Extra dimensions and the strong CP problem
In higher-dimensional theories such as Brane World models with quasi-localized non-Abelian gauge fields the vacuum structure turns out to be trivial. Since the gauge theory behaves at large distances as a $4+δ$-dimensional and thus the topology of the infinity is that of of $S^{3+δ}$ rather than $S^{3}$, the set of gauge mappings are homotopically trivial and the CP-violating $θ$-term vanishes on the brane world-volume. As well there are no contributions to the $θ$-term from the higher-dimensional solitonic configurations. In this way, the strong CP problem is absent in the models with quasi-localized gluons.
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