchange. We observe that scattering in this kinematics domain remains infrared-sensitive even at high center-of-mass energy. By considering a string-inspired model in which high-energy loops can be calculated using unitarity and Regge behavior of tree amplitudes, we uncover a natural mechanism through which $1/m$-enhanced terms perfectly cancel between low and high energy contributions. This concretely explains possible positivity violations in the presence of gravity from the high-energy viewpoint.