Appears in collection : 2023 - T1A - WS1 - Quantum gravity and random geometry

Magnetic amplitudes for strings in a background 3-form H involve a 2-form potential B integrated on surfaces of arbitrary topology. We propose a random matrix model whose topological expansion lead to a discretized version of these amplitudes, in the case of a propagation on a finite space X quotiented by a finite group G. Besides the fluxes induced by B, they also involve topological defects on the surface, weighted by additional fields that are constructed using the group cohomology of G.