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Pion to photon transition form factor: Beyond valence quarks

Xiaoyi Wu1,2, Zhimin Zhu1,2, Ziyang Lin1,2, Chandan Mondal1,2,*, Jiangshan Lan1,2,3,†, Xingbo Zhao1,2,5,6, and James P. Vary4 (BLFQ Collaboration)

  • *Contact author: mondal@impcas.ac.cn
  • †Contact author: jiangshanlan@impcas.ac.cn
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Phys. Rev. D 112, 014035 – Published 22 July, 2025

DOI: https://doi.org/10.1103/qxbp-63qm

Abstract

We investigate the singly virtual transition form factor (TFF) for the π0→γ*γ process in the spacelike region using the hard-scattering formalism within the basis light-front quantization (BLFQ) framework. This form factor is expressed in terms of the perturbatively calculable hard-scattering amplitudes (HSAs) and the light-front wave functions (LFWFs) of the pion. We obtain the pion LFWFs by diagonalizing the light-front QCD Hamiltonian, which is determined for its constituent quark-antiquark and quark-antiquark-gluon Fock sectors with a three-dimensional confinement. We employ the HSAs up to next-to-leading order (NLO) in the quark-antiquark Fock sector and leading order (LO) in the quark-antiquark-gluon Fock sector. The NLO correction to the TFF in the quark-antiquark Fock sector is of the same order as the LO contribution to the TFF in the quark-antiquark-gluon Fock sector. We find that while the quark-antiquark-gluon Fock sector has minimal effect in the large momentum transfer (Q2) region, it has a noteworthy impact in the low-Q2 region. Our results show that, after accounting for both Fock sectors, the TFF within the BLFQ framework aligns well with existing experimental data, particularly in the low Q2 region.

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Phys. Rev. D 112, 014035– Published 22 July, 2025
  • Received 12 February 2025
  • Accepted 20 June 2025
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