Stringy solution to strong CP violation problem

A new solution to the strong-CP problem is discussed. It involves a, stringy inspired, extension of QCD containing an unbroken gauged U(1)X symmetry whose gauge boson gets a Stuckelberg mass term by combining with a pseudoscalar field eta(x). The latter has axion-like couplings to QCD. This system leads to mixed gauge anomalies and we argue that they are cancelled by the addition of an appropriate Wess-Zumino term, so that no SM fermions need to be charged under U(1)(X). Axion like field can be gauged away. The full system possesses an extra global abelian symmetry which allows to rotate away theta parameter. There are generic scenarios from which such interactions can be derived. In particular we show that in certain D-brane Type-II string models (with antisymmetric tensor field strength fluxes) higher dimensional Chern-Simons couplings give rise to the required D = 4 Wess-Zumino terms upon compactification. In one of the possible string realizations of the mechanism the U(1)(X) gauge boson comes from the Kaluza-Klein reduction of the eleven-dimensional metric in M-theory.