Experimental Study of Nucleon Structure and QCD

J. P. Chen, Jefferson LabWorkshop on Confinement Physics, March 12, 2012

Introduction

Selected JLab 6 GeV Experimental Results

Spin Distributions in the High-x (Valence Quark) Region

and Quark-Hadron Duality

Moments of Spin Structure Functions:

Spin Sum Rules and Polarizabilities

Transverse Spin, TMDs

Planned Experiments with JLab 12 GeV

QCD: still unsolved in non-perturbative region

• 2004 Nobel prize for ``asymptotic freedom’’• non-perturbative regime QCD ?• Confinement: one of the top 10 challenges for physics!• QCD: Important for discovering new physics beyond SM• Nucleon structure is one of the most active areas

Introduction• Quarks/Gulons are confined in hadron • To study/understand confinement: both static (spectroscopy) and dynamics• Nucleon: an ideal laboratory to study strong interaction (QCD) • Nucleon = valence quarks (u u d or u d d) + sea + gluons

• Mass, charge, magnetic moment, spin, axial charge, tensor charge • Decomposition of each of the fundamental quantities

Mass: ~1 GeV, but u/d quark mass only a few MeV each! Momentum: quarks carry ~ 50% Spin: ½, quarks contribute ~30% Spin Sum Rule

Orbital Angular Momentum Relations to TMDs and GPDs Tensor charge Lattice QCD

• Quarks and gluon field are in-separable • Multi-parton correlations are important• Transverse dimension is crucial for understanding nucleon structure and QCD, help understanding confinement

• Elastic (Form Factors), Resonances, DIS, Spin, Transverse Spin, TMDs, GPDs

Three Decades of Spin Structure Study• 1980s: EMC (CERN) + early SLAC quark contribution to proton spin is very small = (12+-9+-14)% ! ‘spin crisis’ (Ellis-Jaffe sum rule violated)

• 1990s: SLAC, SMC (CERN), HERMES (DESY) = 20-30% the rest: gluon and quark orbital angular momentum

A+=0 (light-cone) gauge (½) + Lq+ G + Lg=1/2 (Jaffe)

gauge invariant (½) + Lq + JG =1/2 (Ji) New decomposition (X. Chen, et. Al, Wakamatsu, …) What observable directly corresponds to Lz~ bx X py ? Bjorken Sum Rule verified to <10% level

• 2000s: COMPASS (CERN), HERMES, RHIC-Spin, JLab, … : ~ 30%;G probably small, orbital angular momentum probably significant Valence Quark Spin Distributions Sum Rules at low Q2, Higher-Twists Transversity, Transverse-Momentum Dependent Distributions

JLab Spin Experiments

• Results: • Spin in the valence (high-x) region• Spin (g1/g2) Moments: Spin Sum Rules, Spin Polarizabilities• SSA in SIDIS: Transversity, TMDs

• On-going• g2

p at low Q2

• Future: 12 GeV• Inclusive: A1/d2,

• Semi-Inclusive: Transversity, TMDs, Flavor-decomposition

• Reviews: S. Kuhn, J. P. Chen, E. Leader, Prog. Part. Nucl. Phys. 63, 1 (2009)

Valence Quark Spin Structure

A1 at high x and flavor decomposition

Why Are PDFs at High x Important?

• Valence quark dominance: simpler picture

-- direct comparison with nucleon structure models

SU(6) symmetry, broken SU(6), diquark• x 1 region amenable to pQCD analysis

-- hadron helicity conservation?

role of quark orbit angular momentum?

• Clean connection with QCD, via lattice moments (d2)

• Input for search for new physics at high energy collider

-- evolution: high x at low Q2 low x at high Q2

-- small uncertainties amplified

-- example: HERA ‘anomaly’ (1998)

World data for A1 World data for A1

Proton Neutron

JLab E99-117 Precision Measurement of A1

n at Large xSpokespersons: J. P. Chen, Z. Meziani, P. Souder; PhD Student: X. Zheng

• First precision A1n data at high x

• Extracting valence quark spin distributions

• Test our fundamental understanding of valence quark picture

• SU(6) symmetry• Valence quark models• pQCD (with HHC) predictions

• Quark orbital angular momentum• Crucial input for pQCD fit to PDF• PRL 92, 012004 (2004)

• PRC 70, 065207 (2004)

Polarized Quark Distributions

• Combining A1n and A1

p results

• Valence quark dominating at high x

• u quark spin as expected• d quark spin stays negative!

• Disagree with pQCD model calculations assuming HHC (hadron helicity conservation)

• Quark orbital angular momentum

• Consistent with valence quark models and pQCD PDF fits without HHC constraint

Inclusive Hall A and B and Semi-Inclusive Hermes

BBS

BBS+OAM

H. Avakian, S. Brodsky, A. Deur, and F. Yuan, PRL 99, 082001 (2007)

pQCD with Quark Orbital Angular Momentum

Spin-Structure in Resonance Region: E01-012Study Quark-Hadorn Duality

Spokesperson: N. Liyanage, J. P. Chen, S. Choi; PhD Student: P. Solvignon PRL 101, 1825 02 (2008)

A13He (resonance vs DIS)1 resonance vs. pdfs

x Q2 x

A1p at 11 GeV (CLAS12)

Projections for JLab at 11 GeV

A1n at 11 GeV (Hall C/A)

Moments of Spin Structure Functions

Sum Rules, Polarizabilities

First Moment of g1p :1

p

EG1b, arXiv:0802.2232 EG1a, PRL 91, 222002 (2003)Spokespersons: V. Burkert, D. Crabb, G. Dodge,

1p

Total Quark Contribution to Proton Spin (at high Q2)

Twist expansion at intermediate Q2, LQCD, ChPT at low Q2

First Moment of g1n :1

n

E94-010, PRL 92 (2004) 022301 E97-110, preliminaryEG1a, from d-p

1n

1 of p-n

EG1b, PRD 78, 032001 (2008)E94-010 + EG1a: PRL 93 (2004) 212001

Effective Coupling Extracted from Bjorken Sum

s/

A. Deur, V. Burkert, J. P. Chen and W. Korsch PLB 650, 244 (2007) and PLB 665, 349 (2008)

Second Spin Structure Function g2

Burkhardt - Cottingham Sum RuleSpin Polarizabilities

Precision Measurement of g2n(x,Q2): Search for Higher Twist Effects

• Measure higher twist quark-gluon correlations.• Hall A Collaboration, K. Kramer et al., PRL 95, 142002 (2005)

Preliminary results on neutron from E01-012Spokespersons: J. P. Chen, S. Choi, N. Liyanage, plots by P. Solvignon

Burkhardt - Cottingham Sum Rule

P

N

3He

BC = Meas+low_x+Elastic

0<X<1 :Total Integral

very prelim

“low-x”: refers to unmeasured low x part of the integral. Assume Leading Twist Behaviour

Elastic: From well know FFs (<5%)

“Meas”: Measured x-range

Brawn: SLAC E155xRed: Hall C RSS Black: Hall A E94-010Green: Hall A E97-110 (preliminary)Blue: Hall A E01-012 (spokespersons: N. Liyanage, former student, JPC)(preliminary)

0)(1

0 22 dxxgΓ

BC Sum Rule

P

N

3He BC satisfied w/in errors for 3He

BC satisfied w/in errors for Neutron(But just barely in vicinity of Q2=1!)

BC satisfied w/in errors for JLab Proton2.8 violation seen in SLAC data

very prelim

Neutron Spin Polarizabilities LT insensitive to resonance• RB ChPT calculation with resonance for 0 agree with data at Q2=0.1 GeV2 • Significant disagreement between data and both ChPT calculations for LT

• Good agreement with MAID model predictions

0 LT

Q2

Q2

E94-010, PRL 93 (2004) 152301

Spin Polarizabilities Preliminary E97-110 (and Published E94-010)

Spokesperson: J. P. Chen, A. Deur, F. Garibaldi, plots by V. Sulkosky • Significant disagreement between data and both ChPT calculations for LT

• Good agreement with MAID model predictions

0 LT

Q2

Q2

Axial Anomaly and the LT Puzzle

N. Kochelev and Y. Oh; arXiv:1103.4891v1

E08-027 : Proton g2 Structure Function Fundamental spin observable has never been measured at low or moderate Q2

BC Sum Rule : violation suggested for proton at large Q2, but found satisfied for the neutron & 3He.

Spin Polarizability : Major failure (>8 of PT for neutron LT. Need g2 isospin separation to solve.

Hydrogen HyperFine Splitting : Lack of knowledge of g2 at low Q2 is one of the leading uncertainties.

Proton Charge Radius : also one of the leading uncertainties in extraction of <Rp> from H Lamb shift.

BC

Su

m R

ule

Spokespersons: Camsonne, Chen, Crabb, Slifer(contact), 6 PhD students, 3 postdocs

Running until 5/2012

Sp

in P

ola

riza

bili

ty

LT

Single Target-Spin Asymmetries in SIDIS

Transversity/Tensor Charge

Transversity

• Three twist-2 quark distributions:• Momentum distributions: q(x,Q2) = q↑(x) + q↓(x)• Longitudinal spin distributions: Δq(x,Q2) = q↑(x) - q↓(x)• Transversity distributions: δq(x,Q2) = q┴(x) - q┬(x)

• It takes two chiral-odd objects to measure transversity• Semi-inclusive DIS

Chiral-odd distributions function (transversity) Chiral-odd fragmentation function (Collins function)

• TMDs: (without integrating over PT)

• Distribution functions depends on x, k┴ and Q2 : δq, f1T┴ (x,k┴ ,Q2), …

• Fragmentation functions depends on z, p┴ and Q2 : D, H1(x,p┴ ,Q2)• Measured asymmetries depends on x, z, P┴ and Q2 : Collins, Sivers, …

(k┴, p┴ and P┴ are related)

Leading-Twist TMD PDFs

f1 =

f 1T =

SiversSivers

HelicityHelicity

g1 =

h1 =TransversityTransversity

h1 =

Boer-MuldersBoer-Mulders

h1T =

PretzelosityPretzelosity

h1L =

Worm GearWorm Gear(Longi-Tranversity)(Longi-Tranversity)

: Survive trans. Momentum : Survive trans. Momentum integrationintegration

Nucleon Spin

Quark Spin

g1T =

Worm GearWorm GearTrans-Trans-HelicityHelicity

Wpu(x,k

T,r ) Wigner distributions

d2kT

PDFs f1

u(x), .. h1u(x)

GPDs

d2kT drzd3r

TMDs

f1u(x,kT), ..

h1u(x,kT) 3D imaging

6D Dist.

Form FactorsGE(Q2), GM(Q2)

d2rT

dx &Fourier Transformation

1D

Separation of Collins, Sivers and pretzelocity effects through angular dependence

1( , )

sin( ) sin( )

sin(3 )

l lUT h S

h SSiverCollins

Pretzelosi

UT

tyU

sUT h S

h ST

N NA

P N

A

A

N

A

1

1 1

1

1 1

sin( )

sin(3 )

sin( )Co

PretzelosityU

SiversUT

llins

T h S T

h S

UT

UT h S

TU

UT

TA

H

f

A

D

A h H

h

Transverity2011 Franco Bradamante

COMPASS Sivers asymmetry 2010 datax > 0.032 region - comparison with HERMES results

NEW

NEW

Status of Transverse Spin Study • Large single spin asymmetry in pp->X• Collins Asymmetries - sizable for the proton (HERMES and COMPASS) large at high x,- and has opposite sign unfavored Collins fragmentation as large as favored (opposite sign)? - consistent with 0 for the deuteron (COMPASS)• Sivers Asymmetries - non-zero for + from proton (HERMES), new COMPASS data - consistent with zero for - from proton and for all channels from deuteron - large for K+ ?• Collins Fragmentation from Belle• Global Fits/models: Anselmino, Prokudin et al., Vogelsang/Yuan et al.,

Pasquini et al., Ma et al., …• Very active theoretical and experimental efforts RHIC-spin, JLab (6 GeV and 12 GeV), Belle, FAIR, J-PARC, EIC, …• First neutron measurement from Hall A 6 GeV (E06-010)• Solenoid with polarized 3He at JLab 12 GeV Unprecedented precision with high luminosity and large acceptance

E06-010 3He Target Single-Spin Asymmetry in SIDISSpokespersons: J. P. Chen, E. Cisbani, H. Gao, X. Jiang, J-C. Peng, 7 PhD students

3He Sivers SSA:negative for π+,

3He Collins SSA small Non-zero at highest x for +

Blue band: model (fitting) uncertainties Red band: other systematic uncertainties

X. Qian, et al. PRL (2011) 107:072003 (2011)

Results on Neutron

Collinsasymmetries are not large, except at x=0.34

Sivers negative

Blue band: model (fitting) uncertainties Red band: other systematic uncertainties

Asymmetry ALT Result

• 3He ALT

Positive for -

hq

qTLT DgFA shsh

11)cos()cos(

LT

To leading twist:

Preliminary

Asymmetry ALT Result

• 3He ALT : Positive for -

hq

qTLT DgFA shsh

11)cos()cos(

LT

To leading twist:

Preliminary

J. Huang et al., PRL

• – Corrected for proton dilution, fp

– Predicted proton asymmetry contribution < 1.5% (π+), 0.6% (π-)

•

– Dominated by L=0 (S) and L=1 (P) interference

• Consist w/ model in signs, suggest larger asymmetry

Neutron ALT Extraction

Preliminary

hq

qT

n DgA 11LT Trans-helictiy

JLab 12 GeV Era: Precision Study of TMDs

• From exploration to precision study with 12 GeV JLab• Transversity: fundamental PDFs, tensor charge• TMDs: 3-d momentum structure of the nucleon Quark orbital angular momentum• Multi-dimensional mapping of TMDs

• 4-d (x,z,P┴,Q2)

• Multi-facilities, global effort

• Precision high statistics• high luminosity and large acceptance

GEMs

(study done with CDF magnet, 1.5T)

41

12 GeV: Mapping of Collins/Siver Asymmetries with SoLID

• Both + and -

• For one z bin

(0.4-0.45)

• Will obtain many z bins (0.3-0.7)

• Tensor charge

E12-10-006 3He(n), Spokespersons: J. P. Chen, H. Gao, X. Jiang, J-C. Peng, X. QianE12-11-007(p) , Spokespersons: K. Allda, J. P. Chen, H. Gao, X. Li, Z-E. Mezinai

Map Collins and Sivers asymmetries in 4-D (x, z, Q2, PT)

Expected Improvement: Sivers Function

• Significant Improvement in the valence quark (high-x) region• Illustrated in a model fit (from A. Prokudin)

f 1T =

E12-11-107: Worm-gear functions (“A’ rating: )

Spokespersons: Chen/Huang/Qiang/Yan

• Dominated by real part of interference between L=0 (S) and L=1 (P) states

• No GPD correspondence• Lattice QCD -> Dipole Shift in mom. space.

• Model Calculations -> h1L =? -g1T

.

h1L =

g1T =

Longi-transversityTrans-helicity

Cent

er o

f poi

nts:

)()(~ 11 zDxgA TLT )()(~ 11 zHxhA LUL

Discussion• Unprecedented precision 4-d mapping of SSA

• Collins and Sivers• +, - and K+, K-

• New proposal polarized proton with SoLID• Study factorization with x and z-dependences • Study PT dependence• Combining with the world data

• extract transversity and fragmentation functions for both u and d quarks• determine tensor charge• study TMDs for both valence and sea quarks • study quark orbital angular momentum• study Q2 evolution

• Global efforts (experimentalists and theorists), global analysis• much better understanding of multi-d nucleon structure and QCD

• Longer-term future: EIC to map sea and gluon SSAs

Summary

• Nucleon (spin) Structure provides valuable inf on QCD dynamics• A decade of experiments from JLab: exciting results

• valence spin structure , duality• spin sum rules and polarizabilities• precision measurements of g2: high-twist • first neutron transverse spin results: Collins/Sivers/ALT

• Bright future• 12 GeV Upgrade will greatly enhance our capability

• Precision determination of the valence quark spin structureflavor separation

• Precision extraction of transversity/tensor charge/ TMDs