Relative entropy in scattering and the S-matrix bootstrap

Understand

We consider entanglement measures in 2-2 scattering in quantum field theories, focusing on relative entropy which distinguishes two different density matrices.

• Relative entropy is investigated in several cases which include $\phi^4$ theory, chiral perturbation theory ($\chi PT$) describing pion scattering and dilaton scattering in type II superstring theory.
• We derive a high energy bound on the relative entropy using known bounds on the elastic differential cross-sections in massive QFTs.
• In $\chi PT$, relative entropy close to threshold has simple expressions in terms of ratios of scattering lengths.