An even lighter QCD axion

Abstract

We explore whether the axion which solves the strong CP problem can naturally be much lighter than the canonical QCD axion. The $$ {Z}_{\mathcal{N}} $$ Z N symmetry proposed by Hook, with $$ \mathcal{N} $$ N mirror and degenerate worlds coexisting in Nature and linked by the axion field, is considered in terms of generic effective axion couplings. We show that the total potential is safely approximated by a single cosine in the large $$ \mathcal{N} $$ N limit, and we determine the analytical formula for the exponentially suppressed axion mass. The resulting universal enhancement of all axion interactions relative to those of the canonical QCD axion has a strong impact on the prospects of axion-like particle experiments such as ALPS II, IAXO and many others: experiments searching for generic axion-like particles have in fact discovery potential to solve the strong CP problem. The finite density axion potential is also analyzed and we show that the $$ {Z}_{\mathcal{N}} $$ Z N asymmetric background of high-density stellar environments sets already significant model-independent constraints: 3 ≤ $$ \mathcal{N} $$ N ≲ 47 for an axion scale fa ≲ 2.4 × 1015 GeV, with tantalizing discovery prospects for any value of fa and down to $$ \mathcal{N} $$ N ∼ 9 with future neutron star and gravitational wave data, down to the ultra-light mass region. In addition, two specific ultraviolet $$ {Z}_{\mathcal{N}} $$ Z N completions are developed: a composite axion one and a KSVZ-like model with improved Peccei-Quinn quality.

Gamma-ray Cosmology and Tests of Fundamental Physics

“…Additionally, theoretical advancements open up the parameter space of the QCD axion towards lower masses which could be probed with gamma-ray instruments (e.g., Refs. [221][222][223]). The development of open-source software [224,225] facilitates the computation of photon-ALP oscillation probabilities in various magnetic field environments for young scientists entering the domain.…”

Photophilic hadronic axion from heavy magnetic monopoles

“…For example, in the clockwork axion model [13], in order to get an enhancement of the axion-photon coupling by six orders of magnitude compared to the KSVZ model, one has to introduce at least 13 new scalar fields. A similar enhancement by six orders of magnitude in all couplings to SM particles is achievable in the Z N axion model [14,15], but it requires N = 45 copies of the SM. Although quite non-minimal from the theory side, such an enhancement would allow one to explain some uneven astrophysical observations concerning cooling of the horizontal branch stars in globular clusters [16] and anomalous TeV-transparency of the Universe [17,18], not to mention that such photophilic axions can be well probed experimentally in the nearest future.…”

Filtered asymmetric dark matter during the Peccei-Quinn phase transition

“…Connecting the mirror worlds by the axion [58], in the UV axion model the PQ scalar can be transformed as φ → exp(2πi/N)φ. Thus, considering f a = 100 TeV, the PT parameters can be retained at T n 9.19 TeV.…”