On stable rationality of some conic bundles and moduli spaces of Prym curves

Abstract

We prove that a very general hypersurface of bidegree $(2,n)$ in ${\mathbb{P}}^2\times {\mathbb{P}}^2$ for $n$ bigger than or equal to $2$ is not stably rational, using Voisin's method of integral Chow-theoretic decompositions of the diagonal and their preservation under mild degenerations. At the same time, we also analyse possible ways to degenerate Prym curves, and the way how various loci inside the moduli space of stable Prym curves are nested. No deformation theory of stacks or sheaves of Azumaya algebras like in recent work of Hassett–Kresch–Tschinkel is used, rather we employ a more elementary and explicit approach via Koszul complexes, which is enough to treat this special case.