Exploring Inelasticity in the S-Matrix Bootstrap

TL;DR

This paper investigates the role of inelasticity in the S-Matrix Bootstrap, showing that incorporating inelastic effects leads to stronger bounds on low-energy scattering amplitudes beyond the elastic approximation.

Contribution

It introduces a method to include inelasticity in the S-Matrix Bootstrap, extending the primal approach and establishing a dual formulation in two dimensions.

Findings

Inelasticity inclusion results in tighter bounds on low-energy observables.

Solutions saturating bounds are less elastic when inelasticity is considered.

Stronger bounds are observed compared to the standard elastic-only setup.

Abstract

The modern S-Matrix Bootstrap provides non-perturbative bounds on low-energy aspects of scattering amplitudes, leveraging the constraints of unitarity, analyticity and crossing. Typically, the solutions saturating such bounds also saturate the unitarity constraint as much as possible, meaning that they are almost exclusively elastic. This is expected to be unphysical in $d>2$ because of Aks' theorem. We explore this issue by adding inelasticity as an additional input, both using a primal approach in general dimensions which extends the usual ansatz, and establishing a dual formulation in the 2d case. We then measure the effects on the low-energy observables where we observe stronger bounds than in the standard setup.