Supersymmetric partition functions and the three-dimensional A-twist

We study three-dimensional $mathcal{N}=2$ supersymmetric gauge theories on $mathcal{M}_{g,p}$, an oriented circle bundle of degree $p$ over a closed Riemann surface, $Sigma_g$. We compute the $mathcal{M}_{g,p}$ supersymmetric partition function and correlation functions of supersymmetric loop operators. This uncovers interesting relations between observables on manifolds of different topologies. In particular, the familiar supersymmetric partition function on the round $S^3$ can be understood as the expectation value of a so-called fibering operator on $S^2 times S^1$ with a topological twist. More generally, we show that the 3d $mathcal{N}=2$ supersymmetric partition functions (and supersymmetric Wilson loop correlation functions) on $mathcal{M}_{g,p}$ are fully determined by the two-dimensional A-twisted topological field theory obtained by compactifying the 3d theory on a circle. We give two complementary derivations of the result. We also discuss applications to F-maximization and to three-dimensional supersymmetric dualities.