Transcript Experimental Study of Nucleon Structure and QCD J. P. Chen, Jefferson Lab Workshop on Confinement Physics, March 12, 2012  Introduction  Selected JLab.

Experimental Study of Nucleon Structure and QCD
J. P. Chen, Jefferson Lab
Workshop 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
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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
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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
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1980s: EMC (CERN) + early SLAC
quark contribution to proton spin is very small
DS = (12+-9+-14)% !
‘spin crisis’
(Ellis-Jaffe sum rule violated)
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1990s: SLAC, SMC (CERN), HERMES (DESY)
DS = 20-30%
the rest: gluon and quark orbital angular momentum
A+=0 (light-cone) gauge
(½)DS + Lq+ DG + Lg=1/2
gauge invariant
(½)DS + Lq + JG =1/2
New decomposition (X. Chen, et. Al, Wakamatsu, …)
What observable directly corresponds to Lz~ bx X py ?
Bjorken Sum Rule verified to <10% level
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(Jaffe)
(Ji)
2000s: COMPASS (CERN), HERMES, RHIC-Spin, JLab, … :
DS ~ 30%; DG 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
• g2p at low Q2
• Future: 12 GeV
• Inclusive: A1/d2,
• Semi-Inclusive: Transversity, TMDs, Flavor-decomposition
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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
Proton
Neutron
JLab E99-117
Precision Measurement of A1n at Large x
Spokespersons: J. P. Chen, Z. Meziani, P. Souder; PhD Student: X. Zheng
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First precision A1n data at high x
Extracting valence quark spin
distributions
Test our fundamental understanding
of valence quark picture
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SU(6) symmetry
Valence quark models
pQCD (with HHC) predictions
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Quark orbital angular momentum
Crucial input for pQCD fit to PDF
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PRL 92, 012004 (2004)
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PRC 70, 065207 (2004)
Polarized Quark Distributions
• Combining A1n and A1p 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
pQCD with Quark Orbital Angular Momentum
H. Avakian, S. Brodsky, A. Deur, and F. Yuan, PRL 99, 082001 (2007)
Inclusive Hall A and B and Semi-Inclusive Hermes
BBS
BBS+OAM
Spin-Structure in Resonance Region: E01-012
Study Quark-Hadorn Duality
Spokesperson: N. Liyanage, J. P. Chen, S. Choi; PhD Student: P. Solvignon
PRL 101, 1825 02 (2008)
G1 resonance vs. pdfs
x Q2
A13He (resonance vs DIS)
x
Projections for JLab at 11 GeV
A1n
at 11 GeV
(Hall C/A)
A1p at 11 GeV
(CLAS12)
Moments of Spin Structure Functions
Sum Rules, Polarizabilities
First Moment of g1p : G1p
Total Quark Contribution to Proton Spin (at high Q2)
Twist expansion at intermediate Q2, LQCD, ChPT at low Q2
G 1p
EG1b, arXiv:0802.2232
EG1a, PRL 91, 222002 (2003)
Spokespersons: V. Burkert,
D. Crabb, G. Dodge,
First Moment of g1n : G1n
G 1n
E94-010, PRL 92 (2004) 022301
E97-110, preliminary
EG1a, from d-p
G1 of p-n
EG1b, PRD 78, 032001 (2008)
E94-010 + EG1a: PRL 93 (2004) 212001
Effective Coupling Extracted from Bjorken Sum
A. Deur, V. Burkert, J. P. Chen and W. Korsch
PLB 650, 244 (2007) and PLB 665, 349 (2008)
as/p
Second Spin Structure Function g2
Burkhardt - Cottingham Sum Rule
Spin 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 E01-012
Spokespersons: J. P. Chen, S. Choi, N. Liyanage, plots by P. Solvignon
Burkhardt - Cottingham Sum Rule
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0<X<1 :Total Integral
P
N
Γ 2   g 2 ( x)dx  0
0
Brawn: SLAC E155x
Red: Hall C RSS
Black: Hall A E94-010
Green: Hall A E97-110 (preliminary)
Blue: Hall A E01-012 (spokespersons:
N. Liyanage, former student, JPC)
(preliminary)
BC = Meas+low_x+Elastic
“Meas”: Measured x-range
3He
“low-x”: refers to unmeasured low x part
of the integral.
Assume Leading Twist Behaviour
Elastic: From well know FFs (<5%)
BC Sum Rule
P
BC satisfied w/in errors for JLab Proton
2.8 violation seen in SLAC data
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BC satisfied w/in errors for Neutron
(But just barely in vicinity of Q2=1!)
3He
BC satisfied w/in errors for 3He
Neutron Spin Polarizabilities
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dLT insensitive to D resonance
RB ChPT calculation with resonance for g0 agree with data at Q2=0.1 GeV2
Significant disagreement between data and both ChPT calculations for dLT
Good agreement with MAID model predictions
g0
E94-010, PRL 93 (2004) 152301
Q2
dLT
Q2
Spin Polarizabilities
Preliminary E97-110 (and Published E94-010)
Spokesperson: J. P. Chen, A. Deur, F. Garibaldi, plots by V. Sulkosky
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Significant disagreement between data and both ChPT calculations for dLT
Good agreement with MAID model predictions
g0
dLT
Q2
Q2
Axial Anomaly and the dLT 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
Spokespersons: Camsonne, Chen, Crabb, Slifer(contact), 6 PhD students, 3 postdocs
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 dLT. 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 <R p> from m-H Lamb shift.
Spin Polarizability dLT
BC Sum Rule
Running until 5/2012
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)
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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
Quark polarization
Unpolarized
(U)
Nucleon Polarization
U
Longitudinally
Polarized (L)
Transversely Polarized
(T)
h1  =
f1 =
Boer-Mulders
h1L =
g1 =
L
Helicity
Worm Gear
(Longi-Tranversity)
h1 =
T
f 1T =
Transversity
g1T =
Sivers
Nucleon Spin
Quark Spin
Worm Gear
Trans-Helicity
h1T =
Pretzelosity
: Survive trans. Momentum integration
6D Dist.
Wpu(x,kT,r ) Wigner distributions
d2kT drz
d3r
GPDs
TMDs f1u(x,kT),
.. h1u(x,kT)
d2k
3D imaging
T
PDFs
f1u(x), .. h1u(x)
dx &
Fourier Transformation
d 2 rT
1D
Form
Factors
GE(Q2),
GM(Q2)
Separation of Collins, Sivers and pretzelocity effects
through angular dependence
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
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N
N
AUT (hl ,  Sl ) 
P N  N
Collins
Sivers
 AUT
sin(h  S )  AUT
sin(h - S )
ty
 AUPretzelosi
sin(3h - S )
T
Collins
UT
A
Sivers
UT
A
 sin(h  S )
 sin(h - S )
Pretzelosity
UT
A
UT
UT
 h1  H

1T
 f
 sin(3h - S )
UT
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 D1

1T
h H
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COMPASS Sivers asymmetry 2010 data
x > 0.032 region
- comparison with HERMES results
NEW
NEW
Transverity2011
Franco Bradamante
Status of Transverse Spin Study
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Large single spin asymmetry in pp->pX
Collins Asymmetries
- sizable for the proton (HERMES and COMPASS)
large at high x, p- and p has opposite sign
unfavored Collins fragmentation as large as favored (opposite sign)?
- consistent with 0 for the deuteron (COMPASS)
Sivers Asymmetries
- non-zero for p+ from proton (HERMES), new COMPASS data
- consistent with zero for p- 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 SIDIS
Spokespersons: J. P. Chen, E. Cisbani, H. Gao, X. Jiang, J-C. Peng, 7 PhD students
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He-(e, e'h), h = p +, p -
X. Qian, et al.
PRL (2011) 107:072003 (2011)
3He
Collins SSA small
Non-zero at highest x for p+
3He
Sivers SSA:
negative for π+,
Blue band: model (fitting) uncertainties
Red band: other systematic uncertainties
Results on Neutron
Collins
asymmetries are not
large, except at x=0.34
Sivers
p + (ud ) negative
Blue band: model (fitting) uncertainties
Red band: other systematic uncertainties
Asymmetry ALT Result
To leading twist:
cos( h - s )
ALT
 FLTcos(h -s )  g1qT  D1hq
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3He
ALT
Positive for p-
Preliminary
Asymmetry ALT Result
J. Huang et al., PRL
To leading twist:
cos( h - s )
ALT
 FLTcos(h -s )  g1qT  D1hq
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3He
ALT : Positive for p-
Preliminary
Neutron ALT Extraction
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– Corrected for proton dilution, fp
– Predicted proton asymmetry contribution < 1.5% (π+), 0.6% (π-)
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n
ALT
 g1qT  D1hq
Trans-helictiy
– Dominated by L=0 (S) and L=1 (P) interference
• Consist w/ model in signs, suggest larger asymmetry
Preliminary
JLab 12 GeV Era: Precision Study of TMDs
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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
(study done with CDF magnet, 1.5T)
GEMs
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12 GeV: Mapping of Collins/Siver Asymmetries with SoLID
E12-10-006 3He(n), Spokespersons: J. P. Chen, H. Gao, X. Jiang, J-C. Peng, X. Qian
E12-11-007(p) , Spokespersons: K. Allda, J. P. Chen, H. Gao, X. Li, Z-E. Mezinai
• Both p+ and p• For one z bin
(0.4-0.45)
• Will obtain
many z bins
(0.3-0.7)
• Tensor charge
Map Collins and Sivers asymmetries in 4-D (x, z, Q2, PT)
Expected Improvement: Sivers Function
f 1T =
• Significant Improvement in the valence quark (high-x) region
• Illustrated in a model fit (from A. Prokudin)
E12-11-107: Worm-gear functions (“A’ rating: )
Spokespersons: Chen/Huang/Qiang/Yan
g1T =
Longi-transversity
Trans-helicity
Center of points:
• 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 =
ALT ~ g1T ( x) D1 ( z)
AUL ~ h1L ( x) H 1 ( z)
Discussion
• Unprecedented precision 4-d mapping of SSA
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Collins and Sivers
p+, p- and K+, K-
New proposal polarized proton with SoLID
Study factorization with x and z-dependences
Study PT dependence
Combining with the world data
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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
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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 structure
flavor separation
• Precision extraction of transversity/tensor charge/ TMDs