Next-to-leading order analysis of J/ψ + γ production in photon–photon collisions at CEPC

We systematically investigate the production of J/ψ + γ J / ψ + γ in γ γ γ γ collisions within nonrelativistic QCD (NRQCD) factorization, with the direct-photon channel calculated specifically up to the next-to-leading order in α ₛ α s . Calculations for CEPC energy region show the resolved photon contribution is negligible, while the direct photon process dominates, yielding substantial annual J/ψ J / ψ yields. Significant modifications to J/ψ J / ψ polarization parameters emerge from color-octet mechanisms, and different NRQCD long distance matrix elements (LDMEs) further yield distinct polarization patterns. Furthermore, the polarization predictions are highly sensitive to the 3 PJ[8] P J [ 8 ] LDME, while being insensitive to the 1 S₀[8] S 0 [ 8 ] and 3 S₁[8] S 1 [ 8 ] LDMEs. Leveraging the cleaner environment of e⁺e⁻ e + e - collisions versus hadronic processes, the production of J/ψ J / ψ associated with a photon in γ γ γ γ collisions provides a high-precision platform to test LDMEs universality and resolve longstanding J/ψ J / ψ polarization puzzles.