11856 1904.02545 10.1007/JHEP11(2019)140 140 140 Fast-rolling relaxion Regular Article - Theoretical Physics 1 47 2019 11 25 2019 4 17 2019 8 22 2019 11 6 2019 11 25 The Author(s) 2019 Open Access. This article is distributed under the terms of the Creative Commons Attribution License ( CC-BY 4.0 ), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. 13130 2019 2019 11 11 Masahiro Ibe ibe@icrr.u-tokyo.ac.jp Yutaro Shoji yshoji@kmi.nagoya-u.ac.jp Motoo Suzuki m0t@icrr.u-tokyo.ac.jp grid.26999.3d 0000 0001 2151 536X ICRR University of Tokyo 5-1-5 Kashiwanoha, Kashiwa Chiba 277-8582 Japan grid.26999.3d 0000 0001 2151 536X IPMU University of Tokyo 5-1-5 Kashiwanoha, Kashiwa Chiba 277-8583 Japan grid.27476.30 0000 0001 0943 978X KMI Nagoya University Furo-cho, Chikusa-ku Nagoya Aichi 464-8602 Japan Abstract We discuss new mechanisms to stop the relaxion field during inflation. They can be realized in a generic model, including the original model but in a quite different parameter region. We consider a fast-rolling relaxion field, which can go over the bumps created by QCD-like dynamics. Then, in one of the mechanisms, we stop it with a parametric resonance of the Higgs field. The mechanisms are free from a super-Planckian field excursion or a gigantic number of e-folds of inflation. The relaxion has a mass around the weak scale and mixes with the Higgs boson, which enhances the testability of our mechanisms. Keywords Beyond Standard Model Higgs Physics ArXiv ePrint: 1904.02545