Branes within Branes

TL;DR

This paper explores the topological and physical relationships between D-branes and instantons in superstring theory, revealing that instantons can be viewed as bound states of lower-dimensional branes, with implications for moduli space descriptions.

Contribution

It demonstrates that instantons in internal gauge theories on D-branes are equivalent to D-branes themselves, providing new insights into brane bound states and instanton moduli spaces.

Findings

Instantons correspond to bound states of D-branes.

Zero size instantons are equivalent to lower-dimensional branes.

The framework generalizes to U(N) instantons in type II theories.

Abstract

We discuss a set of universal couplings between superstring Ramond-Ramond gauge fields and the gauge fields internal to D-branes, with emphasis on their topological consequences, and argue that instanton solutions in these internal theories are equivalent to D-branes. A particular example is the Dirichlet 5-brane in type I theory, which Witten recently showed is the zero size limit of an SO(32) instanton. Its effective world-volume theory is an Sp(1) gauge theory, unbroken in the zero size limit. We show that the zero size limit of an instanton in this theory is a 1-brane, which can be described as a bound state of the Dirichlet 1-brane with the 5-brane. Considering several 1 and 5-branes provides a description of moduli spaces of Sp(N) instantons, and a type II generalization is given which should describe U(N) instantons.