challenges of the standard model and the nucleon spin puzzle thomas jefferson national accelerator...
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Challenges of the Standard Model and the Nucleon Spin Puzzle
Thomas Jefferson National Accelerator Facility (JLab)
Recent Results from JLab Spin Program
Summary and Outlook
Xiaochao ZhengUniv. of Virginia
October 17, 2009
Selected Results from the Nucleon Spin Program at Jefferson Lab
SU(2)L X
U(1)Y
SU(3)
C
Standard Model of Particle Physics
Success of the Standard Model in the strong interaction sector
QCD tested in the high energy (perturbative, = “weak”) region
Major Challenges within the Standard Model
Understand and test QCD in extreme conditions (RHIC, LHC)
Understand and test QCD in “strong” interaction region (non-perturbative)
Understand the nucleon structure, how quarks and gluons form the nucleon's mass, momentum, and spin
Standard Model of Particle Physics
energy (GeV) (~1/distance)
S
Success of the Standard Model in the strong interaction sector
QCD tested in the high energy (perturbative, = “weak”) region
Major Challenges within the Standard Model
Understand and test QCD in extreme conditions (RHIC, LHC)
Understand and test QCD in “strong” interaction region (non-perturbative)
Understand the nucleon structure, how quarks and gluons form the nucleon's mass, momentum, and spin
Standard Model of Particle Physics
Three Decades of Spin Structure Study• 1980s: EMC (CERN) + early SLAC quark contribution to proton spin is very small % ! ‘spin crisis’ (Ellis-Jaffe sum rule violated)• 1990s: SLAC, SMC (CERN), HERMES (DESY) % the rest: quark orbital angular momentum and gluons
Different decompositions: Jaffe, Ji, X. Chen et al. Bjorken Sum Rule verified to <10% level • 2000s: COMPASS (CERN), HERMES, RHIC-Spin, JLab, … : ~ 30%;G probably small, quark orbital angular
momentum probably significant Test of various Sum Rules Transversity, Transverse-Momentum Dependent
Distributions Generalized Parton Distributions
=12± 9± 14
= 20~ 30
Facilities Accelerator Beam Time
FermiLab Tevatron 1.96 TeV low 1995 - ... ...
SLAC Stanford Linear Accelerator 50 GeV, 80% 1962 - ... ... 0.03%
J Lab “CW”
CERN Large e-/e+ Collider (LEP) 90-209 GeV low 1989-2000
DESY Deutsches Elektronen Synchrotron 27.5 GeV low
MAINZ Mainz Microtron MAMI 0.8/1.6 GeV 1979 - ... ... “CW”
MIT Bates MIT Bates Linear Accelerator 0.8 GeV 1975-2005
Energy,
polarizationLuminosity
(cm-2 s-1)
duty
factor
1036
Continuous Electron Beam
Accelerator Facility (CEBAF)
6 GeV, 85%
12 GeV, 85% 1038-39 1985 - ... ...
2015 - ... ...
1987 - ... ...
(DESY-II)
1038
1037
Medium & High Energy Physics Facilities for Lepton Scattering
e−.
e−.
e−. ,e.
,
e−. ,e.
,
e−. ,e.
High luminosity, and “continuous” polarized beam makes JLab an unique facility.
~ns: “continuous”>>ns: “pulsed”
Employment: ~650
User community: ~1200
Three Experimental HallsHall A:
pair of high resolution spectrometers (HRS), E' up to 4 GeV/c, = 7 msrluminosity up to 1039 cm-2 s-1
Hall C:High Momentum (HMS) and Short-Orbit Spectrometers (SOS)luminosity up to 1039 cm-2 s-1
Hall B:CEBAF Large Acceptance Spectrometer (CLAS) luminosity up to 1034 cm-2 s-1
Hall A polarized 3He target
longitudinal, transverse and verticalLuminosity=1036 (cm-
2s-1) (highest polarized luminosity in the world)High in-beam polarization > 65%Effective polarized neutron target
13 completed experiments 6 approved with 12 GeV (A/C)
15 uA
Hall B/C Polarized proton/deuteron
targets
• Polarized NH3/ND3 targets
• Dynamical Nuclear Polarization
• In-beam average polarization
70-90% for p
30-40% for d
• Luminosity up to
~ 1035 cm-2s-1(Hall C)
~ 1034 cm-2s-1(Hall B)
JLab Spin Experiments • Results:
– Spin in the valence (high-x) region– Quark-Hadron duality– Moments: Spin Sum Rules and Polarizabilities– Higher twists: g2/d2
• Just completed:– GDH on the proton at very low Q2;– Transversity (n)
• Planned at 6 GeV– g2
p at low Q2
• Future: 12 GeV– Inclusive: A1/d2, – Semi-Inclusive: Transversity, TMDs, Flavor-
decomposition
Review: Kuhn, Chen, Leader, arXiv:0812.3535, PPNP 63 (2009) 1
Longitudinal Spin (I)
Spin in Valence (high-x) Region
(where we can test pQCD and models such as RCQM)
Valence (high-x) A1p and A1
n results
Hall B CLAS, Phys.Lett. B641, 11 (2006)
Hall A E99-117, PRL 92, 012004 (2004) PRC 70, 065207 (2004)
pQCD with HHC
RCQMRCQM
RCQMRCQM
Data for q/q Before JLab With E99117
Data
pQCD with HHCpQCD with HHC
(HERMES data at large x not shown)
Inclusive Hall A and B and Semi-Inclusive HermesBBS(pQCDw/ HHC)
BBS+OAM
PRL99, 082001 (2007)
pQCD with Quark Orbital Angular Momentum
A1p at 11 GeV
Projections for JLab at 11 GeV
Quark Hadron Duality in Spin Structure Function
<resonances> = <DIS>?
Longitudinal Spin (II)
Duality in Spin-Structure: CLAS EG1b Results
Phys.Rev.C75:035203,2007
A13He (resonance vs DIS)
Duality in Spin-Structure: Hall A E01-012 Results
PRL 101, 182502 (2008)
1 resonance comparison with pdfs
integrated over resonances covered by the data, from the pion threshold to an xmin corresponding to W=1.905 GeV
Parton Distributions (CTEQ6 and DSSV)
DSSV, PRL101, 072001 (2008)
CTEQ6, JHEP 0207, 012 (2002)
Polarized PDFsUnpolarized PDFs
Moments of Spin Structure Functions
Sum Rules
Global Property
Spin Sum Rules for First Moments
Bjørken Sum Rule
gA: axial vector coupling constant from neutron -decay
CNS: Q2-dependent QCD corrections (for flavor non-singlet)
•A fundamental relation relating an integration of spin structure functions to axial-vector coupling constant
•Based on Operator Product Expansion within QCD or Current Algebra, plus isospin invariance.
• Valid at large Q2 (where higher-twist effects negligible)
•Data are consistent with the Bjørken Sum Rule at 5-10% level
1pQ2−1
n Q2=∫ [ g1px ,Q
2−g 1nx ,Q
2 ] d x=16
g AC NS
Gerasimov-Drell-Hearn Sum RuleCircularly polarized photon on longitudinally polarized
nucleon
• A fundamental relation between the nucleon spin structure and its anomalous magnetic moment
• Based on general physics principles • Lorentz invariance, gauge invariance low energy theorem• unitarity optical theorem• causality unsubtracted dispersion relation
applied to forward Compton amplitude
• First measurement on proton up to 800 MeV (Mainz) and up to 3 GeV (Bonn) agree with GDH with assumptions for contributions from un-measured regions. New measurements from LEGS provided complimentary results on the proton, more precise results on the deuteron.
∫ 0
∞ 1/ 2 − 3 / 2
d
=−2 2 EM
M 2 2
Generalized GDH Sum Rule
Many approaches: Anselmino, Ioffe, Burkert, Drechsel, …
Ji and Osborne (J. Phys. G27, 127, 2001):Forward Virtual-Virtual Compton Scattering Amplitudes: S1(Q2,), S2(Q2,)
Same assumptions: no-subtraction dispersion relation
optical theorem (low energy theorem)
S1Q2=4∫el
∞ G1Q2, d
Connecting GDH and Bjorken Sum Rules
• Q2-evolution of GDH Sum Rule provides a bridge linking strong QCD to pQCD• Bjorken and GDH sum rules are two limiting cases
High Q2, Operator Product Expansion : S1(p-n) ~ gA Bjorken
Q2~0, Low Energy Theorem: S1 ~ 2 GDH
• High Q2 (> ~1 GeV2): Operator Product Expansion• Intermediate Q2 region: Lattice QCD calculations• Low Q2 region (< ~0.1 GeV2): Chiral Perturbation
TheoryCalculations: HBPT: Ji, Kao, Osborne, Spitzenberg, Vanderhaeghen
RBPT: Bernard, Hemmert, Meissner
Reviews: Theory: Drechsel, Pasquini, Vanderhaeghen, Phys. Rep. 378,99 (2003)
Experiments: Chen, Deur, Meziani, Mod. Phy. Lett. A 20, 2745 (2005)
JLab E94-010 (Hall A)
Neutron spin structure moments and sum rules
• Q2 evolution of neutron spin structure moments (sum rules) with pol.3He
• transition from quark-gluon to hadron
• Test PT calculations
• Results published in several PRL/PLB’s
GDH integral on neutron
PRL 89 (2002) 242301 Q2
EG1b, PLB672, 12 (2009) EG1a, PRL 91, 222002 (2003)
1p
JLab CLAS Eg1a/Eg1b (Hall B)
Proton spin structure moments and sum rules
E94-010, from 3He, PRL 92, 022301(2004) E97-110, from 3He, preliminaryEG1a, EG1b: from d-p
very low Q2!
1n
Test fundamental understandingTest PT at very low Q2
GDH Sum and Spin Structure Function Moments at very low Q2
JLab CLAS EG4 (Hall B) Proton and deuteron spin structure moments
and sum rules at very Low Q2
Expected statistical accuracy from EG4
• Ran in 2006• Data being analyzed
1 of p-n – Bjorken Sum
EG1b, PRD 78, 032001 (2008)E94-010 + EG1a: PRL 93 (2004) 212001
agree well with PT
pQCD w/o HT corrections agree with data surprisingly well down to Q2=1 GeV2.
Effective Strong Coupling Constant
A new attempt at low Q2
Experimental Extraction of S from
Bjorken Sum
s (Q) is well defined in pQCD
at large Q2. Can be extracted from data (e.g. Bjorken Sum Rule).
Not well defined at low Q2, diverges at QCD
The strong coupling constant from pQCD
∫ g 1p−g 1
n dx=1p−n=
g A
6 1− s
3.58
s
2
Generalized Bjorken sum rule:
Definition of effective QCD couplingsPLB B95 70 (1980); PRD 29 2315 (1984); PRD 40 680(1989).
Prescription: Define effective couplings from a perturbative series truncated to the first term in
s.
Use to define an effective s
g1.
Process dependent. But can be related through “Commensurate scale relations”
S.J. Brodsky & H.J Lu, PRD 51 3652 (1995)S.J. Brodsky, G.T. Gabadadze, A.L. Kataev, H.J Lu, PLB 372 133 (1996)
Extend it to low Q2 down to 0: include all higher twists.
∫ g1p− g1
n dx= 1p−n=
g A
6 1− s
3.58
s
2 HigherTwists
1p−n=
g A
6 1 − s
g1
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)
first attempt of effective S extraction
at low Q2
no strong Q2 dependence of strong force at large distances
“Comparison” with theory ↔
Fisher et al.Bloch et al.Maris-TandyBhagwat et al. CornwallGodfrey-Isgur: Constituant Quark ModelFurui & Nakajima: Latticede Teramond et al:
AdS/CFT (preliminary)
Schwinger-Dyson
the conformality (no Q2 dependence) may imply that it's possible to use AdS/CFT correspondance to calculate strong interaction at low Q2.
Transverse Spin (I): Inclusive
g2 Structure Function and Moments
Burkhardt - Cottingham Sum Rule
g2: twist-3, q-g correlations
Experiments: transversely polarized targetSLAC E155x, (p/d)
JLab Hall A (n), Hall C (p/d)
g2 leading twist related to g1 by Wandzura-Wilczek relation
g2-g2WW: a clean way to access twist-3
contribution, quantify q-g correlations.
g 2 x , Q 2= g 2WW x , Q 2 g 2 x , Q 2
g 2WW x ,Q
2=−g1x ,Q2∫x
1g1 y ,Q
2dyy
Precision Measurement of g2n(x,Q2):
Search for Higher Twist Effects
• Measure higher twist, study quark-gluon correlations.
PRL 95, 142002 (2005)
BC Sum Rule BC Sum Rule
P
N
3HeBC = Meas+low_x+Elastic
0<X<1 :Total Integral
“low-x”: unmeasured low x part of the integral. Assume Leading Twist behaviour
Elastic: From well known form factors (<5%)
“Meas”: Measured x-range
Brown: SLAC E155xRed: Hall C RSS Black: Hall A E94-010Green: Hall A E97-110 (preliminary)Blue: Hall A E01-012 (preliminary)
very preliminary
2 Q 2 =∫0
2g 2 x , Q 2 dx=0
BC Sum Rule BC Sum Rule
P
N
3He BC satisfied w/in errors for 3He
BC satisfied w/in errors for Neutron (though just barely in vicinity of Q2=1)
BC satisfied w/in errors for JLab Proton, 2.8 violation seen in SLAC data
very preliminary
Spin Polarizabilities
Higher Moments of Spin Structure Functions at Low Q2
Higher Moments: Generalized Spin Polarizabilities
(how nucleons respond to virtual photons)• generalized forward spin polarizability 0
0Q2= 1
2 2 ∫ 0
∞ ,Q 2
TT ,Q2
3 d
0 Q2 0Q
2 0Q2= 16 M 2
Q 6 ∫0
x0
x2[ g1 x ,Q 2− 4 M 2 x2
Q 2 g 2 x ,Q 2] d x
0Q2≡ L T Q
2= 122 ∫ 0
∞ ,Q2
LT ,Q 2
Q 2 d
=16 M 2
Q6 ∫0
x0 [ g1 x ,Q 2g 2 x ,Q 2 ] d x
• generalized longitudinal-transverse spin polarizability LT
Neutron Spin Polarizabilities LT insensitive to resonance• Significant disagreement between data and both
PT calculations for LT
• Good agreement with MAID model predictions
0 LT
E94-010, PRL 93 (2004) 152301
Proton Spin Polarizability
• Only longitudinal data, model for transverse (g2)
• 0 sensitive to resonance
• Large discrepancies with PT!
0p 0
p Q6
PLB672, 12 (2009)
Summary of Comparison with PT
Results on GDH sum, 1p, 1
n, 1p-n in general agree
well with at least one of the PT calculations;
LT puzzle:
LT not sensitive to , one of the best quantities
to test PT,
data disagree with all calculations (HBPT, RBPT/) by several hundred %!
A challenge to PT theorists.
Very low Q2 data g1/g2 on n(3He) (E97-110), also g1 on p and D available soon (EG4)
Recently approved: g2 on proton E08-027
Color Polarizabilities and Higher Twists
Higher Moments of Spin Structure Functions at High Q2
d 2= X E2 X B/8
f 2= X E−2 X B /2
∫ g1p− g1
n dx= 1p−n=
g A
6 1− s
3.58
s
2 HigherTwists
1p−n=
g A
6 1− s
3.58
s
2 4
Q2 6
Q4
4
Q2=M 2
9 a24d 22 f 2
Color Polarizabilities and Higher Twists
leading twist(twist 2)
higher twists
leading twist, can be obtained from moments of g
1twist-3, can be obtained from moments of g2
twist-4
X. Zheng, October 17, 2009
BROWN : E155, PLB. 553 (2003) 18BLACK : E94010, PRL. 92 (2004) 022301RED : RSS. PRL 98(2007)132003.Magenta: E99-117, PRC 70(2004)065207
Existing World Data on d2:PROTON
NEUTRON
d 2 Q2 = 3∫0
1x 2 [ g 2 x ,Q 2 − g 2
WW x , Q 2 ] d x
d2(Q2)d2(Q2)
X. Zheng, October 17, 2009
MAID Model
stat only
NEUTRON
Some preliminary data
RED : RSS. (Hall C, NH3,ND3)arXiv:0812.0031
BLUE: E01-012. (Hall A, 3He) preliminaryGREEN: E97-110. (Hall A, 3He) courtesy of V. Sulkosky very preliminary
d2(Q2)d2(Q2)
other ongoing analysis: Hall C “SANE” - for the proton Hall A “d2n”
50
Proton: nucl-ex/0508022
Phys.Lett.B613:148-153,2005
f 2p −n=− 0.18± 0.05−0.05
0.04
Phys.Rev.Lett.93:212001,2004 6
p−n / M 4=0.12± 0.02± 0.01
fit Q2=0.66-10 GeV2,
f 2n=0.033± 0.043
6n / M 4=−0.019±0.017
En =0.033±0.029
Bn =−0.001±0.016
Bp=0.06±0.08−0.04
0.05
Ep =−0.08±0.02−0.08
0.07f 2p=−0.160±0.027−0.106
0.111
4p /M 2=−0.064±0.012−0.047
0.049
Neutron
Proton-Neutron
Color Polarizabilities and Higher Twists
fit Q2=0.6-10 GeV2,
For both proton and neutron, the value indicates the 4 term roughly cancel with 6 term, i.e. the total higher twist effect is small, down to Q2=1 GeV2.EG1b result in preparation, higher precision data are expected.
CHL-2CHL-2
Upgrade magnets Upgrade magnets and power suppliesand power supplies
Enhance equipment in Enhance equipment in existing hallsexisting halls
6 GeV CEBAF1112Add new hallAdd new hall
Solenoid spectrometer for SIDIS at 11 GeV
GEMs
Proposed for PVDIS at 11 GeV
Polarized 3He Target Performance
figure credit: C. Dutta
Several Target Groups: JLab, UVa, W&M, Temple, Kentucky, UNH, ...
Polarized 3He Target Performance
Summary• Spin structure study full of surprises and puzzles• A decade of experiments from JLab: exciting
results• valence spin structure, quark-hadron duality• spin sum rules, polarizabilities, and extraction of
effective S• test PT calculations, ‘LT puzzle’• precision measurements of g2/d2: higher twists• first quasi-elastic target SSA: 2-photon to probe GPDs• JLab plays a major role in recent experimental
efforts • shed light on our understanding of strong + QCD• Bright future• complete a chapter in spin structure study with 6
GeV• 12 GeV Upgrade will greatly enhance our capability• Goal: a full understanding of nucleon structure and
strong interaction