Understanding the U(1) problem through dyon configuration in the Abelian   projection
  We show 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. For this purpose, we derive an Abelian-projected effective gauge theory written in terms of Abelian degrees of freedom, which is obtained by integrating out all the off-diagonal degrees of freedom involved in the SU(2) Yang-Mills theory with the vacuum angle $\theta$. We evaluate the topological susceptibility by estimating the classical part of the effective dyon action obtained by performing the duality transformation. The obtained result is consistent with the Veneziano--Witten formula. 
Seikou Kato, Kei-Ichi Kondo