Ψ0 production by high energy protons on nuclei and quark fragmentation functions

Data on inclusive Ψ0 production by 400 GeV protons incident on Be and Pb nuclei are used to extract quark fragmentation functions. The forward (x approx.= 0.5) Ψ0 production from wounded quarks is small, but changing its estimate well within errors, one changes the expected single spectator quark fragmentation function by a large factor. This effect, unnoticed in previous analyses, leaves the spectator diquark fragmentation function as our only reliable estimate. This function is similar to the proton into K+ fragmentation function and to the valence quark in the proton structure function, but very different from the proton diquark into Λ fragmentation function. Data are consistent with the assumption that the two quarks in the diquark fragment independently, but the experimental errors are too large to make a strong statement. (author)