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.
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.
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.
118260296
Introduction to QCD - A Bound State Perspective
These lecture notes focus on the bound state sector of QCD. Motivated by data which suggests that the strong coupling \alpha_s(Q) freezes at low Q, and by similarities between the spectra of hadrons and atoms, I discuss if and how QCD bound states may be treated perturbatively. I recall the basic principles of perturbative gauge theory bound states at lowest order in the \hbar expansion. Born level amplitudes are insensitive to the i\epsilon prescription of propagators, which allows to eliminate the Z-diagrams of relativistic, time-ordered Coulomb interactions. The Dirac wave function thus describes a single electron which propagates forward in time only, even though the bound state has any number of pair constituents when Feynman propagators are used. In the absence of an external potential, states that are bound by the Coulomb attraction of their constituents can be analogously described using only their valence degrees of freedom. The instantaneous A^0 field is determined by Gauss' law for each wave function component, i.e., for each position of the valence constituents. Solutions for A^0 obtained with a boundary condition that imposes an asymptotically constant energy density give rise to a linear potential for color singlet q\bar q and qqq states. The strength of the linear potential is determined by the boundary condition and is of lower order in \alpha_s than the gluon exchange interaction, which may then be treated as a higher order perturbative correction. Bound states evaluated to a given order in \alpha_s and \hbar must have the full symmetry of the exact theory, including the dynamic boost invariance of states quantized at equal time. The wave functions are indeed found to have such a hidden invariance, which ensures the correct dependence of the energy eigenvalues on the center of mass momentum. Thus relativistic bound states can be studied using perturbative methods.
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.
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.
122671377
New relations for two-body reactions from inclusive finite-mass sum rules
Finite-mass sum rule and duality considerations for the inclusive process ab→cX lead to a number of interesting relations amongst the quasi-two-body cross sections ab→cdi, where di denotes the prominent resonances in the missing-mass channel. The processes π(K)N→NX are studied in detail, and the main results are the following: 
1. 
(i) A superconvergence relation for reggeon (ϱ)-particle (π) scattering gives a sum rule connecting the ϱ-exchange cross sections (dσ/dt) [π−p→(gp°, ω, A20)n]. 
 
2. 
(ii) Semi local duality considerations in reggeon-particle scattering relate the resonance production cross section (dσ/dt) [π(K)N→N di] to the resonance mass in terms of the trajectory α(t) of the exchangedreggeon. From this the two following corollaries follow: 
 
3. 
(iii) The ratio of π to (f-ω) exchange contributions increases linearly with the resonance (mass)2. 
 
4. 
(iv) The resonance production cross sections get less peripheral with increasing resonance mass. 
 
 
 
 
All the above predictions agree reasonably with the available data on mesonic resonance production.
125593487
NEW DUALITY CONSTRAINTS ON MESON AMPLITUDES
27960385
Hard diffraction from parton rescattering in QCD
We analyze the QCD dynamics of diffractive deep inelastic scattering. The presence of a rapidity gap between the target and diffractive system requires that the target remnant emerges in a color singlet state, which we show is made possible by the soft rescattering of the struck quark. This rescattering is described by the path-ordered exponential (Wilson line) in the expression for the parton distribution function of the target. The multiple scattering of the struck parton via instantaneous interactions in the target generates dominantly imaginary diffractive amplitudes, giving rise to an ''effective pomeron'' exchange. The pomeron is not an intrinsic part of the proton but a dynamical effect of the interaction. This picture also applies to diffraction in hadron-initiated processes. Due to the different color environment the rescattering is different in virtual photon- and hadron-induced processes, explaining the observed non-universality of diffractive parton distributions. This framework provides a theoretical basis for the phenomenologically successful Soft Color Interaction model which includes rescattering effects and thus generates a variety of final states with rapidity gaps.
6876910
Structure Functions Are Not Parton Probabilities
The common view that structure functions measured in deep inelastic lepton scattering are determined by the probability distribution of quarks and gluons in the target is not correct. We show that the leading-twist cross section is affected by the rescattering of the struck quark in the target. This is consistent with the Glauber-Gribov interpretation of shadowing as a rescattering effect.
119846230
Fermion Bound States by the Method of Stationary Phase
The stationary phase method is applied to gauge theory path integrals, taking into account the fermion determinant. This approximation is shown to lead to a unique bound state equation in QED. In the nonrelativistic limit, the equation reduces to the usual Schrodinger form. In 1 + 1 dimensions it leads to "bag"-type wave functions and an energy spectrum which agrees with known exact results. An analytic expression is given for the wave function of states with two massless fermions connected by an unbroken string.