Novel Aspects of Hard Diffraction in QCD Page: 3 of 7
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in any gauge by the soft rescattering. The resulting diffractive contributions leave the target
intact and do not resolve its quark structure; thus there are contributions to the DIS structure
functions which cannot be interpreted as parton probabilities 4; the leading-twist contribution
to DIS from rescattering of a quark in the target is a coherent effect which is not included in
the light-front wave functions computed in isolation. One can augment the light-front wave
functions with a gauge link corresponding to an external field created by the virtual photon qq
pair current.5,6 Such a gauge link is process dependent 7, so the resulting augmented LFWFs are
not universal4,5,8 We also note that the shadowing of nuclear structure functions is due to the
destructive interference between multi-nucleon amplitudes involving diffractive DIS and on-shell
intermediate states with a complex phase. In contrast, the wave function of a stable target is
strictly real since it does not have on-energy-shell intermediate state configurations. The physics
of rescattering and shadowing is thus not included in the nuclear light-front wave functions, and
a probabilistic interpretation of the nuclear DIS cross section is precluded.
Rikard Enberg, Paul Hoyer, Gunnar Ingelman and I9 have shown that the quark structure
function of the effective hard pomeron has the same form as the quark contribution of the gluon
structure function. The hard pomeron is not an intrinsic part of the proton; rather it must
be considered as a dynamical effect of the lepton-proton interaction. Our QCD-based picture
also applies to diffraction in hadron-initiated processes. The rescattering is different in virtual
photon- and hadron-induced processes due to the different color environment, which accounts for
the observed non-universality of diffractive parton distributions. This framework also provides
a theoretical basis for the phenomenologically successful Soft Color Interaction (SCI) model 1
which includes rescattering effects and thus generates a variety of final states with rapidity gaps.
2 Single-Spin Asymmetries from Final-State Interactions
Among the most interesting polarization effects are single-spin azimuthal asymmetries in semi-
inclusive deep inelastic scattering, representing the correlation of the spin of the proton target
and the virtual photon to hadron production plane: $p - q x pH. Such asymmetries are time-
reversal odd, but they can arise in QCD through phase differences in different spin amplitudes.
In fact, final-state interactions from gluon exchange between the outgoing quarks and the target
spectator system lead to single-spin asymmetries in semi-inclusive deep inelastic lepton-proton
scattering which are not power-law suppressed at large photon virtuality Q2 at fixed Xbj 11
In contrast to the SSAs arising from transversity and the Collins fragmentation function, the
fragmentation of the quark into hadrons is not necessary; one predicts a correlation with the
production plane of the quark jet itself. Physically, the final-state interaction phase arises as the
infrared-finite difference of QCD Coulomb phases for hadron wave functions with differing orbital
angular momentum. The same proton matrix element which determines the spin-orbit correla-
tion S - L also produces the anomalous magnetic moment of the proton, the Pauli form factor,
and the generalized parton distribution E which is measured in deeply virtual Compton scatter-
ing. Thus the contribution of each quark current to the SSA is proportional to the contribution
rg/p of that quark to the proton target's anomalous magnetic moment KP = q e~qg/.p11,12 The
HERMES collaboration has recently measured the SSA in pion electroproduction using trans-
verse target polarization.3 The Sivers and Collins effects can be separated using planar cor-
relations; both contributions are observed to contribute, with values not in disagreement with
theory expectations.13,14 A related analysis also predicts that the initial-state interactions from
gluon exchange between the incoming quark and the target spectator system lead to leading-
twist single-spin asymmetries in the Drell-Yan process H1H - +-X. 7,15 The SSA in the
Drell-Yan process is the same as that obtained in SIDIS, with the appropriate identification of
variables, but with the opposite sign. Initial-state interactions also lead to a cos 206 planar corre-
lation in unpolarized Drell-Yan reactions.16 There is no Sivers effect in charged-current reactions
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Brodsky, Stanley J. Novel Aspects of Hard Diffraction in QCD, article, December 14, 2005; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc877566/m1/3/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.