Solving the strong CP problem with horizontal gauge symmetry

Abstract

We present a solution to the strong CP problem, which relies on the horizontal gauge symmetry and CP invariance in a full theory. Similar to other Nelson-Barr type solutions, CP violation in both the strong and weak sectors in the Standard model (SM) is attributed to the condensation of complex scalars $\Phi$ in the model. The model is differentiated by others in that it explains the hierarchy in quark-Higgs Yukawa coupling in the SM based on a series of sequential breaking of the horizontal $SU(3{)}_f$ gauge symmetry. The experimental constraint $\overline{\theta }\lesssim 10^{-10}$ requires $<\Phi >\lesssim 10^{13}-10^{14}\mathrm{GeV}$ (vacuum expectation value of complex scalars) and $\lambda \lesssim 10^{-6}$ (scalar quartic coupling). We show that this small coupling is natural in the sense of 'tHooft naturalness. Compared to other models of Nelson-Barr type with CP breaking scale ${\Lambda }_{CP}\lesssim 10^8\mathrm{GeV}$, our model is more advantageous in terms of consistency with the thermal leptogenesis.

8 pages, 3 figures

References in corpus (1)

• An Improved Experimental Limit on the Electric Dipole Moment of the Neutron

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