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Dynamic versus Static Hadronic Structure Functions

Description: 'Static' structure functions are the probabilistic distributions computed from the square of the light-front wavefunctions of the target hadron. In contrast, the 'dynamic' structure functions measured in deep inelastic lepton-hadron scattering include the effects of rescattering associated with the Wilson line. Initial- and final-state rescattering, neglected in the parton model, can have a profound effect in QCD hard-scattering reactions, producing single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan reactions, nuclear shadowing, and non-universal nuclear antishadowing|novel leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also review how 'direct' higher-twist processes--where a proton is produced in the hard subprocess itself--can explain the anomalous proton-to-pion ratio seen in high centrality heavy ion collisions.
Date: January 9, 2009
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Photon-Photon Collisions -- Past and Future

Description: I give a brief review of the history of photon-photon physics and a survey of its potential at future electron-positron colliders. Exclusive hadron production processes in photon-photon and electron-photon collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes. There are also important high energy {gamma}{gamma} and e{gamma} tests of quantum chromodynamics, including the production of jets in photon-photon collisions, deeply virtual Compton scattering on a photon target, and leading-twist single-spin asymmetries for a photon polarized normal to a production plane. Since photons couple directly to all fundamental fields carrying the electromagnetic current including leptons, quarks, W's and supersymmetric particles, high energy {gamma}{gamma} collisions will provide a comprehensive laboratory for Higgs production and exploring virtually every aspect of the Standard Model and its extensions. High energy back-scattered laser beams will thus greatly extend the range of physics of the International Linear Collider.
Date: December 2, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Orbital Angular Momentum on the Light-Front and QCD Observables

Description: The light-front wavefunction formalism provides a physical, but rigorous, representation for angular momentum in a relativistic quantum field theory. Each n-particle LFWF {psi}{sub n}(x{sub i}, {rvec k}{sub {perpendicular}}i,S{sub i}{sup z}) in the Fock state expansion of a hadron in QCD is frame-independent and satisfies angular momentum conservation J{sup z} = {summation}{sub i=1}{sup n} S{sub i}{sup z} + {summation}{sub i=1}{sup n-1} L{sub i}{sup z}, summed over the n - 1 independent intrinsic orbital angular momenta L{sub i}{sup z} = -i [{rvec k}{sub i}{sup x} {partial_derivative}/{partial_derivative}k{sub i}{sup y} - {rvec k}{sub i}{sup y} {partial_derivative}/{partial_derivative}k{sub i}{sup x}]. Gluons propagate with physical polarization S{sub g}{sup z} = {+-} 1 in light-cone gauge A{sup +} = 0. All of these features are illustrated by the Fock state expansion of the electron in terms of its fermion-boson components.
Date: March 10, 2006
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

New Perspectives for QCD from AdS/CFT

Description: The AdS/CFT correspondence between conformal field theory and string states in an extended space-time has provided new insights into not only hadron spectra, but also their light-front wavefunctions. We show that there is an exact correspondence between the fifth-dimensional coordinate of anti-de Sitter space z and a specific impact variable {zeta} which measures the separation of the constituents within the hadron in ordinary space-time. This connection allows one to predict the form of the light-front wavefunctions of mesons and baryons, the fundamental entities which encode hadron properties and scattering amplitudes. A new relativistic Schroedinger light-cone equation is found which reproduces the results obtained using the fifth-dimensional theory.
Date: July 28, 2006
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Novel Aspects of Hard Diffraction in QCD

Description: Initial- and final-state interactions from gluon-exchange, normally neglected in the parton model have a profound effect in QCD hard-scattering reactions, leading to leading-twist single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, and nuclear shadowing and antishadowing--leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also discuss the use of diffraction to materialize the Fock states of a hadronic projectile and test QCD color transparency.
Date: December 14, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Novel High Transverse Momentum Phenomena in Hadronic and Nuclear Collisions

Description: I discuss a number of novel phenomenological features of QCD in high transverse momentum reactions. The presence of direct higher-twist processes, where a proton is produced directly in the hard subprocess, can explain the 'baryon anomaly' - the large proton-to-pion ratio seen at RHIC in high centrality heavy ion collisions. Direct hadronic processes can also account for the deviation from leading-twist PQCD scaling at fixed x{sub T} = 2 p{sub T}/{radical}s. I suggest that the 'ridge' --the same-side long-range rapidity correlation observed at RHIC in high centrality heavy ion collisions is due to the imprint of semihard DGLAP gluon radiation from initial-state partons which have transverse momenta biased toward the trigger. A model for early thermalization of the quark-gluon medium is also outlined. Rescattering interactions from gluon-exchange, normally neglected in the parton model, have a profound effect in QCD hard-scattering reactions, leading to leading-twist single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan reactions, nuclear shadowing--all leading-twist dynamics not incorporated in the light-front wavefunctions of the target computed in isolation. Anti shadowing is shown to be quark flavor specific and thus different in charged and neutral deep inelastic lepton-nucleus scattering. I also discuss other aspects of quantum effects in heavy ion collisions, such as tests of hidden color in nuclear wavefunctions, the use of diffraction to materialize the Fock states of a hadronic projectile and test QCD color transparency, and the important consequences of color-octet intrinsic heavy quark distributions in the proton for particle and Higgs production at high x{sub F}. I also discuss how the AdS/CFT correspondence between Anti-de Sitter space and conformal gauge theories allows one to compute the analytic form of frame-independent light-front wavefunctions of mesons and baryons and to compute quark and gluon hadronization at the amplitude ...
Date: April 10, 2009
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Novel Nuclear Effects in QCD: Non-Universality of Nuclear Antishadowing and Hidden Color Phenomena

Description: The shadowing and antishadowing of nuclear structure functions in the Gribov-Glauber picture is due to the destructive and constructive interference of amplitudes arising from the multiple-scattering of quarks in the nucleus, respectively. The diffractive contributions to deep inelastic scattering includes Pomeron and Odderon contributions from multi-gluon exchange as well as Reggeon quark-exchange contributions. The coherence of multi-step nuclear processes leads to shadowing and antishadowing of the electromagnetic nuclear structure functions in agreement with measurements. This picture also leads to substantially different antishadowing for charged and neutral current reactions, thus affecting the extraction of the weak-mixing angle {Theta}{sub W}. The fact that Reggeon couplings depend on the quantum numbers of the struck quark implies non-universality of nuclear antishadowing for charged and neutral currents as well as a dependence of antishadowing on the polarization of the beam and target. The implications of hidden color degrees of freedom in the nuclear wavefunction is also briefly discussed.
Date: June 30, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Novel Perspectives for Hadron Physics

Description: I discuss several novel and unexpected aspects of quantum chromodynamics. These include: (a) the nonperturbative origin of intrinsic strange, charm and bottom quarks in the nucleon at large x; the breakdown of pQCD factorization theorems due to the lensing effects of initial- and final-state interactions; (b) important corrections to pQCD scaling for inclusive reactions due to processes in which hadrons are created at high transverse momentum directly in the hard processes and their relation to the baryon anomaly in high-centrality heavy-ion collisions; and (c) the nonuniversality of quark distributions in nuclei. I also discuss some novel theoretical perspectives in QCD: (a) light-front holography - a relativistic color-confining first approximation to QCD based on the AdS/CFT correspondence principle; (b) the principle of maximum conformality - a method which determines the renormalization scale at finite order in perturbation theory yielding scheme independent results; (c) the replacement of quark and gluon vacuum condensates by 'in-hadron condensates' and how this helps to resolve the conflict between QCD vacuum and the cosmological constant.
Date: March 9, 2012
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Novel QCD Phenomena

Description: I discuss a number of novel topics in QCD, including the use of the AdS/CFT correspondence between Anti-de Sitter space and conformal gauge theories to obtain an analytically tractable approximation to QCD in the regime where the QCD coupling is large and constant. In particular, there is an exact correspondence between the fifth-dimension coordinate z of AdS space and a specific impact variable {zeta} which measures the separation of the quark constituents within the hadron in ordinary space-time. This connection allows one to compute the analytic form of the frame-independent light-front wavefunctions of mesons and baryons, the fundamental entities which encode hadron properties and allow the computation of exclusive scattering amplitudes. I also discuss a number of novel phenomenological features of QCD. Initial- and final-state interactions from gluon-exchange, normally neglected in the parton model, have a profound effect in QCD hard-scattering reactions, leading to leading-twist single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam Tung relation in Drell-Yan reactions, and nuclear shadowing and non-universal antishadowing--leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also discuss tests of hidden color in nuclear wavefunctions, the use of diffraction to materialize the Fock states of a hadronic projectile and test QCD color transparency, and anomalous heavy quark effects. The presence of direct higher-twist processes where a proton is produced in the hard subprocess can explain the large proton-to-pion ratio seen in high centrality heavy ion collisions.
Date: July 6, 2007
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Exclusive Two-Photon Processes in QCD

Description: Hadron pair production from two-photon annihilation plays an important role in unraveling the perturbative and non-perturbative structure of QCD, first by testing the validity and empirical applicability of leading-twist factorization theorems, second by verifying the structure of the underlying perturbative QCD subprocesses, and third, through measurements of angular distributions and ratios which are sensitive to the shape of the distribution amplitudes. In effect, photon-photon collisions provide a microscope for testing fundamental scaling laws of PQCD and for measuring distribution amplitudes. The determination of the shape and normalization of the distribution amplitude is particularly important in view of their importance in the analysis of exclusive semi-leptonic and two-body hadronic B-decays. The data from the Belle and CLEO collaborations on single and double meson production are in excellent agreement with the QCD predictions. In contrast, the normalization of the nominal leading-order predictions of PQCD for proton pair production appears to be significantly below recent Belle measurements. I also review issues relating to renormalization scale setting.
Date: December 2, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Hadron Spectroscopy and Structure from AdS/CFT

Description: The AdS/CFT correspondence between conformal field theory and string states in an extended space-time has provided new insights into not only hadron spectra, but also their light-front wavefunctions. We show that there is an exact correspondence between the fifth-dimensional coordinate of anti-de Sitter space z and a specific impact variable {zeta} which measures the separation of the constituents within the hadron in ordinary space-time. This connection allows one to predict the form of the light-front wavefunctions of mesons and baryons, the fundamental entities which encode hadron properties and scattering amplitudes. A new relativistic Schroedinger light-front equation is found which reproduces the results obtained using the fifth-dimensional theory. Since they are complete and orthonormal, the AdS/CFT model wavefunctions can be used as an initial ansatz for a variational treatment or as a basis for the diagonalization of the light-front QCD Hamiltonian. A number of applications of light-front wavefunctions are also discussed.
Date: September 29, 2006
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Hard Diffraction in QCD

Description: Gluon exchange between the outgoing quarks and the target spectators affects the structure functions measured in deep inelastic scattering in a profound way, leading to diffractive lepto-production processes, the shadowing and antishadowing of nuclear structure functions, and target spin asymmetries--leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also discuss the diffraction dissociation of hadrons into jets as a tool for resolving fundamental hadron substructure.
Date: June 30, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Atoms in Flight: The Remarkable Connections between Atomic and Hadronic Physics

Description: Atomic physics and hadron physics are both based on Yang Mills gauge theory; in fact, quantum electrodynamics can be regarded as the zero-color limit of quantum chromodynamics. I review a number of areas where the techniques of atomic physics provide important insight into the theory of hadrons in QCD. For example, the Dirac-Coulomb equation, which predicts the spectroscopy and structure of hydrogenic atoms, has an analog in hadron physics in the form of light-front relativistic equations of motion which give a remarkable first approximation to the spectroscopy, dynamics, and structure of light hadrons. The renormalization scale for the running coupling, which is unambiguously set in QED, leads to a method for setting the renormalization scale in QCD. The production of atoms in flight provides a method for computing the formation of hadrons at the amplitude level. Conversely, many techniques which have been developed for hadron physics, such as scaling laws, evolution equations, and light-front quantization have equal utility for atomic physics, especially in the relativistic domain. I also present a new perspective for understanding the contributions to the cosmological constant from QED and QCD.
Date: February 16, 2012
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

The Conformal Template and New Perspectives for Quantum Chromodynamics

Description: Conformal symmetry provides a systematic approximation to QCD in both its perturbative and nonperturbative domains. One can use the AdS/CFT correspondence between Anti-de Sitter space and conformal gauge theories to obtain an analytically tractable approximation to QCD in the regime where the QCD coupling is large and constant. For example, there is an exact correspondence between the fifth-dimensional coordinate of AdS space and a specific impact variable which measures the separation of the quark constituents within the hadron in ordinary space-time. This connection allows one to compute the analytic form of the frame-independent light-front wavefunctions of mesons and baryons, the fundamental entities which encode hadron properties and allow the computation of exclusive scattering amplitudes. One can also use conformal symmetry as a template for perturbative QCD predictions where the effects of the nonzero beta function can be systematically included in the scale of the QCD coupling. This leads to fixing of the renormalization scale and commensurate scale relations which relate observables without scale or scheme ambiguity. The results are consistent with the renormalization group and the analytic connection of QCD to Abelian theory at N{sub C} {yields} 0. I also discuss a number of novel phenomenological features of QCD. Initial- and .nal-state interactions from gluon-exchange, normally neglected in the parton model, have a profound effect in QCD hard-scattering reactions, leading to leading-twist single-spin asymmetries, diffractive deep inelastic scattering, di.ractive hard hadronic reactions, the breakdown of the Lam Tung relation in Drell-Yan reactions, and nuclear shadowing and non-universal antishadowing--leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also discuss tests of hidden color in nuclear wavefunctions, the use of diffraction to materialize the Fock states of a hadronic projectile and test QCD color transparency, nonperturbative antisymmetric sea quark distributions, anomalous heavy quark e.ects, and the ...
Date: March 6, 2007
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Deep Inelastic Scattering at the Amplitude Level

Description: The deep inelastic lepton scattering and deeply virtual Compton scattering cross sections can be interpreted in terms of the fundamental wavefunctions defined by the light-front Fock expansion, thus allowing tests of QCD at the amplitude level. The AdS/CFT correspondence between gauge theory and string theory provides remarkable new insights into QCD, including a model for hadronic wavefunctions which display conformal scaling at short distances and color confinement at large distances.
Date: August 4, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Dynamic versus Static Structure Functions and Novel Diffractive Effects in QCD

Description: Initial- and final-state rescattering, neglected in the parton model, have a profound effect in QCD hard-scattering reactions, predicting single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam Tung relation in Drell-Yan reactions, and nuclear shadowing and non-universal antishadowing--leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also discuss the use of diffraction to materialize the Fock states of a hadronic projectile and test QCD color transparency, and anomalous heavy quark effects. The presence of direct higher-twist processes where a proton is produced in the hard subprocess can explain the large proton-to-pion ratio seen in high centrality heavy ion collisions. I emphasize the importance of distinguishing between static observables such as the probability distributions computed from the square of the light-front wavefunctions versus dynamical observables which include the effects of rescattering.
Date: November 12, 2008
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Novel QCD Effects from Initial and Final State Interactions

Description: Initial-state and final-state interactions which are conventionally neglected in the parton model, have a profound effect in QCD hard-scattering reactions. The effects, which arise from gluon exchange between the active and spectator quarks, cause leading-twist single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, and the breakdown of the Lam-Tung relation in Drell-Yan reactions. Diffractive deep inelastic scattering also leads to nuclear shadowing and non-universal antishadowing of nuclear structure functions through multiple scattering reactions in the nuclear target. Factorization-breaking effects are particularly important for hard hadron interactions since both initial-state and final-state interactions appear. Related factorization breaking effects can also appear in exclusive electroproduction reactions and in deeply virtual Compton scattering. None of the effects of initial-state and final-state interactions are incorporated in the light-front wavefunctions of the target hadron computed in isolation.
Date: September 12, 2007
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Large x Physics

Description: The large x{sub bj} domain of deep inelastic lepton-proton scattering provides essential information on the structure of the proton when the struck quark is far off-shell and one must take into account inter-quark correlations, higher twist effects, and the breakdown of standard DGLAP evolution. I briefly review predictions from PQCD and AdS/CFT. I also discuss how intrinsic heavy quark Fock states lead to novel production mechanisms for heavy hadrons.
Date: June 30, 2005
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

New Perspectives in Quantum Chromodynamics

Description: In these lectures I will discuss three central topics in quantum chromodynamics: (1) the use of light cone quantization and Fock space methods to determine the long and short-distance structure of quark and gluon distributions within hadrons; (2) the role of spin, heavy quarks, and nuclei in unraveling fundamental phenomenological features of QCD; and (3) a new approach to understanding the scale and scheme dependence of perturbative QCD predictions.
Date: July 1, 1993
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Exclusive two-photon processes: Tests of QCD at the amplitude level

Description: Exclusive two-photon processes at large momentum transfer, particularly Compton scattering {gamma}p{yields}{gamma}p and its crossed-channel reactions {gamma}{gamma}{yields}{bar p}p and {bar p}p{yields}{gamma}{gamma}, can provide definitive information on the bound-state distributions of quarks in hadrons at the amplitude level. Recent theoretical work has shown that QCD predictions based on the factorization of long and short distance physics are already applicable at momentum transfers of order of a few GeV.
Date: July 1, 1992
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Perspectives on Epic Physics

Description: An electron-proton/ion polarized beam collider (EPIC) with high luminosity and center of mass energy square root s = 25 GeV would be a valuable facility for fundamental studies of proton and nuclear structure and tests of quantum chromodynamics, I review a sample of prospective EPIC topics, particularly semi-exclusive reactions, studies of the proton fragmentation region, heavy quark electroproduction, and a new probe of odderon/pomeron interference.
Date: July 14, 1999
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

New Tests of Perturbative QCD Inspired by Hypothetical Tau Leptons

Description: Inspired by the relation between the hadronic decay of the {tau} lepton and the e{sup +}e{sup {minus}} annihilation into hadrons, we derive new tests of perturbative QCD. We design a set of commensurate scale relations to test the self-consistency of leading-twist QCD predictions for any observable which defines an effective charge. This method provides renormalization scheme and scale invariant probes of QCD which can be applied over wide data ranges.
Date: May 24, 1999
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Conformal Symmetry as a Template:Commensurate Scale Relations and Physical Renormalization Schemes

Description: Commensurate scale relations are perturbative QCD predictions which relate observable to observable at fixed relative scale, such as the ''generalized Crewther relation'', which connects the Bjorken and Gross-Llewellyn Smith deep inelastic scattering sum rules to measurements of the e{sup +}e{sup {minus}} annihilation cross section. We show how conformal symmetry provides a template for such QCD predictions, providing relations between observables which are present even in theories which are not scale invariant. All non-conformal effects are absorbed by fixing the ratio of the respective momentum transfer and energy scales. In the case of fixed-point theories, commensurate scale relations relate both the ratio of couplings and the ratio of scales as the fixed point is approached. In the case of the {alpha}{sub V} scheme defined from heavy quark interactions, virtual corrections due to fermion pairs are analytically incorporated into the Gell-Mann Low function, thus avoiding the problem of explicitly computing and resuming quark mass corrections related to the running of the coupling. Applications to the decay width of the Z boson, the BFKL pomeron, and virtual photon scattering are discussed.
Date: June 9, 1999
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department

Hard Exclusive and Processes in QCD

Description: Exclusive and semi-exclusive processes, the diffractive dissociation of hadrons into jets, and hard diffractive processes such as vector meson leptoproduction provide new testing grounds for QCD and essential information on the structure of light-cone wavefunctions of hadrons, particularly the pion distribution amplitude. I review the basic features of the leading-twist QCD predictions and the problems and challenges of studying QCD at the amplitude level. The application of the light-cone formalism to the exclusive semi-leptonic decay of heavy hadrons is also discussed.
Date: August 25, 1999
Creator: Brodsky, Stanley J.
Partner: UNT Libraries Government Documents Department