Screening in the QCD instanton gas and the U(1) problem

Abstract

Interacting instantons in QCD are discussed as a solution to the U(1) problem. Assuming that the η is responsible for the long-range instanton interactions, we show that, for a certain parameter range, screening of instanton number is produced. This shifts the physical η mass away from the other Nambu-Goldstone bosons. The result is consistent with the η-mass formula obtained in the 1/ N expansion without instantons.