New extraction of color-octet NRQCD matrix elements from charmonium hadroproduction

Abstract

We re-analyze Tavatron data on charmonium hadroproduction in the framework of the color-octet model implemented in the event generator PYTHIA taking into account initial-state radiation of gluons and Altarelli-Parisi evolution of final-state gluons fragmenting into $c\bar{c}$ pairs. We obtain new values for the color-octet matrix elements relevant to this production process. We discuss the sensitivity of our results to the transverse momentum lower cut-off employed in the generation to avoid the problematic $p_t\to 0$ region, arguing about the reliability of our previous extraction of the NRQCD matrix elements for the ${}^3{S}_1^{(8)}$ and ${}^1{S}_0^{(8)}{+}^3{P}_J^{(8)}$ contributions. Finally we extrapolate to LHC energies to get predictions on the $J/\psi$ direct production rate.

Comments: Latex, espcrc2, 4 pages, 3 EPS figures, Montpellier QCD99 Conference. to appear in Nucl. Phys. B [Proc. Suppl.]