$\mathcal{O}({\alpha }^3{\alpha }_s)$ Study on the yields and polarizations of $J/\psi ({\rm Y})$ within the framework of non-relativistic QCD via $\gamma \gamma \to J/\psi ({\rm Y})+\gamma +X$ at CEPC

Abstract

Within the framework of the non-relativistic QCD (NRQCD), we make a systematical study of the yields and polarizations of $J/\psi$ and ${\rm Y}$ via $\gamma \gamma \to J/\psi ({\rm Y})+\gamma +X$ in photon-photon collisions at the Circular Electron Positron Collider (CEPC), up to $\mathcal{O}({\alpha }^3{\alpha }_s)$. We find that this process at CEPC is quite "clean", namely the direct photoproduction absolutely dominate over the single- and double- resolved processes, at least 2 orders of magnitude larger. It is found that the next-to-leading order (NLO) QCD corrections will significantly reduce the results due to that the virtual corrections to ${}^3{S}_1^1$ is large and negative. For $J/\psi$, as $p_t$ increases, the color octet (CO) processes will provide increasingly important contributions to the total NLO results. Moreover the inclusion of CO contributions will dramatically change the polarizations of $J/\psi$ from toally transverse to longitudinal, which can be regarded as a distinct signal for the CO mechanism. However, for the case of ${\rm Y}$, the effects of the CO processes are negligible, both for yields and polarizations. For $J/\psi$, the dependence of the yields on the value of the renormalization scale ${\mu }_r$ is moderate, while significant for the polarization. The impact of the variation of ${\mu }_{\lambda }$ is found to be relatively slight. As for the case of ${\rm Y}$, the uncertainties of ${\mu }_r$ and ${\mu }_{\lambda }$ just bring about negligible effects. The future measurements on this semi-inclusive photoproductions of $J/\psi ({\rm Y})+\gamma +X$, especially on the polarization parameters of $J/\psi$, will be a good laboratory for the study of heavy quarkonium production mechanism and helpful to clarify the problems of the $J/\psi$ polarization puzzle.