Production and polarization of direct $J/\psi$ to $\mathcal{O}({\alpha }_s^3)$ in the improved color evaporation model in collinear factorization

Abstract

We calculate the production and polarization of direct $J/\psi$ in the improved color evaporation model at $\mathcal{O}({\alpha }_s^3)$ in the collinear factorization approach. We present the first calculation of polarization parameters ${\lambda }_{\vartheta }$, ${\lambda }_{\varphi }$, and ${\lambda }_{\vartheta \varphi }$ in the helicity and the Collins-Soper frames, as well as the frame-invariant polarization parameter $\tilde{\lambda }$ as a function of transverse momentum. We find agreement with both $J/\psi$ cross sections and the invariant polarization parameters at small and moderate $p_T$.