Multiplicative dimensional reduction

We prove the multiplicative version of the dimensional reduction theorem in cohomological Donaldson–Thomas theory. More precisely, we show that the BPS cohomology associated with the loop stack of a $0$-shifted symplectic stack admits a description analogous to orbifold cohomology, even though our stacks are not necessarily Deligne–Mumford. As an application, we propose a new, purely two-dimensional formulation of the topological mirror symmetry conjecture for the moduli space of $G$-Higgs bundles, which in turn leads to a formulation of the conjecture for logarithmic $G$-Higgs bundles. We also investigate a twisted version of the multiplicative dimensional reduction, which applies, in particular, to the cohomological Donaldson–Thomas theory for $S^1$-bundles over compact oriented surfaces, and more generally to Seifert-fibred $3$-manifolds.