Surprises in Open-String Perturbation Theory

TL;DR

This paper surveys surprising features of open-string perturbation theory, emphasizing how world-sheet boundaries and orientifolds generate novel spectra and non-perturbative structures. It details the open-descendant construction via Klein bottle, annulus, and Möbius strip amplitudes and the required tadpole cancellations that fix Chan-Paton data. A key highlight is the generalized Green-Schwarz mechanism in six dimensions, with multiple tensor multiplets and tensionless-string phenomena that illuminate duality with heterotic models. The discussion also reveals enhanced boundary-state symmetries in rational CFT open descendants, driven by simple currents and invariant structure, enriching the open-sector dynamics beyond naive bulk symmetry expectations.

Abstract

The perturbative analysis of models of open and closed superstrings presents a number of surprises. For instance, variable numbers of antisymmetric tensors ensure their consistency via generalized Green-Schwarz cancellations and a novel type of singularity occurs in their moduli spaces. All these features are related, in one way or another, to the presence of boundaries on the world sheet or, equivalently, of extended objects (branes) interacting with the bulk theory in space time. String dualities have largely widened the interest in these models, that exhibit a wealth of generic non-perturbative features of String Theory.