Positivity Bounds and the Massless Spin-2 Pole

TL;DR

This paper investigates the limitations of applying standard positivity bounds to theories with massless spin-2 fields, revealing contradictions and proposing a modified bound that accounts for gravitational effects.

Contribution

It demonstrates that standard positivity bounds are generally inapplicable in gravitational theories with massless spin-2 fields and proposes a new modified bound.

Findings

Standard positivity bounds conflict with low-energy coupling constants in gravity theories.

Applying these bounds suggests new physics at unexpectedly low scales.

Issues with formulating a three-dimensional amplitude satisfying all physical properties.

Abstract

The presence of a massless spin-2 field in an effective field theory results in a $t$-channel pole in the scattering amplitudes that precludes the application of standard positivity bounds. Despite this, recent arguments based on compactification to three dimensions have suggested that positivity bounds may be applied to the $t$-channel pole subtracted amplitude. If correct this would have deep implications for UV physics and the Weak Gravity Conjecture. Within the context of a simple renormalizable field theory coupled to gravity we find that applying these arguments would constrain the low-energy coupling constants in a way which is incompatible with their actual values. This contradiction persists on deforming the theory. Further enforcing the $t$-channel pole subtracted positivity bounds on such generic renormalizable effective theories coupled to gravity would imply new physics at a scale parametrically smaller than expected, with far reaching implications. This suggests that generically the standard positivity bounds are inapplicable with gravity and we highlight a number of issues that impinge on the formulation of a three-dimensional amplitude which simultaneously satisfies the required properties of analyticity, positivity and crossing symmetry. We conjecture instead a modified bound that ought to be satisfied independently of the precise details of the high energy completion.