M5-branes on S^2 x M_4: Nahm's Equations and 4d Topological Sigma-models

TL;DR

This paper explores the reduction of 6d N=(0,2) superconformal theory on S^2 x M_4, establishing a correspondence with 2d N=(0,2) gauge theories and topological sigma-models into Nahm's equations moduli space, revealing new geometric insights.

Contribution

It derives the dimensional reduction of the 6d theory to a 4d sigma-model into Nahm's equations moduli space, connecting M5-branes, gauge theories, and topological sigma-models in a novel way.

Findings

Reduction from 6d to 4d yields a sigma-model into Nahm's equations moduli space.

For hyper-Kahler M_4, the reduction produces a topological sigma-model with tri-holomorphic maps.

Target space for k=2 is the Atiyah-Hitchin manifold, leading to a twisted topological sigma-model.

Abstract

We study the 6d N=(0,2) superconformal field theory, which describes multiple M5-branes, on the product space S^2 x M_4, and suggest a correspondence between a 2d N=(0,2) half-twisted gauge theory on S^2 and a topological sigma-model on the four-manifold M_4. To set up this correspondence, we determine in this paper the dimensional reduction of the 6d N=(0,2) theory on a two-sphere and derive that the four-dimensional theory is a sigma-model into the moduli space of solutions to Nahm's equations, or equivalently the moduli space of k-centered SU(2) monopoles, where k is the number of M5-branes. We proceed in three steps: we reduce the 6d abelian theory to a 5d Super-Yang-Mills theory on I x M_4, with I an interval, then non-abelianize the 5d theory and finally reduce this to 4d. In the special case, when M_4 is a Hyper-Kahler manifold, we show that the dimensional reduction gives rise to a topological sigma-model based on tri-holomorphic maps. Deriving the theory on a general M_4 requires knowledge of the metric of the target space. For k=2 the target space is the Atiyah-Hitchin manifold and we twist the theory to obtain a topological sigma-model, which has both scalar fields and self-dual two-forms.