Abstract
The cosmological relaxation of the electroweak scale has been proposed as a mechanism to address the hierarchy problem of the Standard Model. A field, the relaxion, rolls down its potential and, in doing so, scans the squared mass parameter of the Higgs, relaxing it to a parametrically small value. In this work, we promote the relaxion to an inflaton. We couple it to Abelian gauge bosons, thereby introducing the necessary dissipation mechanism which slows down the field in the last stages. We describe a novel reheating mechanism, which relies on the gauge-boson production leading to strong electro-magnetic fields, and proceeds via the vacuum production of electron-positron pairs through the Schwinger effect. We refer to this mechanism as Schwinger reheating. We discuss the cosmological dynamics of the model and the phenomenological constraints from CMB and other experiments. We find that a cutoff close to the Planck scale may be achieved. In its minimal form, the model does not generate sufficient curvature perturbations and additional ingredients, such as a curvaton field, are needed.
Highlights
During the past few decades, numerous ideas to solve the hierarchy problem have been put forth
The cosmological relaxation of the electroweak scale has been proposed as a mechanism to address the hierarchy problem of the Standard Model
We couple it to Abelian gauge bosons, thereby introducing the necessary dissipation mechanism which slows down the field in the last stages
Summary
During the past few decades, numerous ideas to solve the hierarchy problem have been put forth. In the original proposal [1], the entire scanning takes place in the background of inflation, which provides constant Hubble friction necessary for the relaxion to maintain slow roll and eventually stop at the local minimum This setup is rather constraining, rendering the cutoff scale significantly below the Planck scale. The available parameter space for the above scenario, a-priori rather large, is reduced by several theoretical constraints that force relations among the different parameters, and by phenomenological bounds These include the validity of the effective theory, the requirement for a relaxation at the correct scale, the suppression of cosmological perturbations, and various limits from colliders, astrophysics, cosmology and 5th-force experiments. We find that a cutoff scale close to the Planck scale may be achieved in this relaxed inflation scenario
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