Probing the relaxed relaxion at the luminosity and precision frontiers
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Springer
Received: April Revised: June Accepted: June Published: July
21, 22, 26, 22,
2020 2020 2020 2020
Abhishek Banerjee,a Hyungjin Kim,a Oleksii Matsedonskyi,a Gilad Pereza and Marianna S. Safronovab,c a
Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel b Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, U.S.A. c Joint Quantum Institute, National Institute of Standards and Technology and the University of Maryland, Gaithersburg, Maryland 20742, U.S.A.
E-mail: [email protected], [email protected], [email protected], [email protected], [email protected] Abstract: Cosmological relaxation of the electroweak scale is an attractive scenario addressing the gauge hierarchy problem. Its main actor, the relaxion, is a light spin-zero field which dynamically relaxes the Higgs mass with respect to its natural large value. We show that the relaxion is generically stabilized at a special position in the field space, which leads to suppression of its mass and potentially unnatural values for the model’s effective low-energy couplings. In particular, we find that the relaxion mixing with the Higgs can be several orders of magnitude above its naive naturalness bound. Low energy observers may thus find the relaxion theory being fine-tuned although the relaxion scenario itself is constructed in a technically natural way. More generally, we identify the lower and upper bounds on the mixing angle. We examine the experimental implications of the above observations at the luminosity and precision frontiers. A particular attention is given to the impressive ability of future nuclear clocks to search for rapidly oscillating scalar ultra-light dark matter, where the future projected sensitivity is presented. Keywords: Beyond Standard Model, Cosmology of Theories beyond the SM ArXiv ePrint: 2004.02899
c The Authors. Open Access, Article funded by SCOAP3 .
https://doi.org/10.1007/JHEP07(2020)153
JHEP07(2020)153
Probing the relaxed relaxion at the luminosity and precision frontiers
Contents 1
2 The relaxion mass and couplings: a closer look
3
3 Relaxation of relaxion at π/2 3.1 Vacuum structure 3.2 Classical evolution 3.3 Quantum evolution
5 5 8 8
4 Naturalness
10
5 Mixing angle and phenomenology 5.1 Maximum mixing angle 5.2 Minimum mixing angle 5.3 Comparison with the naive generic case 5.4 h − φ interactions 5.5 Experimental probes above eV scale 5.6 Experimental probes below eV scale
12 13 14 14 15 17 19
6 Discussion
22
A Detailed analysis of relaxion stopping point
24
1
Introduction
The idea that the electroweak (EW) scale is determined as a result of dynamical relaxation provides a new insight on hierarchy problem in the Standard model (SM) [1]. The electroweak scale is dynamically selected by an evolution of an axion-like field φ, which is referred to as relaxion. Compared to conventional models for the electroweak hierarchy problem, the relaxation mechanism includes o
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