Cubic fourfolds behave in many ways like K3 surfaces. Certain cubics - conjecturally, the ones that are rational - have specific K3s associated to them geometrically. Hassett has studied cubics with K3s associated to them at the level of Hodge theory, and Kuznetsov has studied cubics with K3s associated to them at the level of derived categories. These two notions of having an associated K3 should coincide. We prove that they coincide generically: Hassetts cubics form a countable union of irreducible Noether-Lefschetz divisors in moduli space, and we show that Kuznetsovs cubics are a dense subset of these, forming a non-empty, Zariski open subset in each divisor.
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