Science Overview and the Experimental Program

L. Cardman

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

The Structure of the Science Presentations

• • • • • Overview of the Experimental Program – Scientific Motivation and Progress (LSC) Detailed Talks on Three Cross-Cutting Efforts in the JLab “Campaigns” to understand Hadronic and Nuclear Structure: The Shape and Structure of the Nucleon (Volker Burkert) The Parton-Hadron Transition in Structure Functions and Moments (Rolf Ent) From Nucleons and Mesons to Quarks and Gluons (Kees de Jager) Details on the Hall Research Programs and Technical Developments (Dennis Skopik) Theory (Rocco Schiavilla) Progress and Plans for Nuclear Physics Research at 12 GeV and Beyond (LSC) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

JLab’s Scientific Mission

• • • How are the hadrons constructed from the quarks and gluons of QCD?

What is the QCD basis for the nucleon-nucleon force?

Where are the limits of our understanding of nuclear structure? To what precision can we describe nuclei?

To what distance scale can we describe nuclei?

Where does the transition from the nucleon-meson to the QCD description occur?

To make progress toward these research goals we must address critical issues in “strong QCD”: What is the mechanism of confinement?

Where does the dynamics of the q-q interaction make a transition from the strong (confinement) to the perturbative (QED-like) QCD regime?

How does Chiral symmetry breaking occur?

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Nuclear Physics: The Core of Matter, The Fuel of Stars

(NAS/NRC Report, 1999) Science Chapter Headings: The Structure of the Nuclear Building Blocks The Structure of Nuclei Matter at Extreme Densities The Nuclear Physics of the Universe Symmetry Tests in Nuclear Physics

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JLab Scientific “Campaigns”

The Structure of the Nuclear Building Blocks

1.

2.

3.

How are the nucleons made from quarks and gluons? How does QCD work in the ‘strong’ (confinement) regime?

How does the NN Force arise from the underlying quark and gluon structure of hadronic matter?

The Structure of Nuclei Volker’s and Rolf’s talks

4.

5.

What is the structure of nuclear matter? At what distance and energy scale does the underlying quark and gluon structure of nuclear matter become evident?

Kees’ Talk Symmetry Tests in Nuclear Physics

6.

Is the “Standard Model” complete? What are the values of its free parameters?

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

1. How are the Nucleons Made from Quarks and Gluons?

Why are nucleons interacting via V NN approximation to nature?

such a good How do we understand QCD in the confinement regime ?

A.

B.

C.

D.

What are the spatial distributions of u, d, and s quarks in the hadrons?

G E p /G M p G E n , w/ Super-Rosenbluth coming (2 expts in Hall C) G M n (Hall A; CLAS to high Q 2 ) HAPPEX, w/ G0 & HAPPEX II coming F  , w/ Higher Q 2 extension coming (6, then 12 GeV)

What is the excited state spectrum of the hadrons, and what does it reveal about the underlying degrees of freedom?

ND (All three halls) Higher resonances (CLAS e1: ,  0 ,   production) Missing resonance search (CLAS e1 and g1: ,  production VCS in the resonance region (Hall A)

What is the QCD basis for the spin structure of the hadrons?

Q 2 evolution of GDH integral and integrand for: proton (CLAS) and neutron (Hall A) (w/ low Q 2 A 1 n , g 2 n A 1 p w/ 12 GeV follow-on (Hall A) (Hall C, CLAS) extension coming for neutron)

What can other hadron properties tell us about ‘strong’ QCD?

VCS (Hall A) DVCS (CLAS, Hall A & CLAS coming) Compton Scattering (Hall A) Separated Structure Functions (Hall C) Single Spin Asymmetries (CLAS, Hall A coming) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

1. How are the Nucleons Made from Quarks and Gluons?

Why are nucleons interacting via V NN approximation to nature?

such a good How do we understand QCD in the confinement regime ?

A.

B.

C.

D.

What are the spatial distributions of u, d, and s quarks in the hadrons?

G E p /G M p G E n , w/ Super-Rosenbluth coming (2 expts in Hall C) G M n (Hall A; CLAS to high Q 2 ) HAPPEX, w/ G0 & HAPPEX II coming F  , w/ Higher Q 2 extension coming (6, then 12 GeV)

Rolf (2002) What is the excited state spectrum of the hadrons, and what does it reveal about the underlying degrees of freedom?

ND (All three halls) Higher resonances (CLAS e1: ,  0 ,  Missing resonance search (CLAS e1 and g1: VCS in the resonance region (Hall A)  production) ,  production

Volker (2003) Bernhard (2002) What is the QCD basis for the spin structure of the hadrons?

Q 2 evolution of GDH integral and integrand for: proton (CLAS) and neutron (Hall A) (w/ low Q 2 A 1 n , g 2 n A 1 p w/ 12 GeV follow-on (Hall A) (Hall C, CLAS) extension coming for neutron)

Kees (2002) What can other hadron properties tell us about ‘strong’ QCD?

VCS (Hall A) DVCS (CLAS, Hall A & CLAS coming) Compton Scattering (Hall A) Separated Structure Functions (Hall C) Single Spin Asymmetries (CLAS, Hall A coming)

Rolf (2003) Volker (2003)

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The Proton and Neutron are the “Hydrogen Atoms” of QCD

What we “see” changes with spatial resolution

>1 fm Nucleons 0.1 — 1 fm Constituent quarks and glue < 0.1 fm “bare” quarks and glue S=1/2 S=1/2 S=1/2 Q = 1 Q = 1 Q = 1

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Nucleon and Pion Form Factors

• Fundamental ingredients in “Classical” nuclear theory • The spatial distribution of charge and magnetization provide a testing ground for theories constructing nucleons from quarks and gluons.

• Experimental insights into nucleon structure from the flavor decomposition of the nucleon form factors PRECISION G p E G p M G E n G n M G p,Z E G p,Z M }  G u E G u M G d E G d M G s E s G M • Additional insights from the measurement of the form factors of nucleons embedded in the nuclear medium implications for binding, equation of state, EMC… - precursor to QGP S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

G

E p

/G

M p

as Measured by (e,e’p): Critical Data for Understanding the Proton’s Structure

The combination of high intensity e beams and proton polarimetry has dramatically improved our knowledge of this fundamental system and revived theoretical interest in this important problem  S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Work in Progress to Understand Difference between Original (Rosenbluth) and new (e,e’p) Results

•

JLab (e,e’p) data inconsistent with older SLAC data using Rosenbluth technique G E p at Q only 8% of the cross section 2 =5 GeV 2 if G E =

m

G M

• •

Recent analysis of available Hall C (e,e) data (Rosenbluth technique) is consistent with SLAC data



systematic errors not likely the cause of the differences Hall A New “super-Rosenbluth” data under analysis

•

Possible culprits:

−

Radiative Corrections

− − −

Coulomb Corrections Dispersive Effects ????

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Unraveling Nucleon Structure Through a Consistent Analysis of both G E p and G E n Neutron Electric Form Factor Data is Essentially Final

(Polarization Experiments only)

Explaining both G E p and G E n is Proving to be a Challenge Consistently

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Measurements of the Strange Quark Distribution (just underway) Will Provide a Unique New Window into Hadron Structure

Unlike G E n , the ss pairs come uniquely from the sea; there is no “contamination” from pre-existing u or d quarks S=1/2 S=1/2 Q = 1 Q = 1 As is the case for G E n , the strangeness distribution is very sensitive to the nucleon’s properties

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

G0 Installed, Completed 1

st

Engineering Run Successfully

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

G0 Experiment Status

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G0: False Asymmetries

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G0: Backgrounds

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G0: Status and Plans

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Strange Form Factors G

E s

and G

M s

What we have on the books now S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Strange Form Factors G

E s

and G

M s

Expected Forward Angle Results by mid 2004 S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

1. How are the Nucleons Made from Quarks and Gluons?

Why are nucleons interacting via V NN approximation to nature?

such a good How do we understand QCD in the confinement regime ?

A.

B.

C.

D.

What are the spatial distributions of u, d, and s quarks in the hadrons?

G E p /G M p G E n , w/ Super-Rosenbluth coming (2 expts in Hall C) G M n (Hall A; CLAS to high Q 2 ) HAPPEX, w/ G0 & HAPPEX II coming F  , w/ Higher Q 2 extension coming (6, then 12 GeV)

What is the excited state spectrum of the hadrons, and what does it reveal about the underlying degrees of freedom?

ND (All three halls) Higher resonances (CLAS e1: ,  0 ,   production) Missing resonance search (CLAS e1 and g1: ,  production VCS in the resonance region (Hall A)

What is the QCD basis for the spin structure of the hadrons?

Q 2 evolution of GDH integral and integrand for: proton (CLAS) and neutron (Hall A) (w/ low Q 2 A 1 n , g 2 n A 1 p w/ 12 GeV follow-on (Hall A) (Hall C, CLAS) extension coming for neutron)

What can other hadron properties tell us about ‘strong’ QCD?

VCS (Hall A) DVCS (CLAS, Hall A & CLAS coming) Compton Scattering (Hall A) Separated Structure Functions (Hall C) Single Spin Asymmetries (CLAS, Hall A coming) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

The Search for “Missing States” in the Quark Model Classification of N*

“missing” P 13 (1850) Capstick& Roberts

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

CLAS: ep



epX, E = 4GeV

2 1.5

1 0.

2 thresh.

0.5

1.0

1.5

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

CLAS

Resonances in

g *p  p  +  Analysis performed by Genova-Moscow collaboration step #1: use the best information presently available G N  G N g from PDG AO/SQTM extra strength W(GeV) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Attempts to fit observed extra strength

CLAS Analysis step #2: - vary parameters of known D 13 or - introduce new P 13 P 13 D 13 (1700) W(GeV) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

New Resonances are also Seen with Real Photons in

g

p



p



+



-

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

In Strangeness Production in the Resonance Region

• Small sample of data covering the full kinematic range in energy and angles for K + L and K + S , including recoil polarization • Data indicate significant resonance contributions, interfering with each other and with non-resonant amplitudes. • Extraction of resonance Parameters requires a large effort in partial wave analysis and reaction theory.

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

and in



Production

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Next Steps…..

• •

What is needed now is a coherent, consistent analysis of the data from a broad variety of channels, from photo and electro-production, and for the available values of Q 2

•

It will also be important to incorporate consistently available data obtained using hadronic beams We are requesting support for an analysis center to be created as part of the theory group (Rocco’s talk will present details)

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Chiral Soliton Model and Exotic Baryons

The chiral soliton model by D. Diakonov, M. Petrov, M. Polyakov (1997) predicts an anti-decuplet of penta-quark baryons. The lightest state is predicted as a baryon state with exotic quantum number S=+1, and M=1.53GeV, G =15MeV.

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

CLAS

: Evidence for Exotic Baryon with S=+1

g D pK K + (n) g p  + K K + (n)

M

Q

=1.539 GeV

s Q

= 0.013GeV

PAC24 Just Approved Detailed Measurement with 20x Statistics

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

1. How are the Nucleons Made from Quarks and Gluons?

Why are nucleons interacting via V NN approximation to nature?

such a good How do we understand QCD in the confinement regime ?

A.

B.

C.

D.

What are the spatial distributions of u, d, and s quarks in the hadrons?

G E p /G M p G E n , w/ Super-Rosenbluth coming (2 expts in Hall C) G M n (Hall A; CLAS to high Q 2 ) HAPPEX, w/ G0 & HAPPEX II coming F  , w/ Higher Q 2 extension coming (6, then 12 GeV)

What is the excited state spectrum of the hadrons, and what does it reveal about the underlying degrees of freedom?

ND (All three halls) Higher resonances (CLAS e1: ,  0 ,   production) Missing resonance search (CLAS e1 and g1: ,  production VCS in the resonance region (Hall A)

What is the QCD basis for the spin structure of the hadrons?

Q 2 evolution of GDH integral and integrand for: proton (CLAS) and neutron (Hall A) (w/ low Q 2 A 1 n , g 2 n A 1 p w/ 12 GeV follow-on (Hall A) (Hall C, CLAS) extension coming for neutron)

What can other hadron properties tell us about ‘strong’ QCD?

VCS (Hall A) DVCS (CLAS, Hall A & CLAS coming) Compton Scattering (Hall A) Separated Structure Functions (Hall C) Single Spin Asymmetries (CLAS, Hall A coming) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

New Hall A data on A 1 n (E99-117) Data provide first indication that A 1 n deviates from 0 at large x, but are clearly at variance with pQCD prediction assuming Hadron Helicity Conservation

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM



New A 1 n Data Has Changed our Understanding of Nucleon Helicity-Flavor Distributions Use d/u ratio from F 2 proton and neutron on

A

1

n

D

u u

D

d d

   D

u

4

u

15 4 15   4 D 4

d d A

1

p

 4 D

u

 D 4

u



d A A

1 1

p n

(4 (4  

d u

) 1 /

d u

 1 15 ) 

A

1 15 1

n

(1

A

1

p

 (1 4 

d u

) 4 /

d d u

) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

CLAS :

Helicity Asymmetry A

1

(x)

CLAS data: ~1/2 of full statistics Proton Deuteron Hyperfine-perturbed QM qualitatively describes the x-dependence.

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

2. How Does QCD Work in the ‘Strong’ (Confinement) Regime?

This program is a “bridge” between campaigns 1 and 2, and contributes coherently to both by directly studying key aspects of strong QCD directly A.

What is the origin of quark confinement?

(Understanding this unique property of QCD is the key to understanding the QCD basis of nuclear physics.) Lattice QCD Calculations favor the flux tube model Meson spectra will provide the essential experimental data: use the “two-body” system to measure V(r), spin dependence experimental identification of exotics tests the basic mechanism

B.

Some experiments in progress with CLAS, but 12 GeV and Hall D are essential to this program

Where does the dynamics of the q-q interaction make a transition from the strong (confinement) to the perturbative (QED-like) QCD regime?

F    /  (4 GeV so far; 6 GeV this year, then 11 GeV w/ upgrade) ratio S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Gluonic Excitations and the Origin of Confinement

Theoretical studies of QCD suggest that confinement is due to the formation of “Flux tubes” arising from the self-interaction of the glue, leading to a linear potential (and therefore a constant force)

From G. Bali

Flux tube forms between qq linear potential

Experimentally, we want to “pluck” the flux tube and see how it responds S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Ongoing Analysis of CLAS Data Demonstrates the Promise of Meson Photoproduction ~500x existing data on photo production from a 1 month run with CLAS m(



+



+



) GeV/c 2 m(



+



+



)

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

The Pion Form Factor

Where does the dynamics of the q-q interaction make a transition from the strong (confinement) to the perturbative (QED-like) QCD regime?

• It will occur earliest in the simplest systems; the pion form factor provides our best chance to determine the relevant distance scale experimentally • • To Measure F  (Q 2 ): At low Q 2  R rms (<0.3 (GeV/c) =0.66 fm 2 ): use  At higher Q 2 : use 1 H(e,e  + )n e scattering (scatter from a virtual pion in the proton and extrapolate to the pion pole) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Pion Form Factor

And even further with the upgrade Data to be extended to 2.5 GeV 2 next month

FF of lightest hadron is expected to start scaling at lowest Q 2 S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM



+

/



-

Ratio for the

g

+ N

 

+ N

A Complementary Approach: “Soft” Corrections cancel in



/



+ Ratio (Revealing the underlying transition?) But the theoretical interpretation is more ambiguous GPD-based Calculation 12 GeV Data will reveal whether asymptopia has been reached or the agreement with the GPD calculation is fortuitous

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

3. How Does the NN Force Arise from the Underlying Quark and Gluon Structure of Hadronic Matter?

• • We know:

Kees (2003)

The long-range part of the force is well described by pion exchange The remainder involves the quark-gluon structure of the nucleon: quark exchange, color polarization, and glue-glue interaction Unraveling this structure requires data from a broad range of experiments: A. How well does a meson exchange-based NN force describe the few body form factors?

B.

C.

deuteron A, B, t 20 d(e,e’p)n Is there evidence for the QCD structure of nuclear matter from “color transparency” in nucleon propagation?

Geesaman (e,e’p)  photoproduction coming (CLAS) Milner (e,e’p) to higher Q 2 Are the nucleon’s properties modified in the nuclear medium?

G E p in 16 O and 4 He g n   p in 2 H, 4 He D. Nucleon-meson form factors CLAS g1: g p  K  L(S 0 ) ( submitted to PRL ) CLAS e1: ep  e’    (paper in review) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

4. What is the Structure of Nuclear Matter?

Kees (2003)

A broad program of experiments taking advantage of the precision, spatial resolution, and interpretability of experiments performed using electromagnetic probes to address long-standing issues in nuclear physics and identify the limits of our understanding

A. How well does nuclear theory describe the energy and spatial structure of the single particle wavefunctions?

(use the (e,e’p) reaction to measure these wavefunctions) 16 O(e,e’p) 3,4 He(e,e’p) and 4 He(e,e’p) d(e,e’p), and d(e,e’p)

B. Can the parameterized N-N force adequately describe the short-range correlations among the nucleons?

(use (e,e’p), (e,e’pp), (e,e’pn), …reactions and measure the Coulomb Sum Rule) CLAS e2: 12 C(e,e’Np), 3 He(e,e’pp) 4 He(e,e’p) to high Q 2 and E m Sick (e,e’p) study

C.

What can the introduction of an “impurity” (in the form of a

L

) tell us about the nuclear environment and the N-N force?

(electro-produce hypernuclei and measure their properties) HNSS Experiment Upcoming Hall A and Hall C extensions S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

5. At What Distance and Energy Scale Does the Underlying Quark and Gluon Structure of Nuclear Matter Become Evident? Kees (2003)

• • •

We begin with ‘ab initio’ (“exact”) Calculations of the structure of few body nuclei, in which we assume:

Nucleus has A nucleons interacting via force described by V NN V NN fit to N-N phase shifts Exchange currents and leading relativistic corrections in V NN and nucleus We test these calculations via electromagnetic interaction studies of few-body systems where precise, directly interpretable experiments can be compared with exact calculations The goal is to determine the limits of the meson-nucleon description and to infer where a QCD-based description becomes substantially more straightforward

Push precision,



to identify limits and answer the question

Deuteron: A, B, t 20 form factors photodisintegration (Halls C and A, and now CLAS) Induced polarization in photodisintegration 3 He form factors to high Q 2 S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

6. Is the “Standard Model” Complete? What Are the Values of Its Free Parameters?

The Standard Model (SM) has been broadly successful in describing phenomena in nuclear and particle physics. Traditional tests have been at the Z pole and through high-energy searches for new particles. JLab has launched a program aimed at both testing the theory and determining its constants in both the electro-weak and strong sectors using an alternate approach – precision measurements at low energies.

A.

B.

C.

Is the Standard Model of Electro-weak Interactions Correct?

(Precision measurements at low energy provide tests comparable to moderate precision measurements at very high energies)

Q Weak - Test of Standard Model in Electro-weak Sector 12 GeV extensions

Does QCD Lagrangian accurately describe strongly-interacting matter, or is there physics beyond it?

(Test predictions of QCD at energies just above the pion threshold where Chiral Perturbation Theory [



PT] is expected to be valid)

 0 lifetime measurement (PRIMEX) Q 2 evolution of GDH integral at low Q 2

Complete our experimental information on the Standard Model through experiments that determine precisely its free parameters

Radiative decay of  ,  , and  mesons. (12 GeV proposals) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

The Q p Weak Experiment

The First Measurement of the Weak Charge of the Proton; a Precision Test of the Standard Model via a 10 s Measurement of the Predicted Running of the Weak Coupling Constant, and a Search for Evidence of New Physics Beyond the Standard Model at the TeV Scale Weak Mixing Angle (Scale dependence in MS scheme) • Electroweak radiative corrections  sin 2  W varies with Q Pure leptonic Semi-Leptonic

+ +

 • Extracted values of sin 2  W must Standard Model or new agree with physics is indicated.

p

Q

weak

 1  4 sin 2 

W

~ 0 .

072 • A 4% Q p Weak measurement probes for new physics at energy scales to: L

g

  2

G F

D Q

p W

 4.6 TeV • Q p weak (semi-leptonic) and E158 (pure leptonic) together make a powerful program to search for and identify new physics.

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Major Effort Toward Planning for the 12 GeV Upgrade Continues; CD-0 Needed!!!

• • • • •

Since the NSAC LRP:

Two-year effort to refine equipment designs and expand science program Individual, Hall-specific pCDRs written summarizing science and equipment needs Second PAC review of science and equipment (1/03) Defended before NSAC Facilities Subcommittee (2/03) Science absolutely central Ready for Construction pCDR summarizing all science and experimental equipment posted on the web for review by the entire Jlab Community • •

CD-0 is the next essential step:

Work on the CDR can begin in earnest as soon as we have CD-0 authorization to carry out the remaining needed R&D It will permit serious exploration of non-DOE/NP funding sources S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Summary and Perspectives

• CEBAF’s beam and experimental equipment provide a unique tool for nuclear physics • Exciting physics results continue to emerge: Testing the limits of classical nuclear theory Exploring the QCD basis of the strong interaction and of the structure of nucleons and nuclei • The 12 GeV Energy Upgrade will open many exciting new physics opportunities addressing key issues in nuclear physics (more tomorrow) S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

End of Base Talk

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Extending DIS to High x The Neutron Asymmetry A

1 n

at 12 GeV

g

12 GeV will access the valence quark regime (x > 0.3), and glue where constituent quark properties are not masked by the sea quarks

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Neutron Spin Structure A

1 n

E99-117

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Nucleon Inclusive Spin Structure

• I n 1989 the analysis of the EMC results showed that only a small fraction of the nucleon spin is carried by the quark’s spin DS  D

u +

D

d +

D

s



0.2

in contrast to the Constituent Quark Model

•

SPIN PUZZLE

Measurements at CERN , SLAC, DESY confirmed these results and verified the Bjorken sum rule at the ~5% level (Q 2 >> 1 GeV 2 ) • Gerasimov-Drell-Hearn integral for protons measured at Mainz/Bonn.  results are consistent with the sum rule prediction (Q 2 = 0) • The transition from Q 2 = 0 to Q 2 >> 1 GeV 2 virtually unknown S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Spin Integrals Are Constrained at Extremes of Distance Scales by Sum Rules

Bjorken Sum Rule (Q 2

 

): Basic assumptions: Isospin symmetry Current Algebra or Operator Product Expansion within QCD

G p 1

(

Q 2

)

- G n 1

(

Q 2

)

= т

{

g 1 p 2

)

g 1 n 2

)}

dx = 1 6 g C A NS

,

as Q 2 ® Ґ g A =1.2601 ± 0.0025 neutron C NS b decay coupling constant Q 2 -dependent QCD correction (  1 as Q 2   )

GDH Sum Rule (Q 2



0) : Basic assumptions : Lorentz invariance, gauge invariance, unitarity Dispersion relation applied to forward Compton amplitude

Ґ т

n

in (

s

1 2 -

s

3 2 ) d

n n

= 2 2

p a

EM M 2

k

2  = nucleon anomalous magnetic moment S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Moment of Proton Spin Structure Function vs Distance Scale

Q 2 = 

single partons (Bjorken SR)

G 1 = 0 1 т 1 (

,

2 )

multiple partons DIS, pQCD constituent quarks, N* twist expansion ?

pions, nucleon magnetic moment (GDH SR)

Q 2 = 0

quark models LQCD ?

ChPT ?

GDH sum rule

1

Q 2 (GeV 2 )

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

CLAS

: First Moment

G

1p

= g

1

(x,Q

2

)dx

 Q 2 evolution of G 1p reveals importance of resonances.  Nucleon resonance are needed to explain   falloff for Q 2 < 1.5 GeV 2 zero-crossing near 0.3 GeV 2 .

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Photo-pion production E94-104

Indication of global scaling Resonance behavior at ~ 2 GeV not confirmed S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

Color Transparency – Now and at 12 GeV

Hall C (e,e’p) experiments at 4 and 5.5 GeV show no evidence for color transparency Extending these data to 12 GeV will either reveal color transparency or force us to rethink our understanding of quark-based models of the nucleus 12 GeV will also permit similar measurements using the (e,e’ color transparency at lower Q 2  ) reaction, which is expected to show S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

The Neutron’s Charge Distribution Provides Further Insights into Hadron Structure

Previously available data limited to modest Q 2 , just barely sensitive to details beyond the RMS radius

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM

The Neutron’s Charge Distribution Provides Further Insights into Hadron Structure (cont)

 • New neutron electric form factor data reveal the shape of the charge distribution • • And the importance of relativistic effects in nucleon structure

Both data sets are now nearly final, and in a race to publication

S&T_7-03_Cardman_Science_Overview_final 5/6/2020 6:42 PM