Understanding the U(1) problem through dyon configuration in the Abelian   projection
  We give a short review of the recently obtained result that the magnetic monopole promoted to the dyon due to the vacuum angle $\theta$ resolves the U(1) problem in the sense that the dyon obtained in this way gives a dominant contribution to the topological susceptibility 
Seikou Kato and Kei-Ichi Kondo