Generalized Parton Distribution Studies @ JLab
Franck Sabatié
CEA Saclay
On behalf of the Hall A and Hall B collaborations
APS-DNP mini workshop
Newport News, VA
October 10th 2007
Introduction
Non-dedicated measurements
E00-110 experiment in Hall A
E03-106 experiment in Hall A
E00-110 prelim. deep-p0 results
E1 & E1-6 deep-r and deep-w
E1-DVCS experiment in Hall B
Summary
Exclusive’07
GPDs from Theory to Experiment
x+x
x-x
g*
GPDs
g
t
Theory
Handbag Diagram
Factorization theorem states:
2. The GPDs enter
then DVCS
as an integral over x:
Q2 and
large amplitude
In the suitable asymptotic limit,
- GPDs appear in
real
part throughthe
a PP
integral
over is
x the leading
at the
xB and
t fixed
handbag
diagram
contribution
DVCS.
- GPDs appear in the imaginary part but
at the linetox=x
1. Needs to be checked !!!
1
T
DVCS
g*
GPD( x, x , t )

dx
g
x  x  i
1
1
+
g*
g
GPD( x, x Physical
, t)
process
 P
dx - ip GPD( x  x , x , t ) +
x x
but it’s not so simple…1
Collins, Freund
Experiment
Experimental observables linked to GPDs
Experimentally, DVCS is undistinguishable with Bethe-Heitler
However, we know FF at low t and BH is fully calculable
Using a polarized beam on an unpolarized target, 2 observables can be measured:
d 4
BH 2
BH
D
VCS
DVC
DVCS

T

2
T

Re
T

T


dxB dQ 2 dtd

2




2
d d 
BH
DVC
S
DVCS
DVCS
DVCS

2
T

I
m
T
Im

T

T



dxB dQ 2 dtd
td

4
Ji, Kroll, Guichon, Diehl, Pire, …
4
2



At JLab energies,
|TDVCS|2 supposed small
Into the harmonic structure of DVCS
d 4
1
2
BH
BH
BH


(
x
,
Q
,
t
)
c

c
cos


c

1
B
0
1
2 cos 2 
dxB dQ 2 dtd 1 ( ) 2 ( )


1
 2 ( xB , Q 2 , t ) c0I  c1I cos   c2I cos 2  c3I cos 3
1 ( ) 2 ( )

 ( xB , Q 2 , t ) I
d d 
I

s
sin


s
sin 2

1
2
2
dxB dQ dtd 1 ( ) 2 ( )
4
4
hadronic plane
e-’
e-
|TBH|2
g*
leptonic plane
Belitsky, Mueller, Kirchner

g
Interference term
1
1 ( ) 2 ( )
p
BH propagators  dependence
Special case of the asymmetry
The asymmetry can be written as:


s1I sin   s2I sin 2
  A ( xB , Q , t ) I


BH
I
BH
4
4
c

c

(
c

c
0
0
1
1 ) cos   ...
d d 
d d 
4
4
2
Pro: easier experimentally, smaller RC, smaller systematics
Con: direct extraction of GPDs is model- (or hypothesis-) dependent
(denominator complicated and unknown)
It was naturally the first observable extracted from non-dedicated
experiments…
Published non-dedicated JLab/Hall B results on ALU and AUL
JLab/Hall B - E1
JLab/Hall B - Eg1
ALU
AUL
PRL 97, 072002 (2006)
PRL 87, 182002 (2001)
Both results show, with a limited statistics, a sin  behavior
(necessary condition for handbag dominance)
In the ALU result, models (VGG) tend to over-estimate the data
E00-110 experimental setup and performances
• 75% polarized 2.5uA electron beam
• 15cm LH2 target
• Left Hall A HRS with electron package
• 11x12 block PbF2 electromagnetic calorimeter
• 5x20 block plastic scintillator array
50 days of beam time in
the fall 2004, at 2.5mA
1
Lu

dt

13294
fb

Analysis – Looking for DVCS events
HRS: Cerenkov, vertex, flat-acceptance cut with R-functions
Calo: 1 cluster in coincidence in the calorimeter above 1 GeV
With both: subtract accidentals, build missing mass of (e,g) system
Analysis – po subtraction effect on missing mass spectrum
Using p0→2g events in the calorimeter,
the p0 contribution is subtracted bin by bin
After p0 subtraction
Analysis – Exclusivity check using Proton Array and MC
Using Proton-Array, we compare the missing mass spectrum of the
triple and double-coincidence events.
The missing mass spectrum using the Monte-Carlo gives the same position
and width. Using the cut shown on the Fig.,the contamination from
inelastic channels is estimated to be under 3%.
M
2
X
cut
Normalized
(e,p,g)
Monte-Carlo
triple coincidence
events
(e,g)X – (e,p,g)
Difference of cross-sections
PRL97, 262002 (2006)
Q 2  2.3 GeV 2
xB  0.36
Twist-2
Twist-3
Corrected for real+virtual RC
Corrected for efficiency
Corrected for acceptance
Corrected for resolution effects
Checked elastic cross-section @ ~1%
Extracted Twist-3
contribution small !
New work by P. Guichon !
Q2 dependence and test of scaling
<-t>=0.26 GeV2, <xB>=0.36
Twist-2
Twist-3
No Q2 dependence: strong indication for
scaling behavior and handbag dominance
Twist 4+ contributions are smaller than 10%
Total cross-section
PRL97, 262002 (2006)
Q 2  2.3 GeV 2
xB  0.36
Corrected
Corrected
Corrected
Corrected
for real+virtual RC
for efficiency
for acceptance
for resolution effects
Extracted Twist-3
contribution small !
but impossible to disentangle DVCS2
from the interference term
DVCS on the neutron in JLab/Hall A: E03-106
LD2 target
24000 fb-1
xB=0.36, Q2=1.9 GeV2
MODEL-DEPENDENT
Ju-Jd extraction
Deep-p0 electroproduction in JLab/Hall A
Same data as E00-110, but:
- 2g requirement in the calorimeter at p0 mass,
- ep0X at proton mass.
counts
Fit to data
0
180

d  LT
d L
d TT
d  d T


 2 (1   )
cos   
cos2
dt
dt
dt
dt
dt
360
Cross-sections for deep-p0 production in JLab/Hall A
Q 2  2.3 GeV 2
xB  0.36
VGG
Laget
+rediff.
Laget
-Low-t cross-section largely
overshoots GPD & JML
models.
-TT term is large,
suggesting a large
transverse component.
-But p0 cross-section ratio
for proton and deuteron
targets is found ~ 1
More data needed !
PRELIMINARY
Deep-w data at 6 GeV proves to be unusable for GPD studies…
DESY
Found to be dominated by the tchannel p0 exchange (mostly due to
transverse photons)
E1-6 Hall B
L
VGG
Angular decay analysis show that schannel helicity is NOT conserved:
Impossible to extract L and T
Omega electroproduction is a
challenging reaction for GPD studies.
x5
Laget
Cornell
L
Laget
Deep r0 data largely overshoots GPD models at low W
New !
E1-DVCS with CLAS : a dedicated DVCS experiment in Hall B
Beam energy:
Beam Polarization:
Integ. Luminosity:
~5.8 GeV
75-85%
45 fb-1
Mgg (GeV2)
Inner Calorimeter
+ Moller shielding solenoid
~50 cm
E1-DVCS kinematical coverage and binning
W2 > 4 GeV2
Q2 > 1 GeV2
E1-DVCS : Asymmetry as a function of xB and Q2
<-t> = 0.18 GeV2
<-t> = 0.30 GeV2
<-t> = 0.49 GeV2
<-t> = 0.76 GeV2
Accurate data in a
large kinematical
domain
Integrated over t
E1-DVCS : ALU(90°) as a function of |t| + models
VGG twist-2+3
VGG twist-2
JM Laget
E1-DVCS : Cross-sections over a wide kinematical range
PhD Thesis H.S. Jo (IPNO)
0.09<-t<0.2
0.2<-t<0.4
0.4<-t<0.6
0.6<-t<1
1<-t<1.5
1.5<-t<2
Summary (DVCS)
DVCS BSA (Hall B/CLAS):
 Data in a large kinematical range and good statistics. It will give a very
hard time to models, to fit the whole range in Q2, xB and t.
 Data seem to favor JM Laget model at low-t and VGG model at high t.
DVCS Cross-section difference (Hall A):
 High statistics test of scaling: Strong support for twist-2 dominance
 First model-independent extraction of GPD linear combination from
DVCS data in the twist-3 approximation
 Upper limit set on twist-4+ effects in the cross-section difference:
twist>3 contribution is smaller than 10%
DVCS Total cross-section (Hall A):
 Bethe-Heitler is not dominant everywhere
 |DVCS|2 terms might be sizeable