Complete NLO QCD study of single- and double-quarkonium hadroproduction in the colour-evaporation model at the Tevatron and the LHC

Abstract We study the Single-Parton-Scattering (SPS) production of double quarkonia ( J / ψ + J / ψ , J / ψ + ϒ , and ϒ + ϒ ) in pp and p p ¯ collisions at the LHC and the Tevatron as measured by the CMS, ATLAS, LHCb, and D0 experiments in the Colour-Evaporation Model (CEM), based on the quark-hadron duality, including Next-to-Leading Order (NLO) QCD corrections up to α s 5 . To do so, we also perform the first true NLO –up to α s 4 – study of the p T -differential cross section for single-quarkonium production. This allows us to fix the non-perturbative CEM parameters at NLO accuracy in the region where quarkonium-pair data are measured. Our results show that the CEM at NLO in general significantly undershoots these experimental data and, in view of the other existing SPS studies, confirm the need for Double Parton Scattering (DPS) to account for the data. Our NLO study of single-quarkonium production at mid and large p T also confirms the difficulty of the approach to account for the measured p T spectra; this is reminiscent of the impossibility to fit single-quarkonium data with the sole S 1 [ 8 ] 3 NRQCD contribution from gluon fragmentation. We stress that the discrepancy occurs in a kinematical region where the new features of the improved CEM are not relevant.