['The', 'charmonium', 'production', 'has', 'long', 'been', 'considered', 'as', 'a', 'good', 'process', 'for', 'investigating', 'both', 'perturbative', 'and', 'nonperturbative', 'properties', 'of', 'quantum', 'chromodynamics', '(', 'QCD', ')', ',', 'because', 'of', 'the', 'relatively', 'large', 'difference', 'between', 'the', 'scale', 'at', 'which', 'the', 'charm', '–', 'quark', 'pair', 'is', 'produced', 'at', 'the', 'parton', 'level', 'and', 'the', 'scale', 'at', 'which', 'it', 'evolves', 'into', 'a', 'quarkonium', '.']