  We construct an isomorphism between the preprojective cohomological Hall
algebra of an arbitrary quiver and a positive half of the corresponding
Maulik-Okounkov Yangian, which intertwines the respective actions on the
cohomology of the Nakajima quiver varieties. We use this to prove a conjecture
of Okounkov relating the character of the Maulik-Okounkov Lie algebra to Kac
polynomials.
