Session 2WG: Hadron Structure and Spectroscopy II

Nucleon structure physics at Fermilab and J-PARC

Experiments with a proton or hadron beam and a fixed target give a unique opportunity to study nucleon structure. They can provide information not at a very small Bjorken-x region, but at a mid or large x region. In addition, proton-induced experiments can probe anti-quark distributions via the Drell-Yan reaction. Making use of these characteristics of the proton beams, a series of experiments at Fermilab, a major proton lab in the world, with its 800-GeV proton beams has generated remarkable results. Currently a new experiment with the 120-GeV proton beams, E906/SeaQuest, is being carried out at Fermilab, one of whose goals is to measure the dbar/ubar distribution at a larger x region. Also at J-PARC, another major proton lab, experiments to study nucleon structure with hadron beams are under consideration. As J-PARC is a high-intensity proton accelerator complex, it can generate intense secondary beams such as pions and kaons. A high-momentum beam line is being constructed at the Hadron Experimental Facility of J-PARC, which will be suitable for nucleon structure experiments. In this talk, after reviewing results from past Fermilab experiments, prospects of the SeaQuest and the J-PARC experiments will be discussed, as well as the facility status and plan.   

12 GeV Upgrade and the study of 3D imaging of nucleon

The 12 GeV upgrade of Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab will enable a new experimental program with substantial discovery potential in hadronic and nuclear physics. One of the flagship science programs is the study of multidimensional images of hadrons. A broad experimental program has been developed to map the nucleon's intrinsic correlated spin and momentum distribution through measurement of deeply exclusive and semi- inclusive processes. These multidimensional distributions open a new window on the elusive spin content of the nucleon through observables that are directly related to the orbital angular momentum of quarks and gluons. An overview of the 12 GeV upgrade with focus on the study of the multidimensional imaging of hadrons with the CLAS12 detector will be presented.   

High-energy hadron physics at J-PARC

Model-independent informations on the internal structure of the hadrons, including the compositions of the mass, momentum, and spin of the nucleon in terms of the quark and gluon degrees of freedom, are dominated by those obtained through the parton distribution functions which are deduced from the measurements of the hard processes. This is based on the comparisons of experimental data of the various high-energy scattering cross sections for the hadrons with the corresponding theory predictions expressed by the parton distribution functions (QCD factorization formulas) over wide kinematical range. In particular, the high-intensity beams at J-PARC energies allow the detailed studies of not only inclusive processes but also exclusive processes in the nucleon-nucleon scatterings, nucleon-pion scatterings, etc. The relevant J-PARC processes include the unpolarized and polarized Drell-Yan processes, the exclusive productions of lepton pair, the exclusive 2-to-2 processes, etc. It is known that most of those J-PARC processes are described by the extended versions of the QCD factorization formulas which are associated with the higher twist parton distribution functions, the transverse-momentum-dependent parton distribution functions, the generalized (off-forward) parton distribution functions, the light-cone distribution amplitudes, etc. We discuss the corresponding QCD factorization approaches for the relevant high-energy inclusive and exclusive processes at J-PARC, and novel aspects on the internal structure of the hadrons and the interplay of soft and hard QCD mechanisms, which are expected to be revealed by those experiments.   

Nucleon structure and spectroscopy from lattice QCD

One of the great challenges in our quest to understand QCD is to deduce the spectrum of excited hadronic states predicted by QCD and to compare the properties of those states against experimental evidence. Although great progress is being made in the development of sophisticated models which incorporate many of the known properties of QCD, lattice QCD is the only rigorous way known to solve QCD itself. While for ground states lattice QCD has recently achieved great success, the situation is far more complicated for excited states. In particular, on the lattice the eigenstates are stable, whereas in the real world they may decay into multiple hadronic channels. The approach of Luescher relates energy levels measured on the lattice to phase shifts when there is a single open channel but the generalizations to more realistic cases with multiple open channels are highly non-trivial in that treatment. We review recent work based on a Hamiltonian treatment of the resonant state and its coupled channels on the lattice, showing that it appears to provide a very promising and cost effective alternative method for extracting physical properties from lattice simulations.   

High-Energy Exclusive Reactions at J-PARC

The construction for a high-momentum beam line at J-PARC has been funded. This beam line will provide primary proton beam of 30 GeV, as well as secondary beams (pion, kaon, antiproton) up to $\sim$15 GeV/c. We discuss the physics opportunities offered by this high-momentum beam line. In particular, the prospect for measuring exclusive dilepton production with meson beams, and the complimentarity of such measurements to the exclusive reactions programs at JLab and COMPASS will be presented.