GW/PT-correspondence for CY $4$-fold-II: Virtual Classes

Abstract

Let $B$ be a complex manifold and $f:\bX\to B$ a derived \textit{complex-analytic} (not necessarily \textit{algebraic}) space with an oriented relative $(-2)$-shifted symplectic structure. We suppose that the closed symplectic form is locally exact in the completed relative de Rham complex, and construct a derived smooth manifold over $B$, independent of additional choices. When $f$ is proper and oriented, this gives integral virtual fundamental classes on its fibres. Our argument is heavily based on the strategy of Borisov-Joyce who already solved the \textit{algebraic} version of this problem, and uses \textit{derived differential geometry}. The proof also contains a relative analytic Darboux theorem a-la Brav-Bussi-Joyce, and has a natural analogue in Borel-Moore homology. We provide applications to complex-analytic enumerative geometry: in particular, to counting Pandharipande-Thomas pairs on compact and non-compact complex-analytic Calabi-Yau $4$-folds. This is part of a joint project with Felix Thimm dedicated to the proof of the GW/PT-correspondence for Calabi-Yau $4$-folds by implementing Pardon's programme in dimension $4$.