Afif Omar

Topic

Probing the Dark Universe with Big Bang Nucleosynthesis and Belle II

Department of Physics and Astronomy

Date & location

• Tuesday, August 11, 2026
• 12:30 P.M.
• Clearihue Building, Room B009

Examining Committee

Supervisory Committee

• Dr. David McKeen, Department of Physics and Astronomy, University of Victoria (Co-Supervisor)
• Dr. Adam Ritz, Department of Physics and Astronomy, UVic (Co-Supervisor)
• Dr. Slim Ibrahim, Department of Mathematics and Statistics, UVic (Outside Member)

External Examiner

• Dr. Yue Zhang, Department of Physics, Carleton University

Chair of Oral Examination

• Dr. Donald Juzwishin, School of Health Information Science, UVic

Abstract

This dissertation examines the MeV-to-GeV dark Universe through complementary cosmological and terrestrial tests of new physics. Big bang nucleosynthesis (BBN) is a major focus of the studies presented here. A simple phenomenological model of early dark energy (EDE) is introduced in which its density remains constant until a critical time before redshifting to avoid overclosing the Universe. Three redshifting scenarios are studied: Standard Model (SM)-coupled radiation, dark radiation, and kination, each modifying thermal expansion and predicted light-element abundances. Comparing the predicted deuterium abundance, helium-4 mass fraction 𝑌_𝑝, and effective number of relativistic species 𝑁_eff with abundance measurements and the CMB-inferred 𝑁_eff constrains the allowed EDE fraction and the transition temperature. The SM-coupled scenario injects entropy before the deuterium bottleneck, lowering the predicted 𝑌_𝑝 and improving the fit to recent measurements favouring a smaller primordial helium abundance. Building on the same BBN framework, and restoring standard cosmology, another component of the dark Universe is tested—namely, dark matter (DM). Sub-GeV thermal dark matter candidates with velocity-suppressed annihilation channels evade CMB constraints, but BBN is sensitive to such annihilations, especially with hadronic final states. Charged pions and kaons injected after DM freeze-out bias neutron–proton charge conversion reactions prior to the deuterium bottleneck, altering the predicted primordial abundances. The resulting limits on the DM annihilation cross section surpass those from the CMB and from indirect detection in the Galaxy.

Thermal MeV-GeV dark matter generically requires an additional force carrier connecting the dark sector to the visible SM sector to produce the observed relic abundance while evading direct detection. When cast into the benchmark dark photon portal model, BBN limits on DM annihilation complement direct detection and accelerator-based searches. Such dark mediators are studied further at the terrestrial intensity frontier, where dark-sector spin-1 bosons could be produced through the mono-photon channel 𝑒⁺𝑒⁻→ 𝛾 + invisible at Belle II. The production cross section’s polarization dependence is calculated for three benchmark scenarios—a kinetically mixed dark photon, a mass-mixed dark Z, and a vector coupled only to right-handed fermions. The ±70% longitudinal electron-beam polarization proposed for “Chiral Belle” gives a handle on the Lorentz structure of the mediator’s couplings in the event of a positive signal, breaking degeneracies unresolved by the unpolarized beam.