The QCD axion serves as a well-motivated dark matter candidate and the misalignment mechanism is known to reproduce the observed abundance with a decay constant fa ≃ O(1012) GeV for a misalignment angle θmis ≃ O(1). While fa ≪ 1012 GeV is of great experimental interest, the misalignment mechanism requires the axion to be very close to the hilltop, i.e. θmis ≃ π. This particular choice of θmis has been understood as fine-tuning the initial condition. We offer a dynamical explanation for θmis ≃ π in a class of models. The axion dynamically relaxes to the minimum of the potential by virtue of an enhanced mass in the early universe. This minimum is subsequently converted to a hilltop because the CP phase of the theory shifts by π when one contribution becomes subdominant to another with an opposite sign. We demonstrate explicit and viable examples in supersymmetric models where the higher dimensional Higgs coupling with the inflaton naturally achieves both criteria. Associated phenomenology includes a strikingly sharp prediction of 3 × 109 GeV ≲ fa ≲ 1010 GeV and the absence of isocurvature perturbation.
Bibliographical noteFunding Information:
The authors thank Asimina Arvanitaki, Joshua W. Foster, Andrew J. Long, Aaron Pierce, Benjamin R. Safdi, and Ken van Tilburg for discussions. K.H. thanks the Leinweber Center for Theoretical Physics for the warm hospitality he received during his visit when most of this work was done. The work was supported in part by the DoE Early Career Grant DE-SC0019225 (R.C.) and the DoE grant DE-SC0009988 (K.H.).
© 2019, The Author(s).
- Cosmology of Theories beyond the SM
- Supersymmetric Gauge Theory
- Supersymmetric Standard Model