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SingleSpin Asymmetries at CLAS
Transverse momentum of quarks and spinazimuthal asymmetries Target singlespin asymmetries Beam singlespin asymmetries in
semiinclusive pion productionexclusive pion production
Summary
H. Avakian, L.Elouadrhiri (Jefferson Lab)
Conference on Intersections of Particle and Nuclear Physics
For the CLAS Collaboration
H. Avakian CIPANP 2003 May21 2
Parton Distribution Functions generalized to contain information not only on longitudinal, but also on the transverse distribution of
partons: Generalized Parton Distributions (GPD) H, E ...Transversemomentum dependent (TMD) parton distributions
Orbital Angular Momentum and Transverse momentum of quarks
SSA are sensitive to the orbital motion of quarks, enable measurements of GPDs and kTdependent PDFs (TMDs)
Spin asymmetries + azimuthal dependence new class of DIS measurements SingleSpin Asymmetries:
H. Avakian CIPANP 2003 May21 3
 Dependent Parton Distributions
f1, g1 studied for decades: h1 and others essentially unknown
Gauge invariant definition of TMDs discussed by Collins and Belitsky,Ji & Yuan Nucl.Phys. B656 165, 2003
)kx,(fkd)x(f T1T2
1
Tk
Twist2  dependent PDFsby Mulders et al.
Tk
Tk opens access to mixed hadronparton and partonhadron transitions
TMDs appear in sin and sin2 SSA In polarized semiinclusive DIS
H. Avakian CIPANP 2003 May21 4
Polarized SemiInclusive DIS
Cross section defined by scale variables x,y,z
Two fundamental QCD mecha nisms (Collins and Sivers) identified, to generate SSA:
HadronParton/ PartonHadron transitions described by distribution and fragmentation functions.
H. Avakian CIPANP 2003 May21 5
Contributions to σ in ep→e’X
σ for longitudinally polarized leptons scattering off unpolarized protons:
Different structure functions can be extracted asazimuthal moments of the total cross section.
More spindependent azimuthal moments arise for polarized targets
H. Avakian CIPANP 2003 May21 6
The CLAS Detector
Scattering of 5.7 GeV polarized electrons off polarized NH3 and unpolarized hydrogen
~8M + in DIS kinematics, Q2>1 GeV2, W2>4 GeV2, y<0.85, z>0.5 beam polarization 73% target polarization 72% (f≈0.2)
X n
H. Avakian CIPANP 2003 May21 7
Longitudinally Pol. Target: SSA for +
Target SSA: CLAS (4.3 GeV, and 5.7 GeV ) consistent with HERMES (27.5 GeV)
AUL zdependence also
consistent both in magnitude and sign with predictions based on Collins mechanism
predictions for Sivers effect from BHS Phys.Lett.B53099,2002 Sivers only interpretation require large AUT
H. Avakian CIPANP 2003 May21 8
H1
Longitudinally Pol Target: SSA for +
SSA: xdependenceCLAS (5.7 GeV) is consistent with predictions for Collins effect from Efremov et al. hepph/0208124
┬
 Collins FF
D1 – unpol. FF
)()( 12
,
sin zHxxheQ
MS q
Lqqq
LUL
)()( 112
,
sin zDxfeS qqTq
qqTUT
)()( 112
,
sin zHxheS qq
qqTUT
qqLq
qqLUL HxheS
1
)1(1
2
,
2sin )(
•sin2 moment (KotzinianMulders asymmetry) is a clean source of Collins SSA•x 4 more CLAS data available for analysis
P1= 0.0110±0.0024P2=0.0065±0.0025
H. Avakian CIPANP 2003 May21 9
Unpolarized target: x,z factorization studies at 5.7GeV
The z distribution for different ranges of Bjorken x
No significant variation observed in z dependence for
different x ranges.
ep → e’ X ( Ee =5.7 GeV, MX > 1.1)
H. Avakian CIPANP 2003 May21 10
Factorization studies in CFR at CLAS
DIS:Q2>1W2>4y<0.85
The separation of CFR with 0.5<z<0.8 is not changing significantly with beam energy.
No significant variation observed in z dependence of ALU for different x ranges
H. Avakian CIPANP 2003 May21 11
ALU from CLAS 4.3 and 5.7GeV data
4.3 GeV
5.7GeV
No significant dependence on beam energy
No sign flip in ALU in the exclusive limit (unlike HERMES target SSA)
Low MX in average correspond to large z
H. Avakian CIPANP 2003 May21 12
Beam SSA: sin Moment
ALU
Beam SSA ALU from
CLAS at 4.3GeV andHERMES (SPIN2002) at 27.5 GeV
Beam SSA measurements fordifferent beam energies areconsistent.
H. Avakian CIPANP 2003 May21 13
ALU xdependence: CLAS 4.3 vs. 5.7 GeV
Photon Sivers EffectAfanasev & Carlson (preliminary)
0.5<z<0.8
4.3 GeV
5.7GeV
4.3 GeV and 5.7 GeV data consistent
CLAS preliminary
Beam SSA analyzed also in terms of Collins effect by Efremov et al.
H. Avakian CIPANP 2003 May21 14
Photon Sivers Effect
Target SSA (Brodsky et al.)
Beam SSA (Afanasev & Carlson)
Essential constituents
Lightcone spin wave function of a nucleon
Final state interaction from gluon exchange
Beam SSA has the same source and provides access to essentialparameters with less ambiguities
H. Avakian CIPANP 2003 May21 15
Exclusive pions: CLAS 4.3 vs. 5.7GeV
Sign flip at z ~ 0.5
At z>0.5 struck quark in pion
W2>4 GeV2,Q2>1 GeV2
4.3 GeV
5.7 GeV
LUNDMC
t < 0.5GeV2
H. Avakian CIPANP 2003 May21 16
Beam SSA ALU: x,tdependence
t=(PNPN’)2 <0.5GeV2 ,W2>5GeV2, Q2>2.5GeV2 (<Q2>=3.0GeV2>)
CLAS PRELIMINARY CLAS PRELIMINARY
Beam SSA for exclusive pions may provide access to GPDs

H. Avakian CIPANP 2003 May21 17
Summary
• Beam spin asymmetries observed for the first time in exclusive + electroproduction in hard scattering kinematics.
• No significant dependence on x observed for z dependencies of beam SSA.
• Kinematic dependencies of beam and target SSA measured for different beam energies are consistent, and can be described by a variety of theoretical models.
• Combined analysis of SSA for polarized beam and target needed to separate contributions from different mechanisms and extract corresponding distribution functions.
H. Avakian CIPANP 2003 May21 21
 Dependent Parton Distributions
f1, g1 studied for decades: h1 and others essentially unknown
Gauge invariant definition of TMD PDFs discussed by Collins and Belitsky,Ji & Yuan Nucl.Phys. B656 165, 2003
)kx,(fkd)x(f T1T2
1
Tk
Twist2  dependent PDFsby Mulders et al.
Tk
TMDs appear in sin and sin2 SSA In polarized semiinclusive DIS