Abstract
Composite axion scenarios offer a robust field theoretic justification for the existence of a Peccei–Quinn symmetry of high quality. We present a new class of realizations that are naturally embedded in Grand-Unified Theories, retain asymptotic freedom for all gauge groups, and protect the axion shift symmetry up to operators of dimension 12. Our setup leads to a number of distinctive signatures at low energies. First, additional composite scalars are predicted; some of these are viable dark matter candidates for values of the decay constant that are likely too low for the QCD axion abundance to be relevant. Second, an approximate unification of the Standard Model gauge couplings takes place at the axion scale, while leaving the actual quark-lepton unification at much higher energies as usual. This suggests the existence of GUT relics with Standard Model gauge quantum numbers at potentially accessible scales.
Highlights
Composite axion models can feature both ingredients. In such a framework the axion arises as a Nambu-Goldstone boson (NGB) of some exotic strong dynamics that undergoes chiral symmetry breaking at a scale ∼ fa [13]
A value of fa that is simultaneously parametrically larger than the weak scale and lower than the Planck scale is completely natural in composite axion models, where fa gets generated via dimensional transmutation
Solve the Strong CP Problem, we have to make sure that the NGB associated to the breaking of U (1)PQ, namely the QCD axion a, has a potential dominated by non-perturbative QCD effects
Summary
Consider a model based on the gauge symmetry SU (Nc) × S O(10). The representations of the axion constituents are shown in Table 2, and are all chiral under the weakly-gauged S O(10). Without the gauged S O(10), the approximate global symmetry breaking pattern Gglobal → Hglobal ≡ SU (32)V × U (1)B produces 1023 would-be NGBs in the 32 ⊗ 32 − 1 ∈ SU (32)V We may find their gauge quantum numbers by decomposing the broken generators in irreducible representations of the unbroken gauge group. The factor of 2 in is chosen to conform to the standard definition 0|JPμQ(0)|a( p) = i f pμ Some of these pseudo-scalars have anomalous couplings to the SM gauge fields proportional to the anomaly coefficients Agαaugeδ AB = 4NcTr[Tα TgAaugeTgBauge], where TgAauge are the SM generators, with A, B indices in the adjoint, and the trace runs over all the flavors of the axion constituents. The (misaligning) effect induced by unavoidable higher-dimensional operators is suppressed by at least powers of f 2/ fG2UT gu2nbroken/16π 2 and is much smaller than the (aligning) contributions due to the gauge couplings
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
