119250582
Inclusive Perspectives
I discuss the relation between inclusive and exclusive dynamics suggested by Bloom-Gilman duality. Duality implies the simultaneous applicability of two distinct limits, the standard DIS limit of hard inclusive processes taken at fixed xB and a limit where the hadronic mass is held fixed. I review experimental evidence for the relevance of the fixed mass limit in inclusive processes at high xF. Semi-local duality suggests that inclusive and exclusive processes occur on the same target Fock states. DIS scaling then implies that the Fock states contributing to hard exclusive processes have a large transverse size, i.e., that the hard scattering occurs off a single parton which carries a large fraction of the hadron momentum.
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.
119448188
Structure functions are not Parton probabilities
Abstract Parton distributions given by deep inelastic lepton scattering (DIS) are not equal to the probabilities of finding those partons in the parent wave function. Soft rescattering of the struck parton within the coherence length of the hard process influences the DIS cross section and gives dynamical phases to the scattering amplitudes. This gives rise to diffractive DIS, shadowing in nuclear targets and transverse spin asymmetry.
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.
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.
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.
11346578
QCD EFFECTS IN PARTICLE PRODUCTION ON NUCLEI
I discuss some questions related to hard scattering processes in nuclei and corrections to the leading twist approximation. The QCD factorization theorem requires that high energy partons do not lose energy while traversing the nucleus. I explain the physical reason for this. The theorem also states that spectator partons, not involved in the hard collision, have no influence on the inclusive cross section. Important spectator effects are, however, seen in the data for certain reactions and in some kinematical regions. I discuss the reasons from a phenomenological and theoretical point of view. Finally, I mention some methods for analyzing hard processes in regions of very large x, where the leading twist terms are not dominant. Studies of the dependence of hard collisions on the nuclear number of the target and/or projectile have revealed a large number of interesting, diverse and often surprising QCD effects. The nucleus has been, and will continue to be, a versatile tool for uncovering reaction mechanisms and indicating where our standard methods are inadequate. In this talk I shall confine myself to two particular questions, related to the energy loss of partons in nuclei and the importance of higher twist corrections due to spectator parton interactions. For a wider view of nuclear effects, I refer to recent reviews 1 .
120890782
Mesons and baryons from QCD
Abstract We study a gauge covariant method for quark bound states in 3 + 1 dimensions, based on a stationary phase approximation of the gluon path integral. Assuming thet lim | x |→∞ F μν a ( x ) F a μν ( x ) ≠ 0 we find meson (q q ) and baryon (qqq) states bound by a Coulomb + linear potential. The quark wave function is confined, as in the bag model. The high-spin states lie on asymptotically linear Regge trajectories of universal slope.
117067462
Soft Perturbative QCD
There are indications of a faith transition in QCD: The strong coupling may freeze at a sufficiently low value to make PQCD relevant even in the confining regime. The properties of PQCD at low Q^2 depend on the asymptotic field configuration assumed at t=\pm\infty. A change in the i\epsilon prescription of the free propagators is equivalent to including quarks and gluons in the initial and final states. Perturbative expansions thus generated are formally as justified as standard PQCD and may turn out to be phenomenologically relevant. I discuss examples of the effects of such propagator modifications in QED and QCD.
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