  We prove that the Yangian associated to an untwisted symmetric affine
Kac-Moody Lie algebra is isomorphic to the Drinfeld double of a shuffle
algebra. The latter is constructed by the authors in arXiv:1407.7994 as an
algebraic formalism of the cohomological Hall algebras. As a consequence, we
obtain the Poincare-Birkhoff-Witt (PBW) theorem for this class of affine
Yangians. Another independent proof of the PBW theorem is given recently by
Guay-Regelskis-Wendlandt.
