Quiver Varieties and Yangians

We prove a conjecture of Nakajima (for type A the result was announced by Ginzburg- Vasserot) giving a geometric realization, via quiver varieties, of the Yangian of type ADE (and more in general of the Yangian associated to every symmetric Kac-Moody Lie algebra). As a corollary we get that tthe characters of the simple finite dimensional representations of the quantized affine algebra and that of the yangian coincide.