experimental study of nucleon structure and qcd j. p. chen, jefferson lab workshop on confinement...
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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