A new six-dimensional irreducible symplectic variety

We construct a six-dimensional irreducible symplectic variety with $b_2=8$. Since the known examples of irreducible symplectic varieties have $b_2=7$ or $b_2=23$, our variety is in a new deformation class. The example is obtained as follows. Let $J$ be the Jacobian of a genus-two curve with its natural principal polarization: results of another paper of ours give a symplectic desingularization of the moduli space of semistable rank-two sheaves on $J$ with $c_1=0$ and $c_2=2$. Let $\mathcal {M}_{\mathbf {v}}$ be this symplectic desingularization: there is a natural locally trivial fibration $\mathcal {M}_{\mathbf {v}}\rightarrow J\times \widehat {J}$. Our example is the fiber over $(0,\widehat {0})$ of this map, we denote it by $\widetilde {\mathcal {M}}$. The main body of the paper is devoted to the proof that $\widetilde {\mathcal {M}}$ is irreducible symplectic and that $b_2(\widetilde {\mathcal {M}})=8$. Applying the generalized Lefschetz Hyperplane Theorem we get that low-dimensional homotopy (or homology) groups of $\widetilde {\mathcal {M}}$ are represented by homotopy (or homology) groups of a subset of $\widetilde {\mathcal {M}}$ which has an explicit description. The main problem is to provide the explicit description and to extract the necessary information on homotopy or homology groups.