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UBC Theses and Dissertations

Axion field and the quark nugget's formation at the QCD phase transition Liang, Xunyu


We study a testable dark matter (DM) model outside of the standard WIMP paradigm in which the observed ratio Ωdark ≃ Ω visible for visible and dark matter densities finds its natural explanation as a result of their common QCD origin when both types of matter (DM and visible) are formed at the QCD phase transition and both are proportional to ΛQCD. Instead of the conventional ``baryogenesis" mechanism we advocate a paradigm when the ``baryogenesis'' is actually a charge separation process which occur in the presence of the CP odd axion field a(x). In this scenario the global baryon number of the Universe remains zero, while the unobserved antibaryon charge is hidden in form of heavy nuggets, similar to Witten's strangelets and compromise the DM of the Universe. In the present work we study in great detail a possible formation mechanism of such macroscopically large heavy objects. We argue that the nuggets will be inevitably produced during the QCD phase transition as a result of Kibble-Zurek mechanism on formation of the topological defects during a phase transition. Relevant topological defects in our scenario are the closed bubbles made of the NDW=1 axion domain walls. These bubbles, in general, accrete the baryon (or antibaryon) charge, which eventually result in formation of the nuggets and anti-nuggets carrying a huge baryon (anti-baryon) charge. A typical size and the baryon charge of these macroscopically large objects is mainly determined by the axion mass ma. However, the main consequence of the model, Ωdark ≈ Ωvisible is insensitive to the axion mass which may assume any value within the observationally allowed window 10-⁶ eV ≲ ma ≲ 10-³ eV. We also estimate the baryon to entropy ratio η ≡ nB/nγ ∼ 10-¹⁰ within this scenario. Finally, we comment on implications of these results to the axion search experiments, including microwave cavity and the Orpheus experiments.

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