Quality of the Peccei-Quinn symmetry in the aligned QCD axion and cosmological implications

Abstract

We show that the required high quality of the Peccei-Quinn symmetry can be naturally explained in the aligned QCD axion models where the QCD axion arises from multiple axions with decay constants much smaller than the axion window, e.g., around the weak scale. Even in the presence of general Planck-suppressed Peccei-Quinn symmetry breaking operators, the effective strong CP phase remains sufficiently small in contrast to the standard axion models without the alignment. The QCD axion potential has small or large modulations due to the symmetry breaking operators, which can significantly affect the axion cosmology. When the axions are trapped in different minima, domain walls appear and their scaling behavior suppresses the axion isocurvature perturbations at super-horizon scales. Our scenario predicts many axions and saxions coupled to gluons, and they may be searched for at collider experiments. In particular, the recently found diphoton excess at 750 GeV could be due to one of such (s)axions.

Topological defects and nano-Hz gravitational waves in aligned axion models

“…Recently, four of the present authors (TH, KSJ, NK, and FT) applied the alignment mechanism to the QCD axion, and studied its cosmological and phenomenological implications [35,36]. In this model, the QCD axion is given by a flat direction composed of multiple axions.…”

“…One of the axions or saxions in the model can explain the recently found diphoton excess in LHC [37][38][39][40]. 1 In addition, the required high quality of the PQ symmetry [44,45] can be naturally realized in this model [36].…”

“…One of the virtues of the aligned QCD axion model is that the high quality of the PQ symmetry is naturally explained [35,36]. This is because the actual symmetry breaking scale is much smaller than F a , and so, any Planck-suppressed PQ breaking terms give only…”

“…In some case, such Planck-suppressed PQ breaking terms have interesting implications for the QCD axion dynamics [36]. 3 Another cosmological implication is that the PQ symmetry is easily restored in the early Universe.…”

“…Once the domain-wall network follows the scaling law, a typical curvature radius is of order the Hubble parameter. 6 In the presence of extra PQ breaking terms, each axion is trapped at each potential minima even before the QCD phase transition [36]. In this case, the string-wall network sooner disappears and f can be a larger value.…”