# Nelson-Barr relaxion

Davidi, Oz (Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel) ; Gupta, Rick S. (Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel) (Institute for Particle Physics Phenomenology, Department of Physics, Durham University, DH1 3LE, Durham, United Kingdom) ; Perez, Gilad (Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel) ; Redigolo, Diego (Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel) (Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel) ; Shalit, Aviv (Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel)

12 February 2019

Abstract: Cosmological relaxation models in which the relaxion 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 “relaxion $CP$ problem” based on the Nelson-Barr mechanism. We take $CP$ to be a symmetry of the UV theory, and the relaxion to have no anomalous coupling with QCD. The nonzero vacuum expectation value of the relaxion breaks $CP$ spontaneously, and the resulting phase is mapped to the Cabibbo-Kobayashi-Maskawa phase of 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 flavor experiments.

Published in: Physical Review D 99 (2019)