Fermions in electroweak baryogenesis

Mou, Zong-Gang (2015) Fermions in electroweak baryogenesis. PhD thesis, University of Nottingham.

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Abstract

We study the chiral anomaly by solving the Dirac equation for fermions in parallel electric and magnetic fields. In such case, only the lowest-energy Landau levels are relevant to the anomaly. Specifically, for massless fermions, the chiral anomaly is a result of the production of particles of one chirality, and no creation of particles of the other chirality. For massive fermions, we find that the chiral anomaly equation can be simply obtained via a proper regularization of the range of the momentum. We extend the method to anomaly cancellation, and conclude that the conservation of the baryon number plus lepton number must be violated as a quantum anomaly in the context of the Standard Model. Accordingly, such baryon number non-conservation can play a vital role during the electroweak transition to achieve the baryon asymmetry of the Universe.

Through real-time lattice simulations, we refine the implementation of ensemble fermions for a cold electroweak transition, involving the SU (2) gauge field, Higgs field and one generation of fermions. We find that the dynamics and most observables converge quickly with a reasonable number of fermion realizations, and the method of ensemble fermions for the entire electroweak sector becomes numerically tractable. We apply the method to the computation of the effective preheating temperature during a fast electroweak transition, relevant for Cold Electroweak Baryogenesis. We find that the fermion temperature is never below 20 GeV, and this can indirectly rule out Standard Model CP -violation as the origin of the baryon asymmetry of the Universe, as Standard Model cold baryogenesis requires a temperature of at most of order of 1 GeV. For this reason, new CP -violation source from physics beyond the Standard Model is required in order to explain the baryon asymmetry.

We further present a first-principles numerical computation of the baryon asymmetry in electroweak-scale baryogenesis, where the CP -violation is obtained as a consequence of including another Higgs doublet. For one particularly favourable scalar potential that could provide a high sphaleron transition rate, we calculate the asymmetry through large-scale computer simulations. The numerical signal is at the boundary of what is numerically discernible with the available computer resources, but we tentatively find an asymmetry of |η| ≤ 3.5 × 10−7 .

We also find it is attainable to include the complete electroweak SU (2) × U (1) gauge fields in the reduced Standard Model that we are using in practical simulations, so that in further studies we can measure the cosmic magnetic field generated during the electroweak phase transition.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Saffin, P.
Copeland, E.
Subjects: Q Science > QC Physics > QC770 Nuclear and particle physics. Atomic energy. Radioactivity
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 30597
Depositing User: Mou, Zong-Gang
Date Deposited: 22 Feb 2016 15:01
Last Modified: 17 Sep 2016 14:37
URI: http://eprints.nottingham.ac.uk/id/eprint/30597

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