Window for preferred axion models

Abstract

Publisher's copyright statement:Reprinted with permission from the American Physical Society: Physical Review D 96, 075003 c 2017 by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modied, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We discuss phenomenological criteria for defining "axion windows," namely regions in the parameter space of the axion-photon coupling where realistic models live. Currently, the boundaries of this region depend on somewhat arbitrary criteria, and it would be highly desirable to specify them in terms of precise phenomenological requirements. We first focus on hadronic axion models within post-inflationary scenarios, in which the initial abundance of the new vectorlike quarks Q is thermal. We classify their representations R Q by requiring that (i) the Q are sufficiently short lived to avoid issues with long-lived strongly interacting relics, (ii) the theory remains weakly coupled up to the Planck scale. The more general case of multiple R Q is also studied, and the absolute upper and lower bounds on the axion-photon coupling as a function of the axion mass is identified. Pre-inflationary scenarios in which the axion decay constant remains bounded as f a ≤ 5 × 10 11 GeV allow for axion-photon couplings only about 20% larger. Realistic Dine-Fischler-Srednicki-Zhitnitsky type of axion models also remain encompassed within the hadronic axion window. Some mechanisms that can allow to enhance the axion-photon coupling to values sizeably above the preferred window are discussed.

Possible roles of Peccei-Quinn symmetry in an effective low energy model

“…Apart from the above interesting feature, it is a crucial problem how this axion model could be distinguished from others based on different PQ charge assignments. On this point, we should note that the axion nature determined by the PQ charge assignment might be experimentally examined through the axion-photon coupling [19,20]. The present model predicts it as…”

Flavour Violating Axions

“…By extending the SM with the axion at low-energy, it is somewhat artificial to require that flavour mixing beyond the CKM is unphysical and we should rather let experiments to decide. In the meanwhile, 8 Gauging the horizontal flavour group can also lead to an accidental global U(1) whose spontaneous breaking delivers a flavoured arion [38] or a flavoured axion [39][40][41]. 9 Up to the usual dependence from f a and tan β, as in universal DFSZ models.…”

“…A particularly natural realization is to identify the U(1) PQ with the horizontal U(1) symmetry responsible for the Yukawa hierarchies [34][35][36][37]. 8 Models of Froggatt-Nielsen [42] type have recently regained some attention (see e.g. [43,44]), and they typically predict axion flavour transitions controlled by the CKM matrix, although subject to irreducible O(1) uncertainties common also to other flavour models.…”

TASI Lectures on the Strong CP Problem and Axions

“…Photon couplings parametrically smaller than f a are only achievable if the two terms cancel each other. This cancellation accidentally happens for some GUTs where f UV = f a /2 [58], and can happen if one chooses Casmirs of the UV quarks to give f UV ∼ f a /1.92 [59][60][61]. A way of obtaining parametrically small g aγγ without fine tuning is not known.…”