The Real EFThedron: A Moment Geometry for the Nonperturbative S-Matrix

Abstract

The S-matrix bootstrap restricts low-energy interactions through unitarity, analyticity and crossing symmetry. For effective field theory couplings, positivity bounds have a natural geometric formulation, the EFThedron, in which the allowed couplings form the classical positive moment problem. We show that this geometry extends to full nonlinear unitarity. Multiplying the amplitude by fractional powers of the energy turns its real part into a moment as well, so that unitarity becomes a bounded complex moment problem. Analyticity becomes a set of linear constraints requiring different kernels to reconstruct the same couplings, analogous to the null constraints imposed by crossing symmetry. In two dimensions the problem can be solved exactly, leading to Castillejo--Dalitz--Dyson amplitudes as extremal measures of the moment problem. In four dimensions we obtain closed-form and semi-analytic bounds for the massless EFThedron model and for massive scalar scattering. For massive scalars, the analyticity constraints improve on published dual bounds, reducing the gap to the primal region. By placing the real and imaginary parts of amplitudes on an equal footing, the Real EFThedron provides a powerful framework combining unitarity, analyticity and crossing, opening a new route to mapping the space of consistent quantum field theories.