Next-to-leading order analysis of $$J/\psi + \gamma $$ production in photon–photon collisions at CEPC

Abstract We systematically investigate the production of $$J/\psi + \gamma $$ J / ψ + γ in $$\gamma \gamma $$ γ γ collisions within nonrelativistic QCD (NRQCD) factorization, with the direct-photon channel calculated specifically up to the next-to-leading order in $$\alpha _s$$ α s . Calculations for CEPC energy region show the resolved photon contribution is negligible, while the direct photon process dominates, yielding substantial annual $$J/\psi $$ J / ψ yields. Significant modifications to $$J/\psi $$ 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 $$P_J^{[8]}$$ P J [ 8 ] LDME, while being insensitive to the 1 $$S_0^{[8]}$$ S 0 [ 8 ] and 3 $$S_1^{[8]}$$ S 1 [ 8 ] LDMEs. Leveraging the cleaner environment of $$e^+e^-$$ e + e - collisions versus hadronic processes, the production of $$J/\psi $$ J / ψ associated with a photon in $$\gamma \gamma $$ γ γ collisions provides a high-precision platform to test LDMEs universality and resolve longstanding $$J/\psi $$ J / ψ polarization puzzles.