Transgression to loop spaces and its inverse, III: Gerbes and thin fusion bundles

We show that the category of abelian gerbes over a smooth manifold is equivalent to a certain category of principal bundles over the free loop space. These principal bundles are equipped with fusion products and are equivariant with respect to thin homotopies between loops. The equivalence is established by a functor called regression, and complements a similar equivalence for bundles and gerbes equipped with connections, derived previously in Part II of this series of papers. The two equivalences provide a complete loop space formulation of the geometry of gerbes; functorial, monoidal, natural in the base manifold, and consistent with passing from the setting “with connections” to the one “without connections”. We discuss an application to lifting problems, which provides in particular loop space formulations of spin structures, complex spin structures, and spin connections.