slac-pub-2605 - SLAC - Stanford University

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(8) Some of the most important tests of QCD involve real photons. In
contrast to vector-meson dominance ideas , point-like interactions are predicted
to dominate the hadronic interactions of on-shell photons in large momenta
transfer reactions C511. This feature is particularly clear in the QCD
predictions for the two-photon inclusive [52] and exclusive processes [5]
yy + jet+jet and yy + 6, and the photon structure function at large momentum
transfer. Higher twist terms should be relatively less important in photon
reactions compared to hadron reactions.
(9) The generation of hadron multiplicity is not completely understood
in QCD. However a number of features are anticipated which appear to differ
strongly from parton model expectations: For example, in QCD (a) the final
state multiplicity reflects the color separation set up in the collision [53];
this is believed to even hold in low pT hadron collisions since the basic
interactions involve color exchange; (b) the evolution in space-time corres-
ponds to an inside-outside cascade -- the slowest hadrons are created first;
(c) the final state multiplicity in the Drell-Yan process AB -+ !LzX is expected
to reflect the creation of 3c and 3: spectator systems at the moment of
annihilation C531; and (d) the multiplicity of gluon jets and other color
octet systems should reflect its higher color charge relative to color triplet
systems C531. It is clear that QCD provides a novel perspective for multi-
particle phenomena.
(10) Another critical testing ground of QCD dynamics is the study of
quark and gluon interactions in nuclear targets c551. For example, consider
the Drell-Yan process pU + u+u-X. If the factorization theorem for inclusive
reactions is correct the cross section at large pair mass Q is given by the
convolution of the anti-proton structure function G- -(x ,Q) with the nuclear
qlp a
distribution function G q/U%‘Q) -- which for large Q2 is -AGqjN$ ,Q) .
Thus the factorization theorem predicts no absorption of the constituents
of the anti-proton despite the fact that the F suffers repeated inelastic
collisions in its passage through the nuclear volume! If this is really