Studies of transverse momentum dependent distributions at CLAS

1
Studies of transverse momentum dependent
distributions at CLAS
H. Avakian, L.Elouadrhiri
Jefferson Lab
SPIN 2003

• Polarized semi-inclusive DIS and kT-dependent
  distributions
• Factorization studies
• Spin and Azimuthal Asymmetries
• b) Double spin asymmetry
• c) SSAs with polarized target and beam
• Future measurements
• Summary

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- Dependent Parton Distributions
f1, g1 studied for decades h1 and others
essentially unknown
Twist-2 - dependent PDFs by Mulders et
al.
opens access to mixed hadron-parton and
parton-hadron transitions
Gauge invariant definition of TMDs discussed by
Collins and Belitsky,Ji Yuan Nucl.Phys.
B656 165, 2003
TMDs appear in sinf and sin2f SSA In polarized
semi-inclusive DIS
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Contributions to s in Polarized SIDIS
?e, SL, ST electron and proton long. and trans.
pol. sum over quarks and
anti-quarks.
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Polarized Semi-Inclusive DIS
Cross section defined by scale variables x,y,z
Spin Azimuthal Asymmetries can be extracted as
azimuthal moments of the total cross section.
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Spin-Azimuthal Asymmetries
Spin asymmetries azimuthal dependence
new class of DIS measurements Spin-Azimuthal
Asymmetries
Significant progress made recently in studies of
Single-Spin Azimuthal Asymmetries (SSA) with
longitudinally polarized target (HERMES),
transversely polarized target (SMC), and
polarized beam (CLAS).

• SSA are sensitive to the orbital momentum of
  quarks, enable measurements of
• GPDs and kT-dependend PDFs (TMDs)
• provide a window to the physics of partonic final
  and initial state interactions
• model calculations indicate that SSA are not
  affected significantly by a wide
• range of corrections.
• Good agreement in SSAs measured in a wide energy
  range in electroproduction
• and pp scattering.

SSAs appropriate observable at JLAB beam
energies and Q2
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The CLAS Detector
Scattering of 5.7 GeV polarized electrons off
polarized NH3 and unpolarized hydrogen

• 8M p in DIS kinematics,
• Q2gt1 GeV2, W2gt4 GeV2,
• ylt0.85, 0.8gtzgt0.5, MX2 gt1.1
• beam polarization 73
• target polarization 72 (f0.2)

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LUND-MC vs Polarized CLAS data
DIS kinematics defined Q2gt1 GeV2 W2gt4 ylt0.85 0.5
ltzlt0.8 MX2 gt1.1
x and Q2 distributions from LUND-MC vs 5.7 CLAS
NH3 data.
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Factorization studies in CFR at CLAS
The separation of CFR with 0.5ltzlt0.8 is not
changing significantly with beam energy.
DIS Q2gt1 W2gt4 ylt0.85
No significant variation observed in z
distributions of p for different x ranges
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LUND-MC Dilution in NH3
NH3 dilution factor (f0.21) from data is
consistent with LUND and simple quark counting
(dotted line).
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Polarized target HERMES vs CLAS at 5.7GeV
ep ? e p X ( Ee 5.7 GeV, MX gt 1.1)
x dependence of CLAS A1p (A20) consistent
with HERMES data, world inclusive fit x0.727
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Polarized target x,z factorization studies at
5.7GeV
ep ? e p X ( Ee 5.7 GeV, MX gt 1.1)
No significant variation observed in x
dependence of A1p for different z ranges.
z-independent A1p consistent with factorization!
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Longitudinally Pol Target SSA for p
H1
SSA x-dependence CLAS (5.7 GeV) is consistent
with predictions for Collins effect from
Efremov et al. hep-ph/0208124
-
- Collins FF

• First indication of a non-zero sin2f moment
• (Kotzinian-Mulders asymmetry)
• AULsin2f is a clean source of Collins SSA

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ALU x-dependence CLAS 4.3 vs. 5.7 GeV
Photon Sivers Effect Afanasev Carlson (using
BHS mechanizm)
0.5ltzlt0.8
5.7GeV
Sivers contribution in case of beam SSA has the
same source (FSI) as in case of target SSA,
providing access to essential parameters with
less ambiguities
4.3 GeV
CLAS preliminary
Beam SSA analyzed also in terms of Collins effect
by Efremov et al.
4.3 GeV and 5.7 GeV data consistent
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Factorization studies in CFR at CLAS
In terms of Collins fragmentation
No significant variation observed in z dependence
of ALU for different x ranges
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Long. Pol. Target SSA for p at 12GeV
large x high luminosity
6 GeV

• The sin 2f asymmetry for 2000 h of
  projected CLAS data.

Direct measurement of kT dependent twist-2
distribution function at CLAS
Efremov et al.
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Summary

• Current CLAS data are consistent with a partonic
  picture, and can be described by a variety of
  theoretical models
• No x/z-dependence observed in single and double
  spin asymmetry measurements (consistent with
  factorization).
• Single-Spin asymmetries extracted for SIDIS p
  are in agreement with predictions from ?QSM model
  .
• A non-0 ltsin2fgt, measured for the first time, may
  provide a direct access to Collins fragmentation.

• Global analysis of SSA for polarized beam and
  target needed to separate contributions from
  different mechanisms and extract corresponding
  distribution functions.

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Beam SSA
In similar to polarized target case kinematics,
no sign of significant sin2f term in the data
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MC with Long. Pol. Target SSA for p
D1 unpol. FF
SSA x-dependence from Efremov et
al. Phys.Rev.D67063511,2003 used for Collins
effect simulation
H1
-
- Collins FF
Simulated and reconstructed SSA moments consistent

• sin2f moment (Kotzinian-Mulders asymmetry)
• is a clean source of Collins SSA

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Contributions to s in ep?epX

• s for longitudinally polarized leptons
  scattering off polarized protons

Depolarization function defined as
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Beam SSA sinf Moment

• ALU

Beam SSA ALU from CLAS at 4.3GeV and HERMES
(SPIN-2002) at 27.5 GeV
Beam SSA measurements for different beam energies
are consistent.
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ALU from CLAS 4.3 and 5.7GeV data
Low MX in average correspond to large z
5.7GeV
No significant dependence on beam energy
4.3 GeV
No sign flip in ALU in the exclusive limit
(unlike HERMES target SSA)
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Longitudinally Pol Target Twist-2 SSA
CLAS AULsin2f consistent with ?QSM model
predictions (Efremov et al. Phys.Rev.D67063511,2
003)
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LUND-MC vs Polarized CLAS data
f distributions from MC vs 5.7 CLAS data.