Relativistic corrections to prompt double charmonium hadroproduction near threshold

We calculate the relativistic corrections to prompt J/ pair and J/+ (2S) hadroproduction through the color-singlet channel within the framework of nonrelativistic QCD (NRQCD) factorization. The short-distance coefficients are obtained by matching full-QCD and NRQCD calculations at the partonic level, in which both squared amplitude and phase space are expanded in v². We find that such an expansion of the phase space spoils the convergence of NRQCD factorization near the production threshold. To fix this problem, we propose to modify the matching between full QCD and NRQCD by adopting the physical phase space. In this modified approach, the theoretical uncertainties due to the choice of charm quark mass mc are largely reduced and the overall agreement of our predictions with LHC data is significantly improved, both as for total and differential cross sections.