  Parameterizing the charm and anticharm quark momentum distributions by the
Tsallis distribution, we study the nonequilibrium effect on the charmonium
production rate in a quark-gluon plasma up to the next-to-leading order in
perturbative QCD. We find that nonequilibrium charm and anticharm quarks
suppress the charmonium production rate compared to that from equilibrated
ones. We further show that the suppression factor calculated with the charm
quark relaxation time, which has been frequently used in the literature, is
close to our results.
