inter action ▁processes ▁can ▁not ▁be ▁calculated ▁directly ▁with ▁perturb ative ▁Q CD , ▁since ▁one ▁cannot ▁observe ▁free ▁qu arks ▁and ▁gl u ons ▁due ▁to ▁color ▁conf in ement . ▁For ▁example , ▁the ▁structure ▁had rons ▁has ▁a ▁non - pert urb ative ▁nature . ▁To ▁account ▁for ▁this , ▁phys ic ists ▁developed ▁the ▁Q CD ▁factor ization ▁theorem , ▁which ▁separ ates ▁the ▁cross ▁section ▁into ▁two ▁parts : ▁the ▁process ▁dependent ▁perturb atively - calcul able ▁short - distance ▁part on ▁cross ▁section , ▁and ▁the ▁universal ▁long - distance ▁functions . ▁These ▁universal ▁long - distance ▁functions ▁can ▁be ▁measured ▁with ▁global ▁fit ▁to ▁experiments ▁and ▁include ▁part on ▁distribution ▁functions , ▁fragment ation ▁functions , ▁multi - part on ▁correlation ▁functions , ▁generalized ▁part on ▁distributions , ▁generalized ▁distribution ▁amplit udes ▁and ▁many ▁kinds ▁of ▁form ▁factors . ▁There ▁are ▁several ▁collabor ations ▁for ▁each ▁kind ▁of ▁universal ▁long - distance ▁functions . ▁They ▁have ▁become ▁an ▁important ▁part ▁of ▁modern ▁particle ▁physics . ▁ ▁External ▁links ▁ ▁F actor ization ▁of ▁Hard ▁Process es ▁in ▁Q CD ▁ ▁Category : Quant um ▁chrom od ynam ics <0x0A> </s> ▁Fl at ▁Creek ▁is ▁a ▁stream ▁in ▁Ri ple y ▁County ▁in ▁the ▁U . S . ▁state ▁of ▁Missouri . ▁It ▁is ▁a ▁trib ut ary ▁of ▁the ▁Little ▁Black ▁River . ▁ ▁Fl at ▁Creek ▁was ▁so ▁named ▁for ▁the ▁fact ▁the ▁surrounding ▁land ▁is ▁flat . ▁ ▁See ▁also ▁List ▁of ▁rivers ▁of