Gravitational positivity bounds

TL;DR

This paper investigates how positivity bounds are affected by massless gravitons in effective field theories, showing that violations are suppressed and dependent on Regge behavior, with implications for scalar-tensor theories.

Contribution

It demonstrates that positivity bounds can be relaxed in the presence of a massless graviton, with violations controlled by Regge behavior and suppressed by the Planck scale.

Findings

Positivity bounds are generally violated with a massless graviton.

Violations are suppressed by the Planck scale and Regge scale.

Bounds are applicable only when EFT cutoff is below the Reggeization scale.

Abstract

We study the validity of positivity bounds in the presence of a massless graviton, assuming the Regge behavior of the amplitude. Under this assumption, the problematic $t$-channel pole is canceled with the UV integral of the imaginary part of the amplitude in the dispersion relation, which gives rise to finite corrections to the positivity bounds. We find that low-energy effective field theories (EFT) with "wrong" sign are generically allowed. The allowed amount of the positivity violation is determined by the Regge behavior. This violation is suppressed by $M_{\rm pl}^{-2}\alpha'$ where $\alpha'$ is the scale of Reggeization. This implies that the positivity bounds can be applied only when the cutoff scale of EFT is much lower than the scale of Reggeization. We then obtain the positivity bounds on scalar-tensor EFT at one-loop level. Implications of our results on the degenerate higher-order scalar-tensor (DHOST) theory are also discussed.