Department Colloquium | The Bootstrap Program for the Strong Force

In the 1960s, the dominant approach to the strong interaction was the S-matrix bootstrap: the idea that the hadronic spectrum and scattering amplitudes could be determined from the general principles of causality and unitarity. This program culminated in the Veneziano amplitude which gave birth to string theory, but was abandoned as an approach to the strong force after the identification of Quantum Chromodynamics (QCD) as the microscopic theory of hadron physics. Yet QCD at low energies remains largely unsolved. I will describe how modern bootstrap methods, powered new theoretical insights and computational techniques, allow us to revisit this classic program with unprecedented rigor. Consistency of pion scattering — with minimal assumptions about the lightest resonances — leads to the emergence of Regge trajectories from the bootstrap bounds. The bootstrap approach becomes particularly sharp in the limit of a large number of colors. The low-lying spectrum of the extremal solutions shows a tantalizing, and still somewhat mysterious, quantitative proximity to the real-world meson masses. I will discuss what we are learning from these results and outline open questions on the path toward a bootstrap solution of large N QCD.