Hasse Obstructions to Rationality for Special Fourfolds

Abstract

For every nonempty Hassett divisor ${\mathcal{C}}_d$ parameterizing special cubic fourfolds, we consider the quaternion class ${\beta }_d=(d/2,-3)$ in the two-torsion Brauer group. We prove that if a Hodge-general member of $C_d$ is rational then ${\beta }_d=0$ -- or, equivalently, that Huybrechts' twisted-K3 condition $(\ast {\ast }^{\prime })$ holds. Consequently, a very general member of ${\mathcal{C}}_d$ is irrational if ${\beta }_d\ne 0$. Thus, a very general cubic fourfold containing a smooth cubic scroll or a Veronese surface is irrational, and hence so is a very general Küchle fourfold of type $(c7)$. We also obtain an analogous obstruction for Hodge--special Gushel--Mukai fourfolds: a very general member in the discriminant--$d$ locus can be rational only if $d$ is a sum of two squares. Consequently, a very general Gushel--Mukai fourfold containing a cubic scroll is irrational.