Single-Spin Asymmetries at CLAS

Transverse momentum of quarks and spin-azimuthal asymmetries Target single-spin asymmetries Beam single-spin asymmetries in

semi-inclusive 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 ...Transverse-momentum 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 kT-dependent PDFs (TMDs)

Spin asymmetries + azimuthal dependence new class of DIS measurements Single-Spin 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

Twist-2 - dependent PDFsby Mulders et al.

Tk

Tk opens access to mixed hadron-parton and parton-hadron transitions

TMDs appear in sin and sin2 SSA In polarized semi-inclusive DIS

H. Avakian CIPANP 2003 May21 4

Polarized Semi-Inclusive DIS

Cross section defined by scale variables x,y,z

Two fundamental QCD mecha- nisms (Collins and Sivers) identified, to generate SSA:

Hadron-Parton/ Parton-Hadron 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 spin-dependent 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 z-dependence 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: x-dependenceCLAS (5.7 GeV) is consistent with predictions for Collins effect from Efremov et al. hep-ph/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 (Kotzinian-Mulders 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 (SPIN-2002) at 27.5 GeV

Beam SSA measurements fordifferent beam energies areconsistent.

H. Avakian CIPANP 2003 May21 13

ALU x-dependence: 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

Light-cone 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

LUND-MC

-t < 0.5GeV2

H. Avakian CIPANP 2003 May21 16

Beam SSA ALU: x,t-dependence

t=(PN-PN’)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

Twist-2 - dependent PDFsby Mulders et al.

Tk

TMDs appear in sin and sin2 SSA In polarized semi-inclusive DIS