Charmonium production in proton-nucleon and proton-nucleus collisions

Systematic nuclear effects discovered in the production ofJ/ψ particles in high-energy proton-nucleus collisions cannot be explained within the conventional picture in which a heavy color singlet $Qbar Q$ pair, after its production, experiences multiple scattering from nucleons of the residual nucleus. We propose to take into account color octet intermediate states formed in the primary production of the heavy-quark pairs. Using a perturbative QCD analysis we study the microscopic production mechanism for $Qbar Q$ pairs through gluon-gluon fusion with subsequent color neutralization in a strongly interacting medium. The influence of the surrounding matter on this process is investigated.