models:speculative_models:axion
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models:speculative_models:axion [2018/03/19 11:00] jakobadmin [Criticism] |
models:speculative_models:axion [2019/07/01 09:27] (current) jakobadmin [Criticism] |
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| - | ====== Axion Theories ====== | + | ====== Axion Models ====== |
| - | <tabbox Why is it interesting?> | ||
| - | Axions are proposed as a solution to the [[open_problems:strong_cp_problem|strong CP problem]]. In some sense, it's the simplest solution, because the QCD vacuum energy has a minimum at $\bar{\theta}=0$ and the axion allows the vacuum to relax to this ground state. Without the axion the QCD vacuum is frozen and $\bar{\theta}$ is therefore fixed at some not-necessarily minimum value. | ||
| - | <tabbox Layman> | + | <tabbox Intuitive> |
| - | * A great laymen discussion of the axion mechanism is [[https://www.susanjfowler.com/blog/2016/9/17/from-the-fledgling-physicist-archives-an-introduction-to-axions|From The Fledgling Physicist Archives: An Introduction To Axions]] by Susan J Fowler. | + | * A great laymen discussion of the axion mechanism is [[https://www.susanjfowler.com/blog/2016/9/17/from-the-fledgling-physicist-archives-an-introduction-to-axions|An Introduction To Axions]] by Susan J Fowler. |
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| </blockquote> | </blockquote> | ||
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| - | <tabbox Student> | + | <tabbox Concrete> |
| + | |||
| + | * A really great and short introduction is [[https://www.sciencedirect.com/science/article/pii/S1631070511002039|The strong CP problem by Pierre Sikivie]] | ||
| * [[https://arxiv.org/abs/1712.03018|Axions and ALPs: a very short introduction]] by David J. E. Marsh | * [[https://arxiv.org/abs/1712.03018|Axions and ALPs: a very short introduction]] by David J. E. Marsh | ||
| - | <tabbox Researcher> | + | |
| + | |||
| + | <tabbox Abstract> | ||
| * [[https://arxiv.org/abs/1511.02867|The QCD axion, precisely]] by Giovanni Grilli di Cortona, Edward Hardy, Javier Pardo Vega, Giovanni Villadoro | * [[https://arxiv.org/abs/1511.02867|The QCD axion, precisely]] by Giovanni Grilli di Cortona, Edward Hardy, Javier Pardo Vega, Giovanni Villadoro | ||
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| - | <tabbox FAQ> | + | <tabbox Why is it interesting?> |
| + | |||
| + | Axions are proposed as a solution to the [[open_problems:strong_cp_puzzle|strong CP problem]]. In some sense, it's the simplest solution, because the QCD vacuum energy has a minimum at $\bar{\theta}=0$ and the axion allows the vacuum to relax to this ground state. Without the axion the QCD vacuum is frozen and $\bar{\theta}$ is therefore fixed at some not-necessarily minimum value. | ||
| | | ||
| <tabbox History> | <tabbox History> | ||
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| <cite>[[https://arxiv.org/pdf/1608.08969.pdf|Domestic Axion]] Gia Dvali</cite> | <cite>[[https://arxiv.org/pdf/1608.08969.pdf|Domestic Axion]] Gia Dvali</cite> | ||
| </blockquote> | </blockquote> | ||
| + | |||
| + | <blockquote>The notion of Peccei–Quinn (PQ) symmetry may seem contrived. Why should there be a U(1) symmetry which is broken at the quantum level but which is exact at the classical level? However, the reason for PQ symmetry may be deeper than we know at present. String theory contains many examples of symmetries which are exact classically but which are broken by quantum anomalies, including PQ symmetry [17–19]. Within field theory, there are examples of theories with automatic PQ symmetry, i.e. where PQ symmetry is a consequence of just the particle content of the theory without adjustment of parameters to special values. <cite>[[https://www.sciencedirect.com/science/article/pii/S1631070511002039|The strong CP problem by Pierre Sikivie]]</cite></blockquote> | ||
| + | |||
| + | <blockquote>There, a global U(1) symmetry (the PQ symmetry) which is almost exact but broken by the axial anomaly of QCD plays a crucial role. After spontaneous breaking, the effective θ-angle of QCD is cancelled by the vacuum expectation value (VEV) of the associated pseudo Nambu-Goldstone boson, the axion a. The origin of such a convenient global symmetry is, however, quite puzzling from the theoretical point of view in many aspects. By definition, the PQ symmetry is not an exact symmetry. Besides, the postulation of global symmetries is not comfortable in the sense of general relativity. It is also argued that all global symmetries are broken by quantum gravity effects [5–10]. If we | ||
| + | could regard the PQ symmetry as a U(1) gauge symmetry, there would be no suspicion about the exactness and | ||
| + | the consistency with quantum gravity. The PQ symmetry is, however, broken by the QCD anomaly, and hence, | ||
| + | it cannot be a consistent gauge symmetry as it is. <cite>https://arxiv.org/abs/1703.01112</cite></blockquote> | ||
| </tabbox> | </tabbox> | ||
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