Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session CE: Quark and Gluon Structure of the Nucleon at Large Momentum Fractions |
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Chair: Wally Melnitchouk, Thomas Jefferson National Accelerator Facility Room: 103AB |
Thursday, October 27, 2011 8:30AM - 9:06AM |
CE.00001: Structure of the free neutron Invited Speaker: Sebastian Kuhn Information on the structure of the neutron is indispensible for a full understanding of the static properties, resonance excitations and quark distributions of the nucleon. From elastic form factors over resonance transition amplitudes to deep inelastic structure functions (both unpolarized and polarized), studying both partners of the proton-neutron isospin doublet is necessary to address such fundamental questions as the valence quark structure of the nucleon (in particular the ratio of d/u quark probabilities at large $x)$, higher twist effects and the phenomenon of quark-hadron duality. Measurements on the neutron are hampered by the fact that neutron targets of sufficient densities exist only bound inside nuclei, with the deuteron, the triton and (polarized) $^{3}$He being the most often used ``ersatz targets.'' The need to account for binding effects complicates the extraction of free neutron data from these experiments. Progress requires either a way to avoid model uncertainties (e.g., by focusing on kinematics where the PWIA spectator model works reasonably well for the struck nucleus) or a better understanding of these nuclear effects. In either case, one has to also deal with complications like final state interactions and other contributions. On the other hand, detailed studies of the reaction mechanism can yield important new information on the structure of few-body nuclei and the interplay of nuclear and quark degrees of freedom. In my talk, I will present some recent experimental results on neutron structure functions and some new approaches towards a better understanding of nuclear binding effects. I will concentrate on the large and varied program pursued at Jefferson Lab in this area, both from the present 6 GeV program and for the era after the 12 GeV upgrade. [Preview Abstract] |
Thursday, October 27, 2011 9:06AM - 9:42AM |
CE.00002: Longitudinal structure of the proton and neutron Invited Speaker: M. Eric Christy Lepton scattering has been utilized for more than four decades to study the substructure of protons and neutrons, both free and bound in nuclei. The $F_2$ structure function extracted from such experiments has now been determined over many orders of magnitude in both Bjorken $x$ and the 4-momentum transfer squared, $Q^2$, and such data have been an invaluable tool for the testing and study of Quantum Chromodynamics (QCD) in the perturbative regime. In addition, new precision data on $F_2$ from Jefferson Lab in the region of the nucleon resonances has opened up many new studies of the transition from perturbative to non-perturbative QCD. This includes the nature of quark-hadron (Q-H) duality, in which the resonances seem to average to a smooth scaling curve, similar to the that of deep inelastic scattering. In contrast, the longitudinal structure function, $F_L$, has been measured with much poorer precision and over a much more limited kinematic range. This is due to both the high precision required and the need for measurements at fixed $x$ and $Q^2$ with multiple beam energies for the separation of longitudinal and transverse structure. Such measurements are critical for a full picture of nucleon structure. For instance, in deep inelastic scattering $F_L$ is directly sensitive to the gluon content of the nucleon, unlike $F_2$, which is only sensitive through pQCD evolution. We will present the current experimental status of $F_L$ for both free protons and from nucleons in nuclei and discuss some of the physics which can be addressed with such measurements. In particular, studies of Q-H duality and the determination of the structure function moments will be discussed. [Preview Abstract] |
Thursday, October 27, 2011 9:42AM - 10:18AM |
CE.00003: Impact of low-energy data on global fits of PDFs Invited Speaker: Joseph Owens Traditional global fits for parton distribution functions (PDFs) use cuts on $Q$ and $W$ in deeply inelastic lepton-nucleon scattering (DIS) to eliminate regions where target mass corrections and higher twist contributions are important. Such cuts typically limit the coverage in $x$ to $x < 0.7$, which is also the range covered by most high energy DIS experiments. In order to constrain the PDFs at larger values of $x$, one can relax the cuts imposed on $Q$ and $W$ and make use of lower energy data. This necessitates treating the target mass corrections and higher twist contributions in the fits. Furthermore, the separation of the $u$ and $d$ PDFs requires the use of nuclear targets - typically deuterium - which requires the inclusion of corrections for nuclear effects. This is particularly important in the large-$x$ region. In this talk I will review some recent efforts to include low-energy data in global fits and discuss the resulting PDFs. [Preview Abstract] |
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