Session 3WQA: 3D Hadron Structure from Next-Generation Scattering Experiment and Lattice QCD I

3D tomography of hadrons with hadron and lepton beams

Form factors defined by matrix elements of quark and gluon energy-momentum tensors are called gravitational form factors. They used to be considered as purely academic quantities without any experimental measurement because the gravitational interactions are extremely small in comparison with strong, electromagnetic, and weak interactions. However, due to recent developments of the hadron-tomography field, it became possible to determine these form factors from spacelike generalized parton distributions (GPDs) and their s-t crossed ones, generalized distribution amplitudes (GDAs). The GDAs could be called timelike GPDs. The spacelike and timelike GPDs are investigated in charged-lepton-scattering processes and electron-positron collisions, typically, by deeply virtual Compton scattering and two-photon processes, respectively [1]. In addition, hadron-accelerator [2] and neutrino [3] facility measurements will become possible in the near future. In this talk, I explain these GPD projects.   

Deep-inelastic scattering and Drell-Yan process for the 3D structure of the nucleon

An overview of the study of the 3D structure of the nucleon is described. Both the longitudinal and transverse structures of the nucleon have been investigated with various experimental methods. The longitudinal structure or helicity distribution was studied first as the cross section of polarized deep-inelastic scattering was formulated with quark-parton model. The result reported by EMC in the papers in 1988 and 1989 showed that the contribution of spin of quarks and anti-quarks to the proton spin is only (12 +-  9 +- 14)%. The result was confirmed by the experiments in 1990's and later. Spin is one of the quantum numbers of the nucleon. The concept of 'valence quark' was questioned because the valence quarks were supposed to determine all the quantum numbers of the nucleon. Sea quarks are quarks and anti-quarks generated by the gluon dissociation inside the nucleon. Those sea quarks could also contribute to the nucleon spin. Gluon spin contribution was studied by proton-proton collisions and deep-inelastic scattering.

The transverse-momentum-dependent parton distributions (TMDs) and the orbital angular momenta of partons are another subjects under study. The single-spin asymmetry in meson productions was observed with increasing center-of-mass energy up to RHIC. Sivers- and Collins-asymmetries were separately identified by sin( phi - phi_S ) and sin( phi + phi_S ) modulations of the azimuthal distributions by HERMES in 2005. The coincidence measurement of the scattered lepton and produced hadrons turned out to be the effective method. Exclusive photon and meson productions were measured for generalized parton distributions. The deep-inelastic scattering is sensitive to the sum of parton distributions of quarks and anti-quarks while the Drell-Yan process is sensitive to the products of the two. The properties of anti-quarks can be studied with a combination of these two complementary approaches. The evolution and perspective of the study with these experimental methods are discussed.   

Study of 3D nucleon structure at the J-PARC π20 beamline

One of the keys to understand 3D nucleon structure is precise measurements of Generalized Parton Distribution Functions (GPDs). Measurements in different reactions help to verify universality of GPDs and derive GPDs in wide kinematic region. Hadron induced exclusive reactions are complementary to lepton induced reactions such as deeply virtual Compton scattering (DVCS) and deeply virtual meson production (DVMP).

In the J-PARC hadron experimental hall, measurements of GPDs using hadron beam are planned. At the high momentum beam line, 30 GeV/c primary proton beam from the J-PARC Main Ring is derived. In addition, positive and negative π/K/p secondary beam up to 20 GeV/c will be delivered by installing a new production target. The secondary beam line is called π20 beam line.

At the π20 beam line, measurements of the exclusive Drell-Yan reaction (π^-p→γ^*n→μ⁺μ^-n) and 2→3 reactions (such as pp→pπ⁺n) are planned to access GPDs. The medium beam momentum of the π20 beam line is suited for the measurements of those reactions since cross sections of these reactions decrease with beam momentum. A new high-resolution, high-rate capable spectrometer is under construction. The new spectrometer enables the first measurements of above exclusive reactions thanks to a good momentum resolution under high rate. We will report details and prospects of these experiments.