Eguchi-Hanson harmonic spinors revisited

Abstract

We revisit the problem of determining the zero modes of the Dirac operator on the Eguchi-Hanson space. It is well known that there are no normalisable zero modes, but such zero modes do appear when the Dirac operator is twisted by a $U\left(1\right)$ connection with $L^2$ normalisable curvature. The main novelty of our treatment is that we use the established formalism of spin-c spinors as complex differential forms, which makes the required calculations remarkably straightforward. In particular, to compute the Dirac operator we never need to compute the spin connection. As a result, we are able to reproduce the known normalisable zero modes of the twisted Eguchi-Hanson Dirac operator by relatively simple computations. We also collect various different descriptions of the Eguchi-Hanson space, including its construction as a hyperkähler quotient of $C^4$ with the flat metric. The latter illustrates the geometric origin of the connection with $L^2$ curvature used to twist the Dirac operator. To illustrate the power of the formalism developed, we generalise the results to the case of Dirac zero modes on the Ricci-flat Kähler manifolds obtained by applying Calabi's construction to the canonical bundle of $C{P}^n$.