Moduli spaces on Kuznetsov components are Irreducible Symplectic Varieties

Abstract: This article studies moduli spaces of Bridgeland semistable objects in the Kuznetsov component of a cubic fourfold that don't admit a symplectic resolution, i.e., moduli spaces of objects with non-primitve Mukai vector v=mv_0 that is not of OG10-type and where v_0^2 >0. For a generic stability condition, it is shown that these moduli spaces are projective irreducible symplectic varieties with factorial terminal singularities and that their deformation class is uniquely determined by the integers m and v_0^2. On the one hand, this generalizes the results of arXiv:1703.10839, arXiv:1912.06935, arXiv:2007.14108, which deal with moduli spaces of objects in the Kuznetsov component of a cubic fourfold which are smooth or of OG10-type; on the other hand, this extends to the Kuznetsov component of a cubic fourfold the results of arXiv:1802.01182, arXiv:2012.10649, on Gieseker moduli spaces of sheaves on K3 surfaces with non-primitive Mukai vector.