Evidence for Colour-Octet Mechanism from CERN LEP2 gamma gamma -> J/psi + X Data

Abstract

We present theoretical predictions for the transverse-momentum distribution of J/psi mesons promptly produced in gamma gamma collisions within the factorization formalism of nonrelativistic quantum chromodynamics, including the contributions from both direct and resolved photons, and we perform a conservative error analysis. The fraction of J/psi mesons from decays of bottom-flavoured hadrons is estimated to be negligibly small. New data taken by the DELPHI Collaboration at LEP2 nicely confirm these predictions, while they disfavour those obtained within the traditional colour-singlet model.

11 pages (Latex), 3 figures (Postscript); updated experimental data included, references added, accepted for publication in Phys. Rev. Lett