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  • Open Access

Regulated chiral gauge theory and the strong CP problem

David B. Kaplan1,* and Srimoyee Sen2,†

  • 1Institute for Nuclear Theory, Box 351550, Seattle, Washington 98195-1550, USA
  • 2Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA

  • *Contact author: dbkaplan@uw.edu
  • †Contact author: srimoyee08@gmail.com
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Phys. Rev. D 114, 014508 – Published 16 July, 2026

DOI: https://doi.org/10.1103/wld4-f12c

Abstract

Four-dimensional chiral gauge theory can be formulated as the boundary theory on a five-dimensional manifold in a manner that may be realized on a finite lattice. There are interesting features of these theories that defy a purely four-dimensional conception of universality. We find that QCD when embedded in a chiral gauge theory (the Standard Model) and regulated this way can simultaneously avoid both the U(1)A problem and the strong CP problem, with a central role played by fermion zero modes localized far away in the fifth dimension. In this way it differs from conventional lattice QCD formulated as a stand-alone theory, universality being violated by inaccessible light modes in the five-dimensional bulk. Our analysis builds on recent work by others that highlights the role of global U(1) symmetries in five-dimensional formulations of four-dimensional chiral gauge theories, and the generic appearance of fermion zero modes in the bulk.

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Phys. Rev. D 114, 014508– Published 16 July, 2026
  • Received 28 August 2025
  • Accepted 17 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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