Cohomological Hall algebras and affine quantum groups

We study the preprojective cohomological Hall algebra (CoHA) introduced by the authors in an earlier work for any quiver $Q$ and any one-parameter formal group $\mathbb{G}$. In this paper, we construct a comultiplication on the CoHA, making it a bialgebra. We also construct the Drinfeld double of the CoHA. The Drinfeld double is a quantum affine algebra of the Lie algebra $\mathfrak{g}_Q$ associated to $Q$, whose quantization comes from the formal group $\mathbb{G}$. We prove, when the group $\mathbb{G}$ is the additive group, the Drinfeld double of the CoHA is isomorphic to the Yangian.