Axionic Defect Anomalies and their Cancellation

TL;DR

This paper explores anomaly cancellation mechanisms for axion strings and p-brane defects, presenting a simplified derivation of the Callan-Harvey-Naculich effect and proposing a Green-Schwarz-based alternative that applies broadly.

Contribution

It introduces a new perspective on anomaly cancellation for axion strings and general p-brane defects, extending the Green-Schwarz mechanism to these objects.

Findings

The Callan-Harvey-Naculich effect can be derived simply.

An alternative anomaly cancellation mechanism is possible without reference to the surrounding space.

Anomaly-free effective actions exist for p-brane defects in anomaly-free spacetimes.

Abstract

We present a simple derivation of the Callan-Harvey-Naculich effect, {\it i.e.} the compensation of charge violation on axion strings due to gauge anomalies by accretion of charge onto the string from the surrounding space. We then show, in the case of axion fields without a potential, that an alternative explanation is possible in which no reference to the surrounding space is necessary because the anomalies are cancelled by a version of the Green-Schwarz mechanism. We prove that such an alternative explanation is always possible in the more general context of p-brane defects in d-dimensional field theories, and hence that there always exists an anomaly-free effective worldvolume action whenever the spacetime theory is anomaly free. Our results have implications, which we discuss, for heterotic and type II fivebranes.