118656657
Hadron Structure
I discuss a Born (~ → 0) approximation of hadrons, motivated by a general feature of the data: The spectra of hadrons reflect their valence (qq̄ or qqq) constituents, whereas hard scattering reveals a prominent sea quark distribution. Why do the sea quark d.o.f’s not imply a richer spectrum? I look for an approach that can reconcile the quark and parton model descriptions of hadrons, and consider how this physics could emerge from the QCD Lagrangian. The possibilities are reduced by insisting that the approximation should be simple, yet adhere to the rules of quantum field theory. One might suspect that no such method exists – but the Born approximation presents itself. The description of relativistic bound states that it brings has interesting features which merit further exploration.
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.
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.
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.
16490878
The Quark Model via an hbar expansion of QCD
I discuss the possibility that the quark model emerges as the lowest order of an h expansion of QCD bound states. In a hamiltonian approach the instantaneous A0 potential is determined by the field equations separately for each Fock component. These equations allow also a linear potential as a homogeneous solution. Stationarity of the action sets the direction of the ensuing constant electric field to be along the fermion pair separation. States bound by this non-perturbative, linear A0 potential are analogous to the Born term of standard perturbative expansions of scattering amplitudes, in that they represent the dominant contribution at lowest order in h (no loops) and at O (g) in the potential. The Dirac equation for relativistic fermions bound by an external A0 potential is most easily derived using retarded boundary conditions. I demonstrate why this boundary condition does not affect the bound state energies at lowest order in h. Translated to physical Feynman boundary conditions the Dirac bound states are a superposition of Fock states with any number of fermionantifermion pairs. Applying this approach to relativistic quark-antiquark states in QCD results in a bound state equation which was previously proposed without derivation and shown to provide a reasonable description of the meson spectrum, including linear Regge trajectories. The equal-time wave functions have unique Lorentz transformation properties, which ensure the correct dependence of the bound state energy on the center-of-mass momentum. This indicates that the solution is exact at the Born level, i.e., at lowest order in h and at O (g) in the QCD interaction.
17059488
Subprocess size in hard exclusive scattering
The interaction region of hard exclusive hadron scattering can have a large transverse size due to endpoint contributions, where one parton carries most of the hadron momentum. The endpoint region is enhanced and can dominate in processes involving multiple scattering and quark helicity flip. The endpoint Fock states have perturbatively short lifetimes and scatter softly in the target. We give plausible arguments that endpoint contributions can explain the apparent absence of color transparency in fixed angle exclusive scattering and the dimensional scaling of transverse ρ photoproduction at high momentum transfer, which requires quark helicity flip. We also present a quantitative estimate of Sudakov effects.
126284528
MULTIPERIPHERAL QUARK FRAGMENTATION
14523518
Systematics of J/psi production in nuclear collisions
In a storage device, in particular a piece of baggage or a wallet, having at least one outer surface, it is provided that an additional outer layer is provided on the outer surface, said layer having at least one three-dimensional protrusion formed therein by a thermoforming process and protruding from the outer surface.
14560984
Monte Carlo sensitivity in jet studies - what is the physics
Tests of perturbative QCD in hard processes involving jets have been found to depend on the jet fragmentation model used. We emphasize the need for testing the factorization of hard and soft processes, i.e., the independent fragmentation of jets. A method of analysis is suggested, which allows a model-independent determination of the gluon fragmentation function from e/sup +/e/sup -/ data. We also comment on some simple features expected in anti pp ..-->.. (2 or 3 jets) + X events. 19 references.