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

Abstract We systematically investigate the production of J/ + J / ψ + γ in γ γ collisions within nonrelativistic QCD (NRQCD) factorization, with the direct-photon channel calculated specifically up to the next-to-leading order in 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/ J / ψ yields. Significant modifications to J/ 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/ J / ψ associated with a photon in γ γ collisions provides a high-precision platform to test LDMEs universality and resolve longstanding J/ J / ψ polarization puzzles.