First-Principles Determination of the QCD Contribution to the Axion-Photon Coupling Using Domain-Wall Fermions

Abstract

The axion-photon coupling, crucial for experimental axion searches and tests of the strong CP solution, receives a substantial model-independent contribution 𝒞_QCD from QCD dynamics. Next-to-leading-order chiral perturbation theory (NLO ChPT) in different frameworks has yielded puzzling discrepancies of up to 8%, motivating precise first-principles calculations. We present an independent lattice QCD determination using a method complementary to the recent background-field calculation. Computing pseudoscalar-to-two-photon three-point functions and exploiting anomalous Ward identities, we separate 𝒞_QCD into an exact anomaly contribution and a light-quark-mass-suppressed correction. The latter is computed using domain-wall fermions, whose excellent chiral symmetry strongly suppresses discretization effects. Working on two near-physical N_f=2+1 ensembles with continuum extrapolation, we obtain 𝒞_QCD^IS=1.619(30) (isospin-symmetric), 𝒞_QCD^IB=0.347(22) (isospin-breaking), and 𝒞_QCD=1.965(35) in total. While direct comparisons with published NLO ChPT predictions reveal apparent tensions, we identify their sources and show that the ChPT results can be reconciled with our lattice determination. Our result provides a first-principles benchmark for the QCD contribution to the axion-photon coupling and a quantitative test of competing ChPT descriptions.

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BibTeX RIS

BibTeX

@misc{lin2026firstprinciples,
  title = {{First-Principles Determination of the QCD Contribution to the Axion-Photon Coupling Using Domain-Wall Fermions}},
  author = {Tian Lin and Hao-Yang Cheng and Xu Feng and Lu-Chang Jin and Chuan Liu and Qi-Yuan Luo},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.22698},
  eprint = {2609.22698},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.22698},
}

RIS

TY  - UNPB
TI  - First-Principles Determination of the QCD Contribution to the Axion-Photon Coupling Using Domain-Wall Fermions
AU  - Tian Lin
AU  - Hao-Yang Cheng
AU  - Xu Feng
AU  - Lu-Chang Jin
AU  - Chuan Liu
AU  - Qi-Yuan Luo
PY  - 2026
DA  - 2026/09/19
PB  - arXiv
DO  - 10.48550/arxiv.2609.22698
UR  - https://arxiv.org/abs/2609.22698
AB  - The axion-photon coupling, crucial for experimental axion searches and tests of the strong CP solution, receives a substantial model-independent contribution 𝒞_QCD from QCD dynamics. Next-to-leading-order chiral perturbation theory (NLO ChPT) in different frameworks has yielded puzzling discrepancies of up to 8%, motivating precise first-principles calculations. We present an independent lattice QCD determination using a method complementary to the recent background-field calculation. Computing pseudoscalar-to-two-photon three-point functions and exploiting anomalous Ward identities, we separate 𝒞_QCD into an exact anomaly contribution and a light-quark-mass-suppressed correction. The latter is computed using domain-wall fermions, whose excellent chiral symmetry strongly suppresses discretization effects. Working on two near-physical N_f=2+1 ensembles with continuum extrapolation, we obtain 𝒞_QCD^IS=1.619(30) (isospin-symmetric), 𝒞_QCD^IB=0.347(22) (isospin-breaking), and 𝒞_QCD=1.965(35) in total. While direct comparisons with published NLO ChPT predictions reveal apparent tensions, we identify their sources and show that the ChPT results can be reconciled with our lattice determination. Our result provides a first-principles benchmark for the QCD contribution to the axion-photon coupling and a quantitative test of competing ChPT descriptions.
ER  -

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