Monodromy rank and the semisimple Mumford-Tate conjecture for hyper-Kähler varieties

Abstract: In this paper, we establish two main results concerning the Mumford-Tate conjecture for hyper-Kähler varieties. First, we prove the conjecture for the semisimplified $\ell$-adic Galois representations attached to hyper-Kähler varieties with second Betti number $b_2 \geq 4$. As a direct consequence, we deduce that the Hodge conjecture implies the Tate conjecture for powers of hyper-Kähler varieties. Second, we show that the Mumford-Tate conjecture for hyper-Kähler varieties is invariant under deformation. The proofs rely on comparing the ranks of $\ell$-adic algebraic monodromy groups in higher degrees to those in degree $2$ via the theory of Frobenius tori and the Looijenga-Lunts-Verbitsky Lie algebra.