123475685
Evidence for multiple parton interactions from the observation of multi-muon events in Drell-Yan experiments
Abstract We calculate the production of two lepton pairs via the Drell-Yan mechanism. To establish the existence of multiple parton interactions in a single hadron collision, lepton pair production is superior to jet production from the theoretical, phenomenological and experimental points of view. The calculated rate agrees with the previously unexplained four-muon events observed in πp and pp interactions. Our results lend support to a recent analysis of ISR four-jet events and we discuss some implications of this study.
121596214
Test of duality in baryon--antibaryon amplitudes from Reggeon--particle scattering
Abstract By studying the backward production data for meson resonances we investigate the dual properties of baryon-Reggeised antibaryon elastic scattering. We find strong support for the duality prediction that meson resonances in the baryon antibaryon channel should be dual to an exotic trajectory.
120877951
Sister trajectories in the dual resonance model
Abstract The existence of a Regge tranjectory γ = α /3 − 1 in the eight-point amplitude of the Dual Resonance Model set of demonstrated. It is suggested that the three presently known trajectories α, β and γ are part of an infinite set of “sister” trajectories, the expected properties of which are discussed.
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.
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.
119081579
Perturbative QCD with quark and gluon condensates
The quark 〈 ψ ¯ ψ 〉 and gluon ( F μν F μν ) vacuum expectation values are non-vanishing at low orders in α s when the perturbative ground state includes quark and gluon pairs. This offers possibilities of studying the effects of a non-trivial vacuum using formally exact perturbative methods. The QCD Feynman rules are modified only for the free, on-shell quark and gluon propagators at zero four-momentum. Gauge and lorentz invariance is maintained, while chiral symmetry is spontaneously broken by the ground state.
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.
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.
119388879
Comments on the Relativity of Shape
In this talk I address three topics related to the shape of hadrons:1. The Lorentz contraction of bound states. Few dedicated studies of this exist — I describe a recent calculation for ordinary atoms (positronium).2. Does the A‐dependence of nuclear structure functions indicate a change of proton shape in the nuclear environment? (My short answer is no.)3. The size of Fock states contributing to processes involving large momentum transfers. End‐point configurations can be transversally extended and yet sufficiently short‐lived to contribute coherently to hard scattering.
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.
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.