eta' multiplicity and Witten-Veneziano relation at T>0

Abstract: Recent RHIC results on eta' multiplicity in heavy-ion collisions are of great importance because they clearly signal a partial restoration of U_A(1) symmetry at high temperatures T, and thus provide an unambiguous signature of the formation of a new state of matter. Prompted by these experimental results of STAR and PHENIX collaborations, we discuss and propose the minimal generalization of the Witten-Veneziano relation to finite T.