We explore the signals of axion-like particles (ALPs) in flavor-changing neutral current (FCNC) processes. The most general effective linear Lagrangian for ALP couplings to the electroweak bosonic sector is considered, and its contribution to FCNC decays is computed up to one-loop order. The interplay between the different couplings opens new territory for experimental exploration, as analyzed here in the ALP mass range 0 < ma < ∼ 5 GeV. When kinematically allowed, K → πνν decays provide the most stringent constraints for channels with invisible final states, while B-meson decays are more constraining for visible decay channels, such as displaced vertices in B → K ( * ) µ + µ − data. The complementarity with collider constraints is discussed as well.

We consider an enlarged color sector which solves the strong CP problem via new massless fermions. The spontaneous breaking of a unified color group into QCD and another confining group provides a source of naturally large axion mass m a due to small size instantons. This extra source of axion mass respects automatically the alignment of the vacuum, ensuring a low-energy CP-conserving vacuum. The mechanism does not appeal to a Z 2 "mirror" copy of the SM, nor does it require any fine-tuning of the axion-related couplings at the unification scale. There is no light axion and uncharacteristically the lighter spectrum contains instead sterile fermions. The axion scale f a can be naturally brought down to a few TeV, with an exotic spectrum of colored pseudoscalars lighter than this scale, observable at colliders exclusively via strong interactions. The {m a , f a } parameter space which allows a solution of the strong CP problem is thus enlarged well beyond that of invisible axion models.

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.

We determine the model-independent component of the couplings of axions to electroweak gauge bosons, induced by the minimal coupling to QCD inherent to solving the strong CP problem. The case of the invisible QCD axion is developed first, and the impact on W and Z axion couplings is discussed. The analysis is extended next to the generic framework of heavy true axions and low axion scales, corresponding to scenarios with enlarged confining sector. The mass dependence of the coupling of heavy axions to photons, W and Z bosons is determined. Furthermore, we perform a two-coupling-at-a-time phenomenological study where the gluonic coupling together with individual gauge boson couplings are considered. In this way, the regions excluded by experimental data for the axion-W W , axion-ZZ and axion-Zγ couplings are determined and analyzed together with the usual photonic ones. The phenomenological results apply as well to ALPs which have anomalous couplings to both QCD and the electroweak bosons.

We show that dark matter can be accounted for by an axion that solves the strong CP problem, but is much lighter than usual due to a Z N symmetry. The whole mass range from the canonical QCD axion down to the ultra-light regime is allowed, with 3 ≤ N 65. This includes the first proposal of a "fuzzy dark matter" QCD axion with m a ∼ 10 −22 eV. A novel misalignment mechanism occurs -trapped misalignment-due to the peculiar temperature dependence of the Z N axion potential. The dark matter relic density is enhanced because the axion field undergoes two stages of oscillations: it is first trapped in the wrong minimum, which effectively delays the onset of true oscillations. Trapped misalignment is more general than the setup discussed here, and may hold whenever an extra source of Peccei-Quinn breaking appears at high temperatures. Furthermore, it will be shown that trapped misalignment can dynamically source the recently proposed kinetic misalignment mechanism. All the parameter space is within tantalizing reach of the experimental projects for the next decades. For instance, even Phase I of CASPEr-Electric could discover this axion.