Transcript Transversely polarized target for CLAS and CLAS12 Harut Avakian CLAS Collaboration Meeting March 1  Introduction  Structure of nucleon and 3D parton distributions  Semi-Inclusive.

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
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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
EMC at CERN (85):
RHIC Spin & SIDIS
Jq
Understanding of the orbital motion of quarks
and spin-orbit correlations is crucial!!!
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Quantum Phase-Space Distributions of Quarks
Wpu(k,rT) “Mother” Wigner distributions
TMD PDFs fp
u(x,k
Measure momentum
transfer to quark
Direct info about
momentum distributions
No information about
spatial location of partons
T),
Probability to find a quark u in a
nucleon P with a certain
polarization in a position r and
momentum k
GPDs/IPDs Wpu(x,rT),…
PDFs fpu(x),…
Form Factors
F1pu(t),F2pu(t )..
Measure momentum
transfer to target
Direct info about
spatial distributions
No information about
underlying dynamics
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Hard Processes
FNAL
lepton
JPARC
BNL
proton
lepton
proton
antilepton
lepton
proton
pion
SIDIS/DER
electron
pion
Drell-Yan
Partonic scattering
amplitude
Fragmentation amplitude
positron
e–e+ to pions
pion
Distribution amplitude
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xF>0 (current fragmentation)
Single particle production
in hard scattering
h
xF<0 (target fragmentation)
xF - momentum
in the CM frame
Target fragmentation
Current fragmentation
h
h
h
M
-1
Fracture Functions
PDF
0
kT-dependent PDFs
GPD
1
xF
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 qf=f1u(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).
Beam polarization
Target polarization
U unpolarized
L long.polarized
T trans.polarized
sinf moment of the cross section for
unpolarized beam and transverse target
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FNAL
•
•
Single Spin Asymmetries in p  p  p  X

s=20 GeV, pT=0.5-2.0 GeV/c
p0 – E704, PLB261 (1991) 201.
p+/- - E704, PLB264 (1991) 462.
FermiLab E-704
• Recently, large transverse
single-spin effects were observed
also in p+p collisions (RHIC), at
much higher CM energies.
• In collinear picture, the QCD predict small SSAs with
transversely polarized protons colliding at high energies.
Kane, Pumplin, Repko ‘78
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The Elephant and the village of the blind
correlation between quark
fields and the gluonic fields
(“twist-3” Qiu&Sterman)
Non-PQCD “surface effects
(Ma,Boros et al)
SSA
Asymmetry comes from
modulation of the initial
distribution function
(D.Sivers 1990)
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
u
d d
ud
(favored)
fragmentation of transversely
polarized quarks into unpolarized
hadrons
L=1
Orbital momentum generated in string
breaking and pair creation produces leftright 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
P
P
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
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•Opposite effect in target fragmentation
Transverse momentum of quarks
Mulders & Tangerman (1995, the TMD “bible”)
•kT – required to describe azimuthal
distributions of hadrons and in
particular SSAs.
proton polarization
quark polarization
•kT - important for cross section
description (also for exclusive production)
•kT – leads to 3D description with 8PDFs
Boer-Mulders
Sivers
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
Sivers
AUT ~
• significantly positive p asymmetry
 requires non-zero orbital angular momentum
 first hint of naïve T-odd DF from DIS
Phys.Rev.Lett.94:012002,2005.
(100+ in hep SPIRES citation)
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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?
H1 ( z )unfav.  - H1 ( z) fav.
First extraction of transversity by
Anselmino et al. (hep-ph/0701006)
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BELLE: Collins function measurements
_
BELLE: Asymmetries in e+e-→h1h2X (H┴1 H┴1)
Asymmetric collider
8GeV e- + 3.5GeV e+
KEKB
Belle detector
positivity limit
1s
Efremov et al.
Phys.Rev.D. 73,094025
(2006)
First direct indication of non-0
Collins fragmentation !
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Hunting for Lq in hard exclusive processes
- Műller (1994) -
Generalised Parton Distributions
g *L
- Ji & Radyushkin (1996) -
M
Q2>>, t<<

F
( H + E) x dx = Jq
= 1/2 DS  Lz
~~
H,E,H,E
N’
N
10-30%(DIS)
t
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) 17
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 !
HU Feb 15
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CLAS Transversely Polarized Target
CLAS12
CLAS6
q<22.5 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
Collins
sUT ~
Study the Collins fragmentation for all 3 pions with a transversely
polarized target and measure the transversity distribution function.20
Sivers effect
sUT ~
Sivers
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
ep
epg
epg
E = 11 GeV
Transversely polarized target
Ds ~ GPD-E ~ Ju
epr
 Asymmetry for photons and rho
highly sensitive to the GPD E and uquark contributions to proton spin.
CLAS12
HU Feb 15
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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:
•Transverse
target
design in progress at OXFORD (field maps
Inclusive
DIS
available)
Semi-Inclusive DIS
•GSIM studies
of processes
scattering of high lumi beam with transverse
Exclusive
target in……………..
progress.
(proton & deuteron targets)
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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
n=minimal number of spectator quarks =2
Good agreement of HERMES (filled) and JLab (open) data with BBS curves for
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quarks aligned with proton spin, where the orbital motion is not as important.
Hard Scattering Processes: Kinematics Coverage
HERA
Q2
EIC
collider experiments
H1, ZEUS (EIC)
10-4<xB<0.02 (0.3): gluons (and quarks)
in the proton
fixed target experiments
COMPASS, HERMES
 0.006/0.02<xB<0.3 : gluons/valence
and sea quarks
JLab/JLab@12GeV
 0.1<xB<0.7 : valence quarks
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 nucleon’s 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
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