Chiral models of composite axions and accidental Peccei-Quinn symmetry

Abstract

We introduce a class of composite axion models that provide a natural solution to the strong CP problem, and possibly account for the observed dark matter abundance. The QCD axion arises as a composite Nambu-Goldstone boson (NGB) from the dynamics of a chiral gauge theory with a strongly-interacting and confining SU(N) factor and a weakly-interacting U(1), with no fundamental scalar fields. The Peccei-Quinn (PQ) symmetry is accidental and all the mass scales are generated dynamically. We analyze specific models where the PQ symmetry is broken only by operators of dimension 12 or higher. We also classify several other models where the PQ symmetry can be potentially protected up to the dimension 15 or 18 level. Our framework can be easily extended to a scenario where the Standard Model (SM) is unified into a simple gauge group, and we discuss the case of non-supersymmetric SU(5) unification. The GUT models predict the existence of additional pseudo NGBs, parametrically lighter than the GUT and PQ scales, which could have an impact on the cosmological evolution and leave observable signatures. We also clarify the selection rules under which higher-dimensional PQ-violating operators can generate a potential for the axion in the IR, and provide a discussion of the discrete symmetries in composite axion models associated to the number of domain walls. These results can be of general interest for composite axion models based on a QCD-like confining gauge group.

Integer solutions to the anomaly equations for a class of chiral gauge theories

“…2 )d 2 = 0. (43) Since the possible values that {p i , q i } can take are already determined by (13), it is possible to verify whether any of them satisfy (43) upon direct substitution. After a few algebraic manipulations,…”

“…We consider the case in which D ij = 0 for all pairings, that is realised in all the n f = 3 composite axion models classified in Ref. [13], see Tab. 2.…”

“…We shall do so in the context of the models of composite QCD axion recently introduced in Ref. [13], that we briefly review. We refer the reader to [13] and references therein for a more detailed discussion, references to the original literature and alternative approaches.…”

“…The class of theories we consider in this work includes the theories introduced in Ref. [13] as composite axion models with accidental Peccei-Quinn (PQ) symmetry, and can provide a resolution to the QCD axion quality problem by forbidding local PQ violating operators up to very high dimensions, depending on the integer values of the charges. They generalise previous constructions of models beyond the SM with accidentally stable composite dark matter candidates [14][15][16].…”

“…Our general solution for the anomaly equations can be useful, among other things, to assess which charge assignments give rise to a high quality QCD axion in the models of Ref. [13].…”