Axion field and the quark nugget’s formation at the QCD phase transition

Abstract

We study a testable dark matter (DM) model outside of the standard WIMP paradigm in which the observed ratio Ω dark 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 conventional "baryogenesis" mechanism we advocate a paradigm when the "baryogenesis" is actually a charge separation process which always 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 anti-baryon 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 details 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 anti baryon) 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, Ω dark ≈ Ω visible 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.

Beyond WIMPs: the Quark (Anti) Nugget Dark Matter

“…This talk is mostly based on recent paper [1]. It is generally assumed that the Universe began in a symmetric state with zero global baryonic charge and later, through some baryon number violating process, evolved into a state with a net positive baryon number.…”

“…Both quarks and antiquarks are thermally abundant in the primordial plasma but, in addition to forming conventional baryons, some fraction of them are bound into heavy nuggets of quark matter. Nuggets of both matter and antimatter are formed as a result of the dynamics of the axion domain walls [1,3,4], some details of this process will be discussed later in the text. a e-mail: arz@phas.ubc.ca An overall coherent baryon asymmetry in the entire Universe is a result of the strong CP violation due to the fundamental θ parameter in QCD which is assumed to be nonzero at the beginning of the QCD phase transition.…”

“…One should emphasize that the resulting asymmetry is order of one. It is not sensitive to a relatively small magnitude of the axion field, irrespectively to the magnitude of θ(x) ∼ (10 −2 − 10 −4 ) at the beginning of the QCD phase transition as long as it remains coherent on the scale of the Universe, see [1] for the details. This is precisely the main reason of why the visible and dark matter densities must be the same order of magnitude Ω dark ≈ Ω visible (1) as they both proportional to the same fundamental Λ QCD scale, and they both are originated at the same QCD epoch.…”

“…It is not sensitive to a relatively small magnitude of the axion field, irrespectively to the magnitude of θ(x) ∼ (10 −2 − 10 −4 ) at the beginning of the QCD phase transition as long as it remains coherent on the scale of the Universe, see [1] for the details. This is precisely the main reason of why the visible and dark matter densities must be the same order of magnitude Ω dark ≈ Ω visible (1) as they both proportional to the same fundamental Λ QCD scale, and they both are originated at the same QCD epoch. In particular, if one assumes that the nuggets and anti-nuggets saturate the dark matter density today than the observed matter to dark matter ratio Ω dark 5 · Ω visible corresponds to a specific proportion when number of anti-nuggets is larger than number of nuggets by a factor of ∼ 3/2 at the end of nugget's formation.…”

Dark Matter 2017

“…It is not inconceivable that one or more of these systems could be stable, if the decay is forbidden by energy, flavor and baryon number conservation [66,67]. In the early universe the heavy exotic hadron will be extremely abundant soon after the electroweak (EW) phase transition from quark-gluon plasma (QGP) to the hadron gas (HG), Therefore we can speculate that a population of dark baryons could could be present in the Universe today [68].…”