Higher Spins and String Interactions

TL;DR

Massimo Taronna develops a generating-function framework to compute open-string tree-level amplitudes for the first Regge trajectory, extracting explicit 3-point and 4-point higher-spin couplings and associated currents. He demonstrates gauge-invariant structures in the massless limit, reveals off-shell current exchanges, and analyzes the high-energy behavior showing UV softness when summing an infinite tower of higher spins. The work connects string-derived cubic couplings to broader higher-spin theory ideas, including nonlocal interactions and unfolded formulations, and outlines paths toward nontrivial extensions to quartic order and alternative coupling schemes. Overall, the thesis offers a concrete, compact formalism for understanding how higher-spin dynamics arise in string theory and how they might inform broader higher-spin gauge theories.

Abstract

In this Thesis we address the issue of consistent higher-spin interactions taking String Theory as a "theoretical laboratory". We thus arrive at a simple expression encompassing all three-point amplitudes for states belonging to the first Regge trajectory of the open bosonic string, that are described by fully symmetric tensors, and we explicitly construct the corresponding effective action at the cubic level. A gauge invariant structure is seen to emerge in the massless limit, in line with long-held expectations for high-energy string scattering. We also study some properties of the field theory amplitudes that arise from our effective Lagrangian and involve the exchange of infinitely many higher-spin excitations. These results and other related developments will soon appear in a joint publication with A. Sagnotti