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Structure of multiaxion solutions to the strong CP problem

Mario Fernández Navarro1,*, Marta F. Zamoro2,†, Marko Pesut1,3,‡, and Xavier Ponce Díaz4,§

  • *Contact author: mario.fernandeznavarro@physik.uzh.ch
  • †Contact author: marta.zamoro@uam.es
  • ‡Contact author: marko.pesut@psi.ch
  • §Contact author: xavier.poncediaz@unibas.ch
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Phys. Rev. D 114, 035019 – Published 11 August, 2026

DOI: https://doi.org/10.1103/hw7t-45p2

Abstract

A broad experimental program is targeting the QCD axion band predicted by single-axion solutions to the strong CP problem. Multiaxion theories provide a well-motivated departure from this canonical picture, since additional states generically modify the mass-photon-coupling relation. We investigate the general structure of multiaxion solutions to the strong CP problem and study the different qualitative mass-coupling patterns that arise, including axions to the right of the QCD band, axions in the experimentally accessible region to its left, and scenarios in which the QCD axion band itself is displaced. This general treatment reveals a broad set of phenomenological possibilities that are not captured by more restrictive assumptions. In particular, we identify the structure of Peccei-Quinn (PQ) symmetry breaking and the relative alignment between the QCD and electromagnetic anomalies as key ingredients determining the location of the axions in parameter space. Combining these ingredients, we derive a general sum rule for N-axion systems that incorporates both general PQ breaking and nonuniversal anomaly coefficients. We apply the framework to the two-axion system and to general multiaxion setups, identifying UV-complete theories in which the different phenomenological regimes arise naturally. Our results motivate an extended axion search program and have implications for our understanding of fundamental physics and the ultraviolet completion of the Standard Model.

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Phys. Rev. D 114, 035019– Published 11 August, 2026
  • Received 2 June 2026
  • Accepted 16 July 2026
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