J/ψ production at NLO with a scale-dependent color-evaporation model

Nearly ten years ago, Kang, Ma, Qiu, and Sterman derived an evolution equation for a QQ¯ pair fragmenting into a quarkonium. In this study we explore the consequence of this evolution for the color-evaporation model, focusing on J/ψ transverse-momentum (pt) distributions in proton-proton collisions. We show that, as expected, it softens the spectrum obtained by fixed-order calculations. While next-to-leading-order calculations strongly overestimate data at large pt, ours, including the (approximate) QQ¯ evolution and next-to-leading-order cross sections computed with Madgraph, are in good agreement with experiments. Since our study with the color-evaporation model shows a significant effect of the QQ¯ evolution at large pt, a determination of scale-dependent long-distance-matrix elements of non-relativistic QCD could be necessary. To describe data at small and intermediate pt, we use the kt-factorization approach, and we argue that quarkonia data could help constrain unintegrated parton densities.