Geometric stabilisation via p-adic integration

In this article we give a new proof of Ngô's geometric stabilisation theorem, which implies the fundamental lemma. This is a statement which relates the cohomology of Hitchin fibres for a quasi-split reductive group scheme G to the cohomology of Hitchin fibres for the endoscopy groups H_{\kappa }. Our proof avoids the decomposition and support theorem, instead the argument is based on results for p-adic integration on coarse moduli spaces of Deligne-Mumford stacks. Along the way we establish a description of the inertia stack of the (anisotropic) moduli stack of G-Higgs bundles in terms of endoscopic data, and extend duality for generic Hitchin fibres of Langlands dual group schemes to the quasi-split case.