Cohomological Hall algebras of quivers and Yangians

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.