Lattice QCD solution to the U(1) problem.

Abstract

It is shown in quenched lattice QCD that the mass splitting between \ensuremath{\eta}' and a pion arises from gauge configurations with a nonzero topological charge Q, its magnitude increasing for larger values of \ensuremath{\Vert}Q\ensuremath{\Vert}; the contribution from the disconnected quark loop is strongly hindered unless the topological charge is excited. This demonstrates the explicit relation between the large \ensuremath{\eta}' meson mass and gauge field topology, which is in the line of the argument in the continuum of instantons and the 1/N expansion.