Abstracts for the CSSM Workshop on Lepton Scattering, Hadrons
and QCD
March 26 to April 6, 2001
University of Adelaide
Nucleon form factors in the covariant quark-diquark model
In the covariant quark-diquark model the effective Bethe-Salpeter (BS) equations for the nucleon and the Delta are solved including scalar and axialvector diquark correlations. Their quark substructure is effectively taken into account in both, the interaction kernel of the BS equations and the currents employed to calculate nucleon observables. Electromagnetic current conservation is maintained. The electric form factors of proton and neutron match the data. Their magnetic moments improve considerably by including axialvector diquarks and photon induced scalar-axialvector transitions. The ratio \mu GE/GM and the axial and strong couplings gA, g\pi NN , provide an upper bound on the relative importance of axialvector diquarks confirming that scalar diquarks nevertheless describe the dominant 2-quark correlations inside nucleons.
Single spin asymmetries in semi-inclusive DIS
Semi-inclusive polarized DIS, lN --> lhX, is an ideal process to study new features of spin dependent distribution and fragmentation functions. In particular single spin asymmetries, corresponding to cases when either only the initial nucleon (N) or the final hadron (h) is transversely polarized, reveal new non-perturbative properties of QCD. They may give direct access to the yet unmeasured transversity distributions and to new fragmentation functions of polarized quarks into unpolarized hadrons and of unpolarized quarks into polarized hadrons. A review of the theoretical and experimental situation is presented.
Enrique Ruiz Arriola (Granada, Spain)
Structure functions of mesons and baryons in a chiral quark model
Not available
Wolfgang Bentz (Tokai Uni., Japan)
Quark-diquark structure of the nucleon - static properties, structure functions and nuclear matter equation of state
1) Spin independent structure functions of the nucleon in the quark-diquark picture of the Nambu-Jona-Lasinio (NJL) model: Role of the axial vector diquark channel on the flavor dependence of the quark distributions, pionic cloud effects, etc. 2) Static properties of the nucleon calculated in the same framework. 3) The nuclear matter equation of state, based on the quark-diquark picture of the single nucleon: Problem of matter stability in the NJL model, and a possible solution by including confinement effects phenomenologically.
Exclusive processes at HERMES
Growing theoretical interest has recently surrounded the nature of the unknown Skewed Parton Distribution (SPD) functions of the nucleon which appear in the factorisation scheme of exclusive processes, including their possible relation to parton correlations and orbital angular momentum. HERMES will present a first measurement of exclusive pi+ production on a pure hydrogen target, at Q2 > 1 GeV2. In addition, the azimuthal sin(phi) moments of the single spin (target) asymmetries, with respect to the longitudinal target spin, of this process will be presented. Here, phi is the azimuthal angle of the pion with respect to the lepton scattering plane, around the virtual photon direction. In the exclusive limit a dramatic sign change for the positive pion asymmetry and a large enhancement of the neutral pion asymmetry is observed. The HERMES experiment also reports the first measurement for the single spin asymmetry of the azimuthal (phi) distribution in the exclusive leptoproduction of a real photon; phi is the azimuthal angle of the real photon about the incident virtual photon direction. This asymmetry results from the interference between the Bethe-Heitler and the Deeply Virtual Compton (DVCS) processes. The sin(phi) and sin(2 phi) moments of this measurement are sensitive to the imaginary parts of the helicity amplitudes in the DVCS process. The helicity amplitudes for the DVCS process provides a theoretically clean avenue for probing the Off-Forward/Skewed Parton Distribution Functions (OFPDFs).
Determining Delta g with Polarized Photo- and Hadroproduction of Heavy Quarks
Currently the polarized gluon density Delta g is only weakly constrained. But soon the COMPASS and RHIC experiments will provide new spin-dependent data. Their ability to pin down Delta g using open heavy quark production is discussed using the first complete NLO QCD analyses. The possibility of a "win-win" scenario at RHIC is pointed out, in which one obtains information either on Delta g or on the (TEVATRON+HERA) "bottom enigma".
Matthias Burkardt (NMSU, Las Cruces)
Distribution of partons in the transverse plane
Deeply virtual Compton scattering experiments allow probing `generalized parton distributions' (GPDs). I will discuss some of the physics of GPDs and, in particular, the connection between GPDs and the distribution of partons in the transverse plane.
Physics motivation for the Electron Ion Collider (EIC)
Abstract not available
Fu-Guang Cao (Massey Uni.,Palmerston North, New Zealand)
Flavor asymmetry in the polarized nucleon sea
We present a study of the flavour asymmetry of polarized anti-quarks in the nucleon using the meson cloud model. We include contributions both from the vector mesons and the interference terms of pseudoscalar and vector mesons. Employing the bag model, we first give the polarized valence quark distribution of the rho meson and the interference distributions. Our calculations show that the interference effect mildly increases the prediction for Delta {\bar d}(x) - Delta {\bar u}(x) at intermediate x region. We also discuss the contribution of `Pauli blocking' to the asymmetry.
Stephen Cotanch (Raleigh, North Carolina)
Mesons, glueballs and exotic hybrids in a relativistic many-body approach
Abstract not available
Diffraction and vector meson production at HERA
Abstract not available
Chiral extrapolation of lattice moments of structure functions
Lattice data for the first three nontrivial moments of the non-singlet parton distribution u - d exist at quark masses from 6 to 50 times the physical light quark masses. Previous extrapolations to the physical quark mass have given values for the moments 50--100% larger than experiment. As the quark mass approaches the chiral limit, presence of the pionic cloud of the nucleon becomes increasingly important. We present an improved extrapolation that accounts for these effects and gives results that are in quite good agreement with the experimental values.
Nuclear Medium effects at HERMES
Large nuclear medium effect in the unpolarised structure function R = sigma_L/sigma_T has been observed in the HERMES experiment at low Q2 and small x. The increase of the longitudinal cross section and the simultaneous reduction of the transverse one, can be interpreted as a signature of the coupling of the virtual photon with spin-0 object inside the nucleus. No significant effect has been found for the F2 structure function. The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been also studied by the HERMES experiment. The differential multiplicity of charged hadrons from nuclei relative to that from deuterium has been measured as a function of the virtual photon energy nu and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio at low nu and in the previously unexplored high z region. As the reduction of hadron multiplicity depends on the distance traversed by the struck quark before the hadron is formed, on the (unknown) quark-nucleon cross section and the (known) hadron-nucleon cross section, measurements of the multiplicity of hadrons produced on nuclei can provide information on the space-time structure of the hadronization process.
Lepton scattering and quark-hadron duality studies at JLAB
At high enough energies asymptotic freedom guarantees the deep inelastic scattering cross sections to be calculated as nearly free electron-quark scattering. However, confinement guarantees that the experimentally observed final state particles are hadrons. Low-energy quark-hadron duality suggests that hadronic cross sections, when averaged over an appropriate energy range, neverteless coincide with the naive leading-twist quark-gluon calculations. Deep inelastic inclusive scattering shows that scaling at modest Q2 and nu already arises from very few resonance channels. This is reflected by the striking agreement (< 5 %) between data in the nucleon resonance region and the deep inelastic (W2 > 4 GeV2) region for Q2 > 1 (GeV/c)2, known as Bloom-Gilman duality. Electron-hadron scattering allows for further investigation of quark-hadron duality by virtue of its ability to select resonances, by tagging with either spin or flavor.
Olivier Gayou (William and Mary & Universite de Clermont-Ferrand)
Measurement of the ratio of the electric to magnetic form factors GEp/GMp of the proton up to Q2 = 5.6 GeV2 at Jefferson Lab
The ratio of the electric to magnetic form factors of the proton was measured for the first time up to Q2=5.6 GeV2 using the recoil polarization technique, at Jefferson Lab, in November and December 2000. I will present the experimental method, and discuss the results and their theoritical implications.
Nucleon and Few Body Electromagnetic Form Factors
Not available
First estimate of nuclear Gottfried sum rule for the 3He-3H mirror nuclei
We estimate nuclear shadowing and antishadowing corrections at small Bjorken x for DIS on 3He and 3H nuclei. Due to the different electromagnetic radii (charge symmetry breaking) of 3He and 3H, the shadowing correction increases significantly the Gottfried sum. We predict that, when integrated from zero, the Gottfried sum diverges.
Deconfining by Winding: The Magnetic Monopole Vacua at High Temperatures
Characterizing the vacuum of a thermalized SU(3) Yang-Mills theory in the dual Ginzburg-Landau description, the possibility of topologically nontrivial, classical monopole fields in the deconfining phase is explored. These fields are assumed to be Bogomol'nyi-Prasad-Sommerfield (BPS) saturated solutions along the compact, euclidean time dimension. A corresponding, gauge invariant monopole interaction is constructed. The model passes first tests. In particular, a reasonable value for the critical temperature is obtained, and the partial persistence of nonperturbative features in the deconfining phase of SU(3) Yang-Mills theory, as it is measured on the lattice, follows naturally.
Noriyoshi Ishii (Riken, Japan)
Glueball Properties in Anisotropic SU(3) Lattice QCD with Improved Action
We study the properties of the glueballs (mass, size, etc.) in the anisotropic SU(3) lattice QCD with the Symantik improved action by using the smearing technique.
Domain-like Structures in the QCD Vacuum and Meson Properties
A self-dual constant gluon field is a candidate configuration which might dominate the physical QCD vacuum, manifesting colour confinement and reproducing basic features of meson spectroscopy. This field however breaks various global symmetries, which we restore by considering an ensemble of domains of constant field strength of random orientation. Consequences for area law confinement and the pion-eta' splitting are discussed.
A regularization-independent method in non-perturbative field theory
The inherent divergences in quantum field theories have plagued physicists for years. The infinities are removed in a two-step process; regularization followed by a renormalization. Depending on the type of regulator induced, different problems can occur. The difficulties that arise in the studies of non-perturbative field theory using Schwinger-Dyson equations may be circumvented by removing the regulator analytically rather than numerically. Hence we proposed a regularization-independent method for studying a renormalizable field theory non-perturbatively through its Dyson-Schwinger equations. Using QED4 as an example, we showed how the coupled equations determining the nonperturbative fermion and photon propagators can be written entirely in terms of renormalized quantities, which renders the equations manifestly finite in a regularization-independent manner. As an illustration of this technique, we apply it to a study of the fermion propagator in quenched QED4 with the Curtis-Pennington vertex. These results were compared with previous calculations which used the alternative regularization schemes of modified ultraviolet cut-off and dimensional regularization. Our new results are in excellent numerical agreement with these.
Polarized structure functions in QCD
Not available
Hadron Physics from Lattice QCD
In this talk I will discuss the role of chiral physics in connecting today's lattice QCD simulation results to experiment. I will focus on electromagnetic observables including charge radii and magnetic moments. The contributions of quarks to baryon properties display a strong environment sensitivity. For example, when the up quark resides in an environment of two down quarks in the neutron it makes a contribution which is significantly different from when it resides in the environment of two strange quarks in the cascade baryon. These environmental effects have a direct connection to the strange quark contribution to nucleon magnetic moments. It is often said that a vanishing strangeness moment will vindicate the constituent quark model for its magnetic moment predictions. However, it will become clear that a vanishing strangeness moment in the nucleon undermines a cornerstone of the constituent quark model.
Leading Baryon Production in ep Collisions at HERA
Not available
Simonetta Liuti (Charlottesville, VA)
Unraveling the Low Q2 Structure of Hadrons: From Partons to Constituents
The deep inelastic structure of the proton is conventionally described using parton distribution functions, their Q2 dependence being regulated by pQCD. Recent measurements from HERA [1], Jefferson Lab [2], and from jet studies at CDF [3], seem to suggest that pQCD may be applicable down to very low values of Q2 (Q2 ~ 1 GeV2), with a small contribution from power corrections, thus implying that a partonic structure can still be observed at large distances. In order to explain this situation, and its connection with the observed phenomenon of duality, we explore a two step model in which at low Q2, partons appear to be clustered together into extended objects or constituents-with-structure. This model for the low Q2 structure of hadrons is on one side still amenable to a partonic interpretation - it provides information on the form of the initial parton distributions - and at the same time it describes the non perturbative effects which modify the Q2 dependence predicted within pQCD, as resulting from the change in scale for the partons inside constituents which are now (pre)confined in a smaller radius. Comparisons with power corrections recently extracted from current Jlab data will be shown, as well as predictions for a wider kinematical regime attainable at 12 GeV.
[1] J. Breitweg et al., hep-ex/0005018 (2000).
[2] I. Niculescu et al., Phys. Rev. Lett. 85 (2000) 1186.
[3] CDF Collaboration, A.N. Safonov for the collaboration, hep-ex/0007037.
[4] R. Ent, C.E. Keppel, S. Liuti, G. Niculescu and I. Niculescu, in preparationJ. Timothy Londergan (Indiana Uni., Bloomington)
Charge Symmetry in Parton Distributions
Not available
Jean-Francois Mathiot (Universite Blaise-Pascal, France)
Electromagnetic interactions in Light-Front Dynamics
Not available
Wally Melnitchouk (TJNAF, Virginia/CSSM)
Origin of quark-hadron duality
Quark-hadron duality addresses some of the most fundamental issues in strong interaction physics, in particular the nature of the transition from the perturbative to non-perturbative regions of QCD. We use a simple model in which the hadronic spectrum becomes dominated by narrow resonances to expose the essential features of the dynamics behind duality. With a suitable local averaging, we demonstrate that hadronic scaling in the Bjorken limit is a straightforward property of quantum mechanical (as opposed to field-theoretic) systems. Furthermore, using local duality for the case of elastic scattering, we derive model independent relations between structure functions at large x and elastic electromagnetic form factors, and predict the x -> 1 behavior of nucleon polarization asymmetries and the neutron to proton structure function ratios from available data on nucleon electric and magnetic form factors.
Soft pion production in Deeply Virtual Compton Scattering
For the experimental realisation of DVCS, it is essential to evaluate the associated soft pion production. An estimate is performed in the framework of current algebra.
Experimental Studies of the Hadron Spectrum
I will emphasize how we use PWA techniques to unravel the resonant states in multiparticle reactions, and illustrate with examples from experiment E852 at Brookhaven and CLAS at Jefferson lab, as well as our upcoming plans for meson spectroscopy in photoproduction using the new Hall D facility at JLab. In particular, the physics topics will include
* Observations of quark-model states from E852
* Mesons with exotic quantum numbers from E852
* Strong decay dynamics in b1->omega \pi Decay from E852
* Meson photoproduction using the CLAS at Jefferson Lab
* Excited baryon spectroscopy using the CLAS at Jefferson Lab
* Plans to search for exotics and other meson phenomena using Hall D at Jefferson LabExperimental review of nucleon spin structure functions
Not available
The Character of Goldstone bosons
Not available
Structure functions and hadron properties from lattice QCD
Not available
Applications of partially quenched chiral perturbation theory
Chiral extrapolations are an essential adjunct to present and forseeable lattice simulations, since physical up and down quarks are too light to be included as dynamical quarks. I discuss the issues facing such extrapolations, and describe how one can make use of partially quenched simulations (those with differing valence and dynamical quark masses) in order to improve the extrapolations. This leads to the surprising conclusion that one can use simulations of unphysical theories (non-unitary, non-causal) to determine physical parameters, without introducing uncontrolled errors.
Tony Signal (Massey Uni., New Zealand)
Estimating low energy model parameters from deep inelastic structure functions
Model calculations of parton distributions are one of the ways to connect our knowledge of physics at low energy scales with the high energy scales where alphaQCD is small. There now exist a number of these calculations giving qualitative agreement with experimental data. We present a best-fit analysis for the valence u and d quark distributions calculated in the MIT bag model and in a chirally symmetric extension of the bag model. This analysis yields best-fit model parameters in agreement with other low energy fits, and provides new constraints on the parameters governing the pion cloud of the nucleon.
Adam Szczepaniak (Indiana Uni., Bloomington)
Confinement from Coulomb Gauge QCD, and Exotic Phenomenology
Not available
Quark-hadron duality and electroproduction experiments
Not available
Soft QCD modelling of electromagnetic form factors
Not available
Insight into Nucleon Structure from Deep Inelastic Scattering
Not available
Distortions in the negative energy Dirac sea: Violation of the Gottfried sum rule
Using the Dirac equation in 1+1 dimensions, we show that a phenomenon known as the fractional charge (irrational fermion number) which is induced by distortions in the negative energy Dirac sea, is also responsible for a part of the violation of the Gottfried sum rule. That is, even if charge symmetry and flavor SU(2) symmetry are assumed to be valid, the presence of the quark confining potential changes the vacuum structure, and leads to a violation of the valence quark normalization condition.
Gluon dynamics at small x and the initial conditions for heavy ion collisions
Not available
Gluons, quarks, and the transition from nonperturbative to perturbative QCD
Recent advances in lattice techniques using a combination of improved actions and tree-level correction have made possible detailed studies of the momentum dependence of gluon and quark propagators. This gives an excellent opportunity to establish the momentum regime where the transition from nonperturbative to perturbative QCD occurs. The latest lattice data will be described and their implications discussed.