Smooth 2-Group Extensions and Symmetries of Bundle Gerbes

We study bundle gerbes on manifolds M that carry an action of a connected Lie group G. We show that these data give rise to a smooth 2-group extension of G by the smooth 2-group of hermitean line bundles on M. This 2-group extension classifies equivariant structures on the bundle gerbe, and its non-triviality poses an obstruction to the existence of equivariant structures. We present a new global approach to the parallel transport of a bundle gerbe with connection, and use it to give an alternative construction of this smooth 2-group extension in terms of a homotopy-coherent version of the associated bundle construction. We apply our results to give new descriptions of nonassociative magnetic translations in quantum mechanics and the Faddeev-Mickelsson-Shatashvili anomaly in quantum field theory. We also propose a definition of smooth string 2-group models within our geometric framework. Starting from a basic gerbe on a compact simply-connected Lie group G, we prove that the smooth 2-group extensions of G arising from our construction provide new models for the string group of G.

Automated summary

The study explores the relationship between bundle gerbes on manifolds carrying an action of a connected Lie group G and the smooth 2-group extensions, and applies the results to new descriptions in quantum mechanics and quantum field theory. Additionally, a definition of smooth string 2-group models within the geometric framework is proposed. Starting from a basic gerbe on a compact simply-connected Lie group G, the construction provides new models for the string group of G.