Abstract
The strength of electroweak symmetry breaking may substantially differ in the early Universe compared to the present day value. In the Standard Model, the Higgs vacuum expectation value (vev ) vanishes and electroweak symmetry gets restored at temperatures above ∼ 160 GeV due to the Higgs field interactions with the high-temperature plasma. It was however shown that new light singlet scalar fields may change this behaviour. The key feature is the non-standard dependence on the Higgs vev of the new particles mass which can vanish at large Higgs vev, inducing a negative correction to the Higgs thermal mass, leading to electroweak symmetry non-restoration at high temperature. We show that such an effect can also be induced by new singlet fermions which on the other hand have the advantage of not producing unstable directions in the scalar potential at tree level, nor bringing additional severe hierarchy problems. As temperature drops, such a high-temperature breaking phase may continuously evolve into the zero-temperature breaking phase or the two phases can be separated by a temporary phase of restored symmetry. We discuss how our construction can naturally arise in motivated models of new physics, such as Composite Higgs. This is particularly relevant for baryogenesis, as it opens a whole class of possibilities in which the baryon asymmetry can be produced during a high temperature phase transition, while not being erased later by sphalerons.
Highlights
In the early universe, but only if this transition results in the growth of the Higgs vacuum expectation value to a value higher than the temperature
We show that such an effect can be induced by new singlet fermions which on the other hand have the advantage of not producing unstable directions in the scalar potential at tree level, nor bringing additional severe hierarchy problems
This is relevant for baryogenesis, as it opens a whole class of possibilities in which the baryon asymmetry can be produced during a high temperature phase transition, while not being erased later by sphalerons
Summary
The Standard Model Higgs doublet induces spontaneous breaking of the EW symmetry at zero temperature, provided by a negative mass parameter in the scalar potential. Respectively for one thermalized bosonic degree of freedom and one Dirac fermion with mass m Their interactions with the Higgs field are encoded inV 00t/hTe2(hm(-hd)2e)p00 endent masses m = m(h). We assume that the particle mass gradually decreases with h, reaches zero and increases. Such a behaviour is easy to realize for fermionic mass terms, which we concentrate on in this work. The plots in figure 1 are only partly applicable to the case of scalar fields, as their squared mass would typically become negative after reaching zero, leading to an instability. We want to investigate which type of modifications to the SM does not lead to this symmetry restoration, using the thermal effects of fermionic fields
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