Transversely polarized target for CLAS and CLAS12

1
Transversely polarized target for CLAS and CLAS12
Harut Avakian
CLAS Collaboration Meeting March 1

• Introduction
• Structure of nucleon and 3D parton distributions
• Semi-Inclusive processes and TMD distributions
• Hard exclusive processes and GPDs
• Projections
• Summary

2
Physics Motivation
Describe the complex nucleon structure in terms
of partonic degrees of freedom of QCD
QCD evolution tells us how parton distributions
evolve, but not original distributions
J q
Understanding of the orbital motion of quarks and
spin-orbit correlations is crucial!!!
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Quantum Phase-Space Distributions of Quarks
Probability to find a quark u in a nucleon P with
a certain polarization in a position r and
momentum k
Wpu(k,rT) Mother Wigner distributions
d2r
d2kT
GPDs/IPDs Wpu(x,rT),
TMD PDFs fpu(x,kT),
d2r
d2kT
dx (FT)
Measure momentum transfer to quark Direct info
about momentum distributions No information about
spatial location of partons
Measure momentum transfer to target Direct info
about spatial distributions No information about
underlying dynamics
Form Factors F1pu(t),F2pu(t )..
PDFs fpu(x),
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Hard Processes
proton
SIDIS/DER
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Single particle production in hard scattering
xFgt0 (current fragmentation)
h
xFlt0 (target fragmentation)
xF - momentum in the CM frame
Target fragmentation
Current fragmentation
h
h
M
GPD
1
-1
0
xF
Fracture Functions
kT-dependent PDFs
Generalized PDFs
Wide kinematic coverage of large acceptance
detectors allows studies of hadronization both in
the target and current fragmentation regions
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Semi-Inclusive DIS
Parton-Hadron transition by fragmentation
function Dp(p-) (z) probability for a u-quark
to produce a p(p-) with momentum fraction z
Favored Fragmentation Unfavored
Hard scattering
Hadron-Parton transition by distribution
function qff1u(x) probability to find a u-quark
with a momentum fraction x
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SIDIS kinematical plane and observables
Trento Conventions
Phys.Rev. D70, 117504 (2004).
U unpolarized L long.polarized T trans.polarized
sinf moment of the cross section for unpolarized
beam and transverse target
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Single Spin Asymmetries in

?s20 GeV, pT0.5-2.0 GeV/c

• ??0 E704, PLB261 (1991) 201.
• ??/- - E704, PLB264 (1991) 462.

FermiLab E-704

• Recently, large transverse single-spin effects
  were observed also in pp collisions (RHIC), at
  much higher CM energies.

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The Elephant and the village of the blind
Non-PQCD surface effects (Ma,Boros et al)
SSA
Asymmetry comes from the final state interactions
in fragmentation (J.Collins 1993)
Sivers effect forbidden by time reversal
invariance (Collins 1993)
kT crucial for spin structure studies.
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Mechanisms for SSA
Collins Fragmentation
fragmentation of transversely polarized quarks
into unpolarized hadrons
d
u
u
(favored)
L1
Orbital momentum generated in string breaking
and pair creation produces left-right asymmetry
from transversely polarized quark fragmentation
(Artru-93)

• L/R SSA generated in fragmentation
• Unfavored SSA with opposite sign
• No effect in target fragmenation

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Mechanisms for SSA
Sivers Distribution f1T- unpolarized quarks in
transversely polarized nucleon
T-odd f1T-, requires final state interactions
interference between different helicity states
(Brodsky et al., Collins, Ji et al. 2002)

• L/R SSA generated in distribution
• Hadrons from struck quark have the same sign SSA
• Opposite effect in target fragmentation

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Transverse momentum of quarks
Mulders Tangerman (1995, the TMD bible)
quark polarization

• kT required to describe azimuthal distributions
  of hadrons and in
• particular SSAs.
• kT - important for cross section description
  (also for exclusive production)
• kT leads to 3D description with 8PDFs

proton polarization
Transversity
Off diagonal PDFs related to interference between
states with different orbital momentum

• Transverse target provides access to spin-orbit
  correlations with all possible polarizations of
  quarks.

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Sivers function First measurement

• significantly positive p asymmetry
• requires non-zero orbital angular momentum
• first hint of naïve T-odd DF from DIS

(100 in hep SPIRES citation)
Phys.Rev.Lett.94012002,2005.
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Collins effect First measurements
Soffer bound

• significantly positive p and negative p-
  asymmetries
• unexpected large p-
• ?role of unfavoured (u? p-) fragmentation
  function?

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BELLE Collins function measurements
Asymmetric collider 8GeV e- 3.5GeV e
First direct indication of non-0 Collins
fragmentation !
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Hunting for Lq in hard exclusive processes
- Muller (1994) -
Generalised Parton Distributions
- Ji Radyushkin (1996) -
Different final states selects different
combinations of GPDs
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GPDs and spatial distributions
Transverse position distribution
Transverse position shift
Transversely polarized proton
Unpolarized quark
Shift in the transverse space of quarks in the
transversely polarized proton first predicted in
GPD framework, confirmed by Lattice
(hep-ph/0612032)
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HERMES DVCS with Transverse target and GPD E
AUT sin(f-fS) cos(f) Im H - E ..
L 64 pb-1
? First (model dependent) constraints on Ju and
Jd !
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CLAS Transversely Polarized Target
CLAS6
CLAS12
qlt22.5 degree impact
22.5o
Measurements at different beam energies will
allow study of Q2 dependence for a fixed x in a
wide range.
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Collins Effect
clas12
Study the Collins fragmentation for all 3 pions
with a transversely polarized target and measure
the transversity distribution function.
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Sivers effect
clas12
Requires non-trivial phase from the FSI
interference between different helicity states
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Sivers effect in the target fragmentation
A.Kotzinian
Significant effect predicted in the target
fragmentation region, in particular for baryons
(target remnant also asymmetric)
CLAS12 will allow studies of kinematic
dependences of the Sivers effect in the target
fragmentation region
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CLAS12 - Exclusive Target Asymmetries
E 11 GeV
epg
Transversely polarized target
epr
CLAS12
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Summary
Studies of exclusive and semi-inclusive hard
processes with transverse target at CLAS6 and
CLAS12 related to the spin, spin orbit
correlations and orbital angular momentum (in
particular Sivers function and GPD-E) are crucial
for understanding of the transverse structure of
the nucleon
Few independent proposals for PAC32 Inclusive
DIS Semi-Inclusive DIS Exclusive
processes .. (proton deuteron targets)

• Transverse target design in progress at OXFORD
  (field maps available)
• GSIM studies of scattering of high lumi beam with
  transverse target in progress.

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Support slides..
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PDFs and orbital momentum
BBS (NP B441, 197 (1995) using helicity
structure of perturbative QCD coupling at x?1
Transverse position shift using GPD-E
nminimal number of spectator quarks 2
Good agreement of HERMES (filled) and JLab (open)
data with BBS curves for quarks aligned with
proton spin, where the orbital motion is not as
important.
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Hard Scattering Processes Kinematics Coverage
HERA

• collider experiments
• H1, ZEUS (EIC)
• 10-4ltxBlt0.02 (0.3) gluons (and quarks) in the
  proton
• fixed target experiments
• COMPASS, HERMES
• ? 0.006/0.02ltxBlt0.3 gluons/valence and sea
  quarks
• JLab/JLab_at_12GeV
• ? 0.1ltxBlt0.7 valence quarks

27 GeV
Q2
EIC
JLab (upgraded)
compass
hermes
JLab_at_6GeV
Study of high xB domain requires high luminosity
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Boer-Mulders Asymmetry
Transversely polarized quarks in the unpolarized
nucleon
CLAS12
EIC
Nonperturbative TMD
Perturbative region
CLAS12 and EIC studies of transition from
non-perturbative to perturbative regime will
provide complementary info on spin-orbit
correlations.
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Summary

• CLAS12 a full acceptance, general purpose
  detector for high luminosity electron scattering
  experiments, is essential for high precision
  measurements of GPDs and TMDs in the valence
  region.
• Provide new insight into
• - quark orbital angular momentum contributions
• to the nucleon spin
• - 3D structure of the nucleons interior and
  correlations
• - quark flavor polarization
• EIC will extend studies of 3D nucleon structure,
  to low x and high Q2 , important for all
  processes of interest
• - deeply virtual exclusive processes (DVCS,
  DVMP)
• - semi-inclusive meson production with polarized
  beam
• and polarized targets