Studies of OAM at JLAB

1
Studies of OAM at JLAB
Harut Avakian Jefferson Lab
UNM/RBRC Workshop on Parton Angular Momentum ,
NM, Feb 2005

• Introduction
• Exclusive processes
• Semi-Inclusive processes
• Summary

In collaboration with V.Burkert and
L.Elouadrhiri
2
Parton picture Longitudinal and transverse
variables
long before
3
Quark Angular Momentum Sum Rule
GPDs Hu, Hd, Eu, Ed provide access to total
quark contribution to proton angular momentum.
½ ½ (DuDdDs) Lq Jg
Protons spin
J q
Large x contributions important.
4
3D Parton Distributions
TMD PDFs fpu(x,kT),
GPDs Hpu(x,x,t)..
dx
d2kT
x0,t0
Measure momentum transfer to quark
Measure momentum transfer to target
PDFs fpu(x,kT), g1, h1
FFs F1pu(t),F2pu(t)..
Analysis of SIDIS and DVMP are complementary
5
Form Factor Studies
Sachs Form Factors
9 0
GE(t)F1(t)t/4M2F2(t) GM(t)F1(t)F2(t)
n
More data expected in 2006/2007
6
Form Factor Studies
Use various parameterizations for GPDs to fit the
existing form factor data
A.Afanasev hep-ph/9910565 Diehl et al,
Eur.Phys.J c39 (2005) M.Guidal et al PRD (2005)
Different parameterizations yield different
contributions for quarks to the OAM
A)Large Ld and small Lu B)Sum of Lu and Ld small
Issues different realistic fits to FFs produce
different values for Lq fits done at high t,
need to be extrapolated to t?0
More observables needed for detailed studies of
GPDs and the OAM (RCS,DVCS,DVMP)
7
Hard Exclusive Processes and GPDs
DVMP
DVCS
hard gluon
hard vertices
DVCS for different polarizations of beam and
target provide access to different combinations
of GPDs H, H, E
DVMP for different mesons is sensitive to flavor
contributions (r0/r select H, E, for u/d
flavors, p, h, K select H, E)

Study the asymptotic regime and guide theory in
describing HT.
8
Deeply Virtual Compton Scattering ep-gtepg
DVCS
BH
GPD
TBH given by elastic form factors TDVCS
determined by GPDs
Polarized beam, unpolarized target

DsLU sinfImF1H x(F1F2)H kF2E
DVCS
Kinematically suppressed
BH
Unpolarized beam, longitudinal target

DsUL sinfImF1Hx(F1F2)(H ..
Kinematically suppressed
x xB/(2-xB ),k t/4M2
Unpolarized beam, transverse target

• Different GPD combinations accessible as
  azimuthal moments of the total cross section.

DsUT sinfImk1(F2H-F1E ) ..
Kinematically suppressed
9
Deeply Virtual Compton Scattering ep?epg
Interference responsible for SSA, contain the
same lepton propagator P1(f) as BH
Way to access to GPDS
GPD combinations accessible as azimuthal moments
of the total cross section.
10
f-dependent amplitude
5.7 GeV
f0
BH
f45
f90
DVCS
x0.25
Strong dependence on kinematics of prefactor
f-dependence, at yycol P1(f)0 Fraction of pure
DVCS increases with t and f
11
DVCS Experiments
CLAS at 4.3 GeV
HERMES 27 GeV
A(f) asinf bsin2f
S. Stepanyan et al. Phys. Rev. Lett. 87 (2001)
A. Airapetian et al. Phys. Rev. Lett. 87 (2001)
12
GPDs from ep-gtepg
Requirements for precision (lt15) measurements
of s2I and GPDs from DVCS SSA

• Define relation between ALU and s2I
• effect of other non-0 moments 5-10
• effect of finite bins 10
• Define background corrections
• pion contamination 10
• radiative background
• ADVCS lt3 at CLAS

More relevant when proton is not detected
13
DVCS event samples

• 3 event samples(after data quality cuts)

epg(DVCS)

• ep 0 photons (2M events)
• tight cuts on PID,missing mass MX
• 2) epg 1 photon in Calorimeter (150000
  events)
• cut on the direction qgXlt0.015,
• 3) epgg 2 photon(p0) in Calorimeter (70000
  events)
• cut on the direction qpXlt0.02,

epg(p0)
Kinematic coverage of 5.75 GeV(red) and
5.48(blue) CLAS data sets
Angular cut most efficient in separating p0
14
p0 MC vs Data

• Exclusive pi0 production simulated using a
  realistic MC
• Kinematic distributions in x,Q2,t tuned to
  describe the CLAS data

15
p0 beam SSA cross section
Main unknown in corrections of photon SSA are
the p0 contamination and its beam SSA.
Use epgg to estimate the contribution of p0 in
the ep and epg samples
1.6ltQ2lt2.6, 0.22ltxlt0.32
CLAS 5.7 GeV
PRELIMINARY
Contamination from p0 photons increasing at large
t and x and also at large f. Significant SSA
measured for exclusive p0s also should be
accounted
16
BH cosf moment
BH cosf moment can generate 3 sin2f in the ALU
17
DVCS SSA kinematic dependences at 5.7 GeV
PRELIMINARY
Fine binning allows to observe the x and Q2
dependence
ALU for ep-gtepg sample with -tlt0.5 GeV2
Preliminary data for fully exclusive epg is
consistent with the ep data and consistent with
GPD base predictions
18
Dedicated DVCS experiments
? Dedicated detection of 3 particles e, p and ?
in final state ? Firmly establish scaling laws
(up to Q2 5 GeV2), if observed, or
deviations thereof understood,
first significant measurement of GPDs. ? Large
kinematical coverage in xB and t
JLab/CLAS
JLab/Hall A
Calorimeter and superconducting magnet within
CLAS torus
424 PbWO4 crystals
HRS PbF2 Plastic scintillator H(e,egp) D(e,e
gN)N
dedicated calorimeters
19
Extraction of GPD H from ALU moment
cLU
ALU/cLU
epg

x(F1F2)H kF2E
20
2ltQ2lt2.4 GeV

• Redblue points correspond to projected ALU
  uncorrected for p0 (bin by bin)
• H stands for the ratio of the ALU and prefactor
  calculated for all events in a bin (averaged over
  f)
• Curves are for a simple model for CFF H (blue)
  and H(red)

20
Target Spin Asymmetry t- Dependence
Unpolarized beam, longitudinal target

DsUL sinfImF1Hx(F1F2)(H .. DsLL
cosfReF1Hx(F1F2)(H ..

Kinematically suppressed
First data available(5 CLAS days), more(60 days)
to come at 6 GeV
Measurements with polarized target will constrain
the polarized GPD and combined with beam SSA
measurements would allow precision measurement of
unpolarized GPDs.
21
Exclusive ? meson production ep ? ep?0
CLAS (4.2 GeV)
CLAS (5.75 GeV)
Regge (JML)
GPD formalism (beyond leading order) describes
approximately data for xBlt0.4, Q2 gt1.5 GeV2
GPD (MG-MVdh)
Analysis in progress
C. Hadjidakis et al., PLB 605
Two-pion invariant mass spectra
Decent description in pQCD framework already at
moderate Q2
22
Exclusive pp- and pp0
e p e p ?
p p-
n
r
r0
Provide access to different combinations of
orbital momentum contributions Ju,Jd r0 -gt 2Ju
Jd, r -gt Ju - Jd
23
Exclusive r0 production on transverse target
2D -(Im(AB))/p

AUT -
A2(1-x2) - B2(x2t/4m2) - Re(AB)2x2
A 2Hu Hd B 2Eu Ed
r0
A Hu - Hd B Eu - Ed
r
r0
Eu, Ed needed for angular momentum sum rule.
K. Goeke, M.V. Polyakov, M. Vanderhaeghen, 2001
Asymmetry is a more appropriate observable for
GPD studies at JLab energies as possible
corrections to the cross section are expected to
cancel
24
TMD measurements in SIDIS (gp?pX)
TMD PDFs related to interference between L0 and
L1 light-cone wave functions.
Process
TMD
FF
Moment
f1T- ep ? epX D1 sin(fh-fS)
ST(qPT)
h1- ep ? epX H1- sin(fh-fS)
fSp/2-fh
h1L- ep ? epX H1- sin(fh-fS)
fSp-fh
fL- ep ? epX D1 sinfh
Survive in jet limit
g- ep ? epX D1 sinfh
hL ep ? epX H1- sinfh
Significant beam and target SSA were observed in
all listed channels, more data under way
25
Sivers Effect studies with Transversely
polarized target
E06-010 and E06-011
Proposal approved, to study the Sivers function
at JLab (Hall-A)
26
Sivers SSA at CLAS _at_5.7GeV
Expected precision of the AUT with transversely
polarized target
Measurement of p0 AUT at CLAS would allow model
independent extraction of the Sivers function
p
Simultaneous measurement of SIDIS, exclusive
r,r,w and DVCS asymmetries with a transversely
polarized target.
27
Polarized target SSA using CLAS at 6 GeV
curves, cQSM from Efremov et al
Hunf-5Hfav
Hunf-1.2Hfav
Hunf0

• Provide measurement of SSA for all 3 pions,
  extract the Mulders TMD and study Collins
  fragmentation with longitudinally polarized
  target
• Allows also measurements of 2-pion asymmetries

28
Target SSA measurements at CLAS
ep?epX
W2gt4 GeV2
CLAS PRELIMINARY
p1sinfp2sin2f
Q2gt1.1 GeV2
ylt0.85
0.4ltzlt0.7
MXgt1.4 GeV
PTlt1 GeV
0.12ltxlt0.48
p1 0.0590.010 p2-0.0410.010
p1-0.0420.015 p2-0.0520.016
p10.0820.018 p20.0120.019

• Significant SSA measured for pions with
  longitudinally polarized target
• Complete azimuthal coverage crucial for
  separation of sinf, sin2f moments

29
ALU x-dependence CLAS _at_ 5.7 GeV
p,0.5ltzlt0.8
Parton distribution g-(x) is calculated within
the same dynamical model of Afanasev, Carlson

• Assume kT is small
• Assume NLO corrections small

Beam SSA for p0 may provide a FF independent
access to g-
30
Measuring the Q2 dependence of SSA
ssinfLU(UL) FLU(UL) 1/Q (Twist-3)
For fixed x, 1/Q behavior expected
Wide kinematic coverage and higher statistics
will allow to check the higher twist nature of
beam and longitudinal target SSAs
31
CLAS12
High luminosity polarized (80) CW beam
Wide physics acceptance (exclusive,
semi-inclusive current and target fragmentation)
Wide geometric acceptance
Provides new insight into - quark orbital
angular momentum contributions - to the
nucleon spin - 3D structure of the nucleons
interior and correlations - quark flavor
polarization
12GeV significantly increase the kinematic
acceptance and accessible luminosity
32
Summary

• Current JLab data are consistent with a partonic
  picture, and can be described by a variety of
  theoretical models.
• High luminosity, polarized CW beam, wide
  kinematic and geometric acceptance allow studies
  of exclusive and semi-inclusive processes,
  providing data needed to constrain relevant 3D
  distribution functions (TMDs,GPDs)
• Experimental investigation of properties of 3D
  PDFs at JLab, complementary to planed studies at
  HERMES, COMPASS, RHIC, BELLE, GSI, would serve as
  an important check of our understanding of
  nucleon structure in terms of quark and gluon
  properties.

• CLAS12 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.