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Nugget Dark Matter EPJ Web of Conferences 137, 09014 (2017) DOI: 10.1051/ epjconf/201713709014 XIIth Quark Confinement & the Hadron Spectrum Beyond WIMPs: the Quark (Anti) Nugget Dark Matter Ariel Zhitnitsky1;a 1Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada Abstract. We review a testable dark matter (DM) model outside of the standard WIMP paradigm. The model is unique in a sense that the observed ratio Ωdark ' Ωvisible 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 during the QCD phase transition and both are proportional to single dimensional parameter of the system, ΛQCD. We argue that the charge separation effect also inevitably occurs during the same QCD phase transition in the presence of the CP odd axion field a(x). It leads to pref- erential formation of one species of nuggets on the scales of the visible Universe where the axion field a(x) is coherent. A natural outcome of this preferential evolution is that only one type of the visible baryons (not anti- baryons) remain in the system after the nuggets complete their formation. Unlike conventional WIMP dark matter candidates, the nuggets and anti-nuggets are strongly interacting but macroscopically large objects. The rare events of annihilation of the anti-nuggets with visible matter lead to a number of observable effects. We argue that the relative intensities for a number of measured ex- cesses of emission from the centre of galaxy (covering more than 11 orders of magnitude) are determined by standard and well established physics. At the same time the absolute intensity of emission is determined by a single new fundamental parameter of the theory, −6 −3 the axion mass, 10 eV . ma . 10 eV. Finally, we comment on implications of these studies for the axion search experiments, including microwave cavity and the Orpheus experiments. 1 Introduction 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. As an alternative to this scenario we advocate a model in which “baryogenesis” is actually a charge separation process in which the global baryon number of the Universe remains zero. In this model the unobserved antibaryons come to comprise the dark matter in form of the dense heavy nuggets, similar to the Witten’s strangelets [2]. 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. ae-mail: [email protected] © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 137, 09014 (2017) DOI: 10.1051/ epjconf/201713709014 XIIth Quark Confinement & the Hadron Spectrum 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. This source of strong CP violation is no longer available at the present epoch as a result of the axion dynamics, see original [5–7] and more recent papers [8–15] on the subject. Were CP symmetry to be exactly preserved an equal number of matter and antimatter nuggets would form resulting in no net “baryogenesis". However, CP violating processes associated with the axion θ(x) term in QCD result in the preferential formation of one type of species. This preference is essentially determined by the dynamics of coherent axion field θ(x) at the initial stage of the nugget’s formation. Asymmetric production of the nuggets directly translates into asymmetry between visible baryons and anti-baryons because the total baryon charge is a conserved quantity in this framework. If more anti-nuggets are produced in the system then less conventional anti-baryons remain in the system. These remaining anti-baryons in the plasma then annihilate away leaving only the baryons whose antimatter counterparts are bound in the excess of anti-nuggets and thus unavailable to annihilate. 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 Ωdark ≈ Ωvisible (1) as they both proportional to the same fundamental ΛQCD 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 Ωdark ' 5 · Ωvisible 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. This would result in a matter content with baryons, quark nuggets and antiquark nuggets in an approximate ratio jBvisiblej : jBnuggetsj : jBantinuggetsj ' 1 : 2 : 3; (2) with no net baryonic charge. If these processes are not fundamentally related the two components Ωdark and Ωvisible could easily exist at vastly different scales. Unlike conventional dark matter candidates, dark-matter/antimatter nuggets are strongly interact- ing but macroscopically large. They do not contradict the many known observational constraints on dark matter or antimatter for three main reasons [16]: • They carry a huge (anti)baryon charge jBj & 1025, and so have an extremely tiny number density; • The nuggets have nuclear densities, so their effective interaction is small σ/M ∼ 10−10 cm2/g, well below the typical astrophysical and cosmological limits which are on the order of σ/M < 1 cm2/g; • They have a large binding energy such that the baryon charge in the nuggets is not available to participate in big bang nucleosynthesis (bbn) at T ≈ 1 MeV. To reiterate: the weakness of the visible-dark matter interaction in this model due to the small ge- ometrical parameter σ/M ∼ B−1=3 rather than due to the weak coupling of a new fundamental field to standard model particles. It is this small effective interaction ∼ σ/M ∼ B−1=3 which replaces the conventional requirement of sufficiently weak interactions for WIMPs. A fundamental measure of the scale of baryogenesis is the baryon to entropy ratio at the present time n − n ¯ n η ≡ B B ' B ∼ 10−10: (3) nγ nγ 2 EPJ Web of Conferences 137, 09014 (2017) DOI: 10.1051/ epjconf/201713709014 XIIth Quark Confinement & the Hadron Spectrum If the nuggets were not present after the phase transition the conventional baryons and anti-baryons would continue to annihilate each other until the temperature reaches T ' 22 MeV when density would be 9 orders of magnitude smaller than observed. This annihilation catastrophe, normally thought to be resolved as a result of “baryogenesis," is avoided in our proposal because more anti- baryon charges than baryon charges are hidden in the form of the macroscopical nuggets and thus no longer available for annihilation. Only the visible baryons (not anti-baryons) remain in the system after nugget formation is fully completed. In our proposal (in contrast with conventional models) the ratio η is determined by the formation temperature Tform at which the nuggets and anti-nuggets basically have completed their formation and below which annihilation with surrounding matter becomes negligible. This temperature is de- termined by many factors: transmission/reflection coefficients, evolution of the nuggets, expansion of the universe, cooling rates, evaporation rates, the dynamics of the axion domain wall network, etc. In general, all of these effects will contribute equally to determining Tform at the QCD scale. Technically, the corresponding effects are hard to compute as even basic properties of the QCD phase diagram at nonzero θ are still unknown. However, an approximate estimate of Tform is quite simple as it must be T Figure 1. The conjectured phase diagram. The plot is taken from [1]. QGP Possible cooling paths are denoted as path 1, 2 or 3. The phase diagram is in Tc 3 170MeV ≈ fact much more complicated as the dependence on the third essential 1 2 parameter, the θ is not shown as it is largely unknown. It is assumed that the CS final destination after the nuggets are formed is the CS region with Tform ≈ 41 Hadron T 41MeV form ≈ MeV, µ > µc and θ ≈ 0, corresponding to the presently observed ratio (3), see µ (Phase Unknown) text for the details. θ expressed in terms of the gap ∆ ∼ 100 MeV when the colour superconducting phase sets in inside the nuggets. The observed ratio (3) corresponds to Tform ' 41 MeV which is indeed a typical QCD scale slightly below the critical temperature Tc ' 0:6∆ when colour superconductivity (CS) sets in. In different words, in this proposal the ratio (3) emerges as a result of the QCD dynamics when process of charge separation stops at Tform ' 41 MeV, rather than a result of baryogenesis when a net baryonic charge is produced.
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