Bootstrapping Extremal Scalar Amplitudes With and Without Supersymmetry

TL;DR

This paper explores how positivity bounds and extremal amplitudes can constrain the spectrum of UV theories, revealing that maximal supersymmetry might determine the entire space of consistent four-point scalar amplitudes.

Contribution

It introduces a convex hull approach to characterize the space of UV amplitudes and suggests that maximal supersymmetry can fully determine the allowed amplitude space.

Findings

The amplitude space is a convex hull of massive and extremal scalarless amplitudes.

Maximal supersymmetry may suffice to determine all consistent four-point amplitudes.

Spectrum input narrows the allowed Wilson coefficients to small regions near string theory amplitudes.

Abstract

We re-examine positivity bounds on the $2\to2$ scattering of identical massless real scalars with a novel perspective on how these bounds can be used to constrain the spectrum of UV theories. We propose that the entire space of consistent weakly-coupled (and generically non-supersymmetric) UV amplitudes is determined as a convex hull of the massive scalar amplitude and a one-parameter family of scalarless "extremal amplitudes" parameterized by the ratio of the masses of the two lightest massive states. Further, we propose that the extremal amplitudes can be constructed from a similar one-parameter set of maximally supersymmetric amplitudes, leading to the surprising possibility that the S-matrix bootstrap with maximal supersymmetry may be sufficient to determine the entire allowed space of four-point amplitudes! Finally, we show that minimal spectrum input reduces the allowed space of Wilson coefficients to small islands around the open string Dirac-Born-Infeld tree amplitude and the closed string Virasoro-Shapiro amplitude.