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The common view that structure functions measured in deep inelastic lepton scattering are determined by the probability of finding quarks and gluons in the target is not correct in gauge theory. We show that gluon exchange between the fast, outgoing partons and target spectators, which is usually assumed to be an irrelevant gauge artifact, affects the leading twist structure functions in a profound way. This observation removes the apparent contradiction between the projectile (eikonal) and target (parton model) views of diffractive and small x B phenomena. The diffractive scattering of the fast outgoing quarks on spectators in the target causes shadowing in the DIS cross section. Thus the depletion of the nuclear structure functions is not intrinsic to the wave function of the nucleus, but is a coherent effect arising from the destructive interference of diffractive channels induced by final state interactions. This is consistent with the Glauber-Gribov interpretation of shadowing as a rescattering effect.

Charm and bottom production near threshold is sensitive to the multi-quark, gluonic, and hiddencolor correlations of hadronic and nuclear wavefunctions in QCD since all of the target's constituents must act coherently within the small interaction volume of the heavy quark production subprocess. Although such multi-parton subprocess cross sections are suppressed by powers of 1/m 2 Q , they have less phase-space suppression and can dominate the contributions of the leading-twist single-gluon subprocesses in the threshold regime. The small rates for open and hidden charm photoproduction at threshold call for a dedicated facility. PACS: 13.60. Le, 12.40.Nn, 12.40.Lg The threshold regime of charmonium and open charm production can provide a new window into multi-quark, gluonic, and hidden-color correlations of hadronic and nuclear wavefunctions in QCD. For example, consider charm photoproduction γ p → J/ψ p at the threshold energy E lab γ = 8.20 GeV. [See Fig. 1.] The available production energy cannot be wasted at threshold, so all three valence quarks of the target nucleon must interact coherently within the small interaction volume of the heavy quark production subprocess. In the case of threshold charm photoproduction on a deuteron γ d → J/ψ d, all color configurations of the six valence quarks will be involved at the short-distance scale 1/m c . Thus the exchanged gluons can couple to a color-octet quark cluster and reveal the "hidden-color" part of the nuclear wave function, a domain of short-range nuclear physics where nucleons lose their identity [1][2][3].At high energies the dominant contribution to an inclusive process involving a hard scale Q comes from "leading twist" diagrams, characterized by only one parton from each colliding particle participating in the large momentum subprocess. Since the transverse size scale of the hard collision is 1/Q, only partons within this distance can affect the process. The likelihood that two partons of the incident hadrons can be found so close to each other is typically proportional to the transverse area 1/Q 2 and leads to the suppression of higher-twist, multi-parton contributions. However, in contrast to charm production at high energy, charm production near threshold requires all of the target's constituents to act coherently in the heavy quark production process: only compact proton Fock states with a radius of order of the Compton wavelength of the heavy quark can contribute to charm production at threshold. Although the higher-twist subprocess cross sections are suppressed by powers of 1/m 2 c , they have much less phase-space suppression at threshold. Thus charm production at threshold is sensitive to short-range correlations between the valence quarks of the target, and higher-twist multi-gluon exchange reactions can dominate over the contributions of the leadingtwist single-gluon subprocesses. All of the partons of the target wavefunction have to transfer their energy to the charm quarks within their proper creation time 1/mc, and must be within this tran...

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