10426839
Parton propagation in a gluon field
The phenomenological success of PQCD is based on processes where the effects of the color field environment on parton propagation can be eliminated or is universal. In hard diffraction and quarkonium production the PQCD subprocess is the same as in fully inclusive scattering, but the sensitivity to reinteractions is different. I discuss how this may be exploited to give new information on the dynamics of hard collisions.
18528565
Factorization at fixed Q^2(1-x)
We consider QCD factorization between hard and soft subprocesses in inclusive reactions where the momentum fraction x of one parton approaches unity as the hard scale Q^2 -> \infty, such that Q^2(1-x) is fixed. In this ``BB limit'' the entire (multi-parton) Fock state containing the high x parton is coherent with the hard subprocess. The soft contribution is given by a forward multiparton matrix element. The BB limit corresponds to a fixed (large or small) missing mass and is thus closely connected to exclusive production. We analyze the Drell-Yan process h + N -> \gamma^* + X in detail, explaining why the virtual photon is longitudinally polarized for h = \pi and transversely polarized for h = p. The BB limit may be relevant also for other phenomena observed at high x, such as the large single spin asymmetries of p p -> \Lambda^\uparrow X and in p p^\uparrow -> \pi X.
117547375
Filling Perturbative Ground States
I discuss a degree of freedom in formulating perturbation theory that is often neglected: the in- and out-states need not be empty. The inclusion of (free) particles in the asymptotic states modifies the on-shell prescription of the free propagator. This affects loop contributions -- but the modified expansion is {\it a priori} as justified as the standard one with Feynman prescription. 
It is possible to dress the quark propagator to all orders with zero-momentum gluons from the perturbative ground state. The dressed quark has no pole and thus cannot appear as an external particle in the S-matrix. Chiral symmetry may be spontaneously broken, but Lorentz and gauge symmetry is exact. Adding loop corrections to this ``dressed tree approximation'' gives a formally exact PQCD expansion.
7794876
QCD Green functions in a gluon field
We formulate a dressed perturbative expansion of QCD, where the standard diagrams are evaluated in the presence of a constant external gluon field whose magnitude is gaussian distributed. The approach is Poincare and gauge invariant, and modifies the usual results for hard processes only by power suppressed contributions. Long distance propagation of quarks and gluons turns out to be inhibited due to a branch point singularity instead of a pole at p 2 = 0 in the quark and gluon propagators. The dressing keeps the (massless) quarks in q fluctuations of the photon at a finite distance from each other.
829521
The Unexpected Role of Final State Interactions in Deep Inelastic Scattering
Rescattering of the struck quark in Deep Inelastic Scattering implies that measured parton distributions are not directly related to the Fock state probabilities of the target wave function. The production amplitudes acquire dynamical phases, which gives rise to shadowing and diffraction in DIS. I review the kinematics and dynamics of DIS as seen in various frames and gauges.
412880
Duality in Semi-Exclusive Processes
Semi-exclusive processes like \gamma p \to \pi^+ Y are closely analogous to DIS, ep \to eX, in the limit where the momentum transfer |t| to the pion and the mass of the inclusive system Y are large but still much smaller than the total CM energy. We apply Bloom-Gilman duality to this semi-exclusive process. The energy dependence of the \gamma p \to \pi^+ n cross section given by semi-local duality agrees with data for moderate values of |t|, but its normalization is underestimated by about two orders of magnitude. This indicates that rather high momentum transfers are required for the validity of PQCD in the hard subprocess \gamma u \to \pi^+ d. In the case of Compton scattering \gamma p \to \gamma p the analogous discrepancy is about one order of magnitude. In electroproduction the virtuality of the incoming photon can be used to directly measure the hardness of the subprocess.
117968626
Comover Enhancement of Quarkonium Production
Quarkonium data suggest an enhancement of the hadroproduction rate from interactions of the heavy quark pair with a comoving color field generated in the hard gg -> Q\bar{Q} subprocess. We review the motivations and principal consequences of this comover enhancement scenario (CES).
119444323
The dynamics of quarkonium production
Quarkonium production is a sensitive probe of the dynamics of hard scattering, which can ‘measure’ the environment of the heavy quark pair after it is created in a hard process. Quarkonium hadroproduction data indicates that the produced pair experiences a secondary, hard interaction and then ceases to interact at a stage when the pair is still compact (compared to the size of the quarkonium wave function). These features differ from scenarios mostly discussed so far. An approach which relies on an early, perturbative rescattering of the pair with a comoving color field can explain many observed features, including the polarization, the χ 1 / χ 2 ratio and the nuclear target A -dependence, which are difficult to understand otherwise.
18418856
Physics at ELFE
I review some central physics opportunities at the 15…30 GeV continuous beam electron accelerator ELFE, proposed to be built in conjunction with the DESY linear collider. Our present detailed knowledge of single parton distributions in hadrons and nuclei needs to be supplemented by measurements of compact valence quark configurations, accessible through hard exclusive scattering, and of compact multiparton subsystems which contribute to semi-inclusive processes. Cumulative (x > 1, xF > 1) processes in nuclei measure short-range correlations between partons belonging to different nucleons in the same nucleus. The same configurations may give rise to subthreshold production of light hadrons and charm. The challenges of understanding high energy charmonium production indicate that charmonium will be a sensitive probe of color dynamics at ELFE. At low energies, charmonium forms inside the target nucleus, allowing a determination of cc bound state interactions in nuclear matter.
120212150
Measuring the triple-gluon coupling in hadron collisions
The fourth moment of the pT distribution (pT4) of jets or hadrons can be reliably predicted in QCD at the pp and pp collider energies. It depends on the gluon distribution function only via its first moment, i.e., the energy fraction carried by gluons. The dominant contribution to pT4 is from gluon-gluon and gluon-quark collisions. Hence, a measurement of pT4 can be used to determine the gluon self-interaction.
119424657
Rescattering in DIS and other QCD processes
I discuss two issues related to interactions of partons moving in a color field. In deep inelastic scattering reinteractions of the struck quark in the color field of the target may neutralize the overall color exchange from the target. This allows the target to emerge with a rapidity gap to the rest of the produced hadrons: Diffractive DIS. In hard hadron-hadron interactions similar rescattering takes place, and again allows rapidity gap formation. Because of the different color environment in lepton vs. hadron induced processes the rescattering probabilities are not the same, hence the diffractive parton distributions are process-dependent. For both lepton and hadron induced diffraction the hard scattering subprocesses and their logarithmic scaling violations are, however, the same as in inclusive scattering.     Another type of rescattering occurs when quarks and gluons move in the QCD vacuum condensate. Their long distance propagation is likely to be strongly affected by the condensate, which may explain why usual perturbation fails for soft processes. We have modelled the condensate by a ‘vacuum’ gluon field which carries zero four-momentum and couples to quarks and gluons in a gauge invariant way. The dressing of the free propagators by the vacuum gluons can be calculated exactly and shows that colored partons have a finite life-time. This framework may help address issues of analyticity and unitarity in theories where the fundamental fields differ from the asymptotic states of the S-matrix.PACS: 12.38.-t Quantum chromodynamics – 13.60.-r Photon and charged-lepton interactions with hadrons – 13.85.-t Hadron-induced high- and super-high-energy interactions