UCLA

The recent measurement of the Higgs boson mass implies a relatively slow rise of the Standard Model Higgs potential at large scales, and a possible second minimum at even larger scales. Consequently, the Higgs field may develop a large vacuum expectation value during inflation. The relaxation of the Higgs field from its large postinflationary value to the minimum of the effective potential represents an important stage in the evolution of the Universe. During this epoch, the time-dependent Higgs condensate can create an effective chemical potential for the lepton number, leading to a generation of the lepton asymmetry in the presence of some large right-handed Majorana neutrino masses. The electroweak sphalerons redistribute this asymmetry between leptons and baryons. This Higgs relaxation leptogenesis can explain the observed matter-antimatter asymmetry of the Universe even if the Standard Model is valid up to the scale of inflation, and any new physics is suppressed by that high scale. The baryonic isocurvature perturbations generated by the relaxation leptogenesis can also explain the excess found in the cosmic infrared background (CIB) anisotropy.

We begin this dissertation by reviewing the development of the large vacuum expectation value (VEV) of the Higgs and other scalar fields during inflation. We then discuss the postinflationary relaxation of the Higgs field in full detail, and present the relaxation leptogenesis framework using the Standard Model Higgs field as an example.

Next, we extend the relaxation leptogenesis to the elementary Goldstone Higgs (EGH) framework and the pseudoscalar scenario. In the EGH paradigm, the electroweak (EW) scale is not fundamental but radiatively generated. This allows one to disentangle the EW scale from the vacuum expectation of the elementary Higgs field, and construct a very flat scalar potential directions along which the relaxation leptogenesis mechanism can be implemented with larger parameter space.

In December 2015, the ATLAS and CMS Collaborations have reported evidence of a diphoton excess which may be interpreted as a pseudoscalar boson S with a mass around 750 GeV. To explain the diphoton excess, such a boson is coupled to the Standard Model gauge fields via SFF-dual operators, which provide the chemical potential to the lepton asymmetry. Although the diphoton excess turns out to be a statistical fluctuation in 2016, a similar pseudoscalar with greater mass remains a viable model for relaxation leptogenesis mechanism.

Finally, we discuss the imprint of relaxation leptogenesis on the CIB anisotropy. Observations of CIB exhibit significant fluctuations on small angular scales, whose origin remains a question. We consider the possibility that small-scale fluctuations in matter-antimatter asymmetry could lead to variations in star formation rates which are responsible for the CIB fluctuations. We show that the Higgs relaxation leptogenesis mechanism can produce such small-scale baryonic isocurvature perturbations which can explain the observed excess in the CIB fluctuations.