Groups of symplectic involutions on symplectic varieties of Kummer type and their fixed loci

Abstract: We describe the Galois action on the middle $\ell$-adic cohomology of smooth, projective fourfolds $K_A(v)$ that occur as a fiber of the Albanese morphism on moduli spaces of sheaves on an abelian surface $A$ with Mukai vector $v$. We show this action is determined by the action on $H^2_{\'et}(A_{\bar{k}},\mathbb{Q}_\ell(1))$ and on a subgroup $G_A(v) \leqslant (A\times \hat{A})[3]$, which depends on $v$. This generalizes the analysis carried out by Hassett and Tschinkel [HT13] over $\mathbb{C}$. As a consequence, over number fields, we give a condition under which $K_2(A)$ and $K_2(\hat{A})$ are not derived equivalent. The points of $G_A(v)$ correspond to involutions of $K_A(v)$. Over $\mathbb{C}$, they are known to be symplectic and contained in the kernel of the map $\mathrm{Aut}(K_A(v))\to \mathrm{O}(H^2(K_A(v),\mathbb{Z}))$. We describe this kernel for all varieties $K_A(v)$ of dimension at least $4$. When $K_A(v)$ is a fourfold over a field of characteristic 0, the fixed-point loci of the involutions contain K3 surfaces whose cycle classes span a large portion of the middle cohomology. We examine the fixed loci in fourfolds $K_A(0,l,s)$ over $\mathbb{C}$ where $l$ is a $(1,3)$-polarization, finding the K3 surface to be elliptically fibered under a Lagrangian fibration of $K_A(0,l,s)$.