Photon-photon scattering from a UV-complete gravity QFT

Abstract: Quantum quadratic gravity (QQG) produces a tree-level differential cross section for $\gamma\gamma\to\gamma\gamma$ that is well-behaved at all energies. From this we can study how the corrections to low energy scattering amplitudes are related to the UV physics, in particular to the exchange of the massive graviparticles. An effective forward scattering amplitude is obtained by separating out the effects of the $t$-channel graviton pole. This is possible due to the UV-completeness, and even though the Froissart bound is not satisfied. We then consider photon-photon scattering to two graviparticles and a further imaginary contribution to the $\gamma\gamma\to\gamma\gamma$ forward scattering amplitude. Unitarity without positivity is a key property of QQG and it impacts all our results.