Tyurin Degenerations, Derived Lagrangians and Categorification of DT Invariants

Abstract

We consider the moduli space of rigidified perfect complexes with support on a general complete intersection Calabi-Yau threefold $X$ and its Tyurin degeneration $X\rightsquigarrow X_1\cup_SX_2$ to a complete intersection of Fano threefolds $X_1,X_2$ meeting along their anti-canonical divisor $S$. The corresponding derived dg moduli scheme over the generic fiber degenerates to the (Fano) moduli spaces $\mathcal{M}_{1}, \mathcal{M}_{2},$ of perfect complexes supported on each Fano which glue after derived restriction to the relative divisor $S$. We prove that the total moduli space of the degeneration family carries a relative Lagrangian foliation structure, which implies the existence of a flat Gauss-Manin connection on periodic cyclic homology of the category of the matrix factorizations associated with fiber-wise moduli spaces, realized locally as the derived critical loci of suitable potential functions. The Fano moduli spaces each define derived Lagrangians in the (ambient) moduli space of restricted complexes to the relative divisor $S$. The flatness of the Gauss-Manin connection implies the derived geometric deformation invariance of the categorified DT-invariants associated to fiberwise matrix factorization categories, hence, the categorified DT-invariants of the generic fiber are expressed in terms of a derived intersection cohomology of the corresponding Fano moduli spaces on the special fiber.