General Discussion

• some general remarks

• some questions

Perspective• The proton is a very different world to the atom

and the nucleus- mass dominated by binding (glue and sea quarks)- gluons contribute half of proton momentum- quarks account for only about 25% of proton’s

spin• QCD is believable only as well as it is tested• Hadron structure requires

- experimental precision- theoretical precision

Clear Scientific Themes to theStudy of Hadron Structure

• accessing the glue that binds us all

• understanding the spin structure of the nucleon

Deep inelastic scattering• DIS is the technique to image the structure of the proton• These images are typically displayed as a function of parton

momentum x and image resolution Q2 STRUCTURE FUNCTIONS

• The images are measured in the infinite momentum frame• Lattice QCD calculates moments of structure functions• Gluon distributions are determined from quark distributions

using the evolution equations of QCD LOW x• Integrals of structure functions are used to determine global

properties of the proton • The study of hadron structure largely consists of measuring

structure functions over a large range of x and Q2

JLab• DIS works well at low energies in many

JLab experiments ~ 5 GeV !• 12 GeV energy upgrade will make

available the valence quark region at low Q2 with high luminosity

- high x- study of generalized parton

distributions- study of transversity- EMC effect in nuclei

JLab Questions

• What are the limits of the applicability of the DIS diagram – test factorization, scaling, etc.

• What are the deliverables on nucleon structure from the GPD program?

• What are the critical elements of the GPD What are the critical elements of the GPD program to make progress towards program to make progress towards measuring the Ji sum rule?measuring the Ji sum rule?

• Can we make progress in understanding Can we make progress in understanding the EMC effect?the EMC effect?

RHIC• RHIC-spin will provide

- direct information on the contribution of gluons to the spin of the proton

- determination of valence antiquark polarizations

• RHIC p/d-A may provide qualitative information on gluon distribution in nucleus

RHIC questions• How `clean’ is the extraction of ΔG

with proton-proton scattering: theoretical uncertainties?

• How low in x will ΔG be measured? • What is the focus of the p/d-A

program?

EIC

• Is the next generation accelerator to primarily access the low x region

• Direct access to the glue in the nucleon (to x ~ 10-4) and in nuclei (to x ~ 10-3) over a large Q2 region (≤ 104 GeV2)

• Spin structure of nucleon - extension of g1

p,g1n to low x: precision Bjorken Sum Rule

- gluon polarization at low x => integral ΔG- strange quark polarization at low x

• Role of gluons in the nucleus• Test QCD

EIC Questions

• Beyond JLab@12 GeV and RHICII-spin, is there another alternative facility for the US QCD community?

• How low in x do we need to go to do the science?

• What maximum luminosity do we need? How significant is measurement of exclusive processes at low x and high Q2?

QCD and Hadron Structure

Overarching Goal: Explore and Understand QCD

Consists of Gaining Insight• map the basic features of the proton/NP-QCD• discover the best degrees of freedom to fully describe the proton• explore how hadrons emerge from the QCD vacuum

and of Precision Tests of QCD

QCD and Hadron Structure

Key Questions

1. What is the role of gluons in nucleons and nuclei?

2. What is the internal spin and flavor landscape of hadrons?

3. How do hadronic final-states form in QCD?

Overarching Goal: Explore and Understand QCD

• Why are gluons so evident in the proton but lacking in spectroscopy?

QCD and Hadron Structure

Key Questions

1. What is the role of gluons in nucleons and nuclei?

• How do the gluonic fields in nuclei differ from those in the proton?

• How much do the gluons contribute to the proton’s spin?

• Do we know gluon distributions well enough for other particle-physics applications?

Overarching Goal: Explore and Understand QCD

• How are charge and magnetization distributed at small distances?

2. What is the internal spin and flavor landscape of hadrons?

QCD and Hadron Structure

Key Questions

• How are flavor symmetries in nucleon and meson structure broken?

• What are the quark and gluon spatial, momentum and spindistributions in the nucleon?

• What are the spin-orbit correlations of quarks and gluons?

Overarching Goal: Explore and Understand QCD

3. How do hadronic final-states form in QCD?

QCD and Hadron Structure

Key Questions

• Can we understand the physical mechanism of fragmentation and how do we calculate it quantitatively?

• What role do spin and angular momentum play in hadron formation?

Overarching Goal: Explore and Understand QCD

Proposed Priorities of U.S. QCD community for ~ next

decade• Realize planned upgrades in energy,

luminosity, scientific instrumentation of JLab and RHIC

• A vigorous theory/computation capability is essential

• Establish a high luminosity electron ion collider (EIC) as the next generation accelerator in the U.S. beyond the upgrades of JLab and RHIC to study QCD