Abstract
We study the potential of future lepton colliders, running at the Z-pole and above, and the High-Luminosity LHC to search for the relaxion and other light scalars ϕ. We investigate the interplay of direct searches and precision observables for both mathcal{C}mathcal{P} -even and -odd couplings. In particular, precision measurements of exotic Z-decays, Higgs couplings, the exotic Higgs decay into a relaxion pair and associated Zϕ and γϕ production are promising channels to yield strong bounds.
Highlights
Additional degrees of freedom are light and have ultra-weak couplings to the SM, and are hard to find
We study the potential of future lepton colliders, running at the Z-pole and above, and the High-Luminosity LHC to search for the relaxion and other light scalars φ
A new mechanism has been proposed that addresses the hierarchy problem in a way that goes beyond the conventional paradigm of a symmetry-based solution to fine-tuning
Summary
In the following we will briefly review the relaxion mechanism. The effective scalar potential of the theory depends both on the Higgs doublet H and the relaxion φ,. The relaxion scans the Higgs mass parameter μ2(φ) from a large and positive cutoff energy Λ2 down to negative values because of the slow-roll potential. This non-zero vev activates a backreaction potential Vbr which eventually stops the rolling of the relaxion at a value φ0, where v(φ0) = 246 GeV. [9], Vbr is generated by low-energy QCD and the relaxion is identified with the QCD axion. This setup typically predicts a too large phase θQCD and is ruled out by the upper bound on the neutron electric dipole moment. One can introduce a new sector with strongly [9] or weakly [15] fermions, which generates a Higgs-dependent backreaction potential of the form. Other realizations of the relaxion mechanism have been introduced for instance in refs. [10, 42, 43], where our analysis does not apply
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