Lattice QCD solution to the U(1) problem

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