The "relaxion CP problem" is related to the fact that cosmological relaxation models, where the relaxion field is identified with the QCD axion, generically fail to account for the smallness of the strong CP phase. We present a simple alternative solution to this problem which incorporates the Nelson-Barr mechanism within the relaxion framework. We take CP to be a symmetry of the UV theory and the relaxion to have no anomalous coupling with QCD. The non-zero relaxion vacuum expectation value breaks CP spontaneously, and the resulting phase is mapped to the Cabibbo-Kobayashi-Maskawa phase in the Standard Model. The extended Nelson-Barr quark sector generates the relaxion "rolling" potential radiatively, relating the new physics scale with the relaxion decay constant. With no new states within the reach of the LHC, our relaxion can still be probed in a variety of astrophysical and cosmological processes, as well as in fifth forces experiments.

 https://arxiv.org/abs/1711.00858