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Deeply Virtual Compton Scattering on the neutron
Malek MAZOUZ
LPSC Grenoble
EINN 2005 September 23rd 2005
GPDs properties, link to DIS and elastic form factors
),,(~
,~
, , txEHEH qqqq Generalized Parton distributions
Link to DIS at =t=0
)()()0,0,(~
)()()0,0,(
xqxqxH
xqxqxHq
q
Link to form factors (sum rules)1
1
1
1
2
1
1 1
1 1
( , , ) ( )
( , , ) ( )
( , , ) ( ) , ( , , ) ( )
q q
q q
q q q qA A
dxH x t F t
dxE x t F t
dx H x t g t dx E x t h t
1
1
1 1( , ,0) ( , ,0)
2 2q q
q q qJ L xdx H x E x
Access to quark angular momentum (Ji’s sum rule)Quark correlations !
Brief overview of the theory
222
2222
)'(
)'(
Qppt
MkkqQ
k k’ q’
GPDsp p’
Simplest hard exclusive process involving GPDs
pQCD factorization theorem
Perturbative description
(High Q² virtual photon)
Non perturbative description by
Generalized Parton Distributions
Bjorken regime
X. Ji, Phys. Rev. DS56 (1997) 5511A. Radyushkin, Phys. Lett. B380 (1996) 417
x
2
22
22
B
B
B
x
x
M
Q
pq
Qx
fraction of longitudinal momentum
),,( txGPDix
dxDVCS
amplitude
What can be done at JLab Hall A
(DVCS)BHσdσd Im.55 �
Using a polarized electron beam:Asymmetry appears in Φ
K. Goeke, M.V. Polyakov and M. Vanderhaeghen
Direct handle on the imaginary part of the DVCS amplitudeEnhanced by the full magnitude of the BH amplitude
Purely real
BHDVCSBHd ).Re(.225
-High luminosity
-High precision measurement
cross-section difference in the handbag dominance
22 2
2
( , , , ) sin sin 2
with / 2 , / , ' ,
and the angle between the leptonic and photonic planes
BB B
B
d dx y A B
dx dyd d dx dyd d
x Q p q y q p k p p p
��
Γ contains BH propagators and some kinematics
B contains higher twist terms
A is a linear combination of three GPDs evaluated at x=ξ
1 1 2 22( ) ( ) ( ) ( )
2 4B
B
x tA F t F t F t F t
x M
H H E
0.1 1.34 0.81 0.38 0.04
0.3 0.82 0.56 0.24 0.06
0.5 0.54 0.42 0.17 0.07
0.7 0.38 0.33 0.13 0.07
Proton Target
Proton
2 ( )pF t 1 ( )pF t 1 2( ) ( ) /(2 )p pB BF t F t x x 2
2( / 4 ) ( )pt M F t t
t=-0.3
Target
Proton 1.13 0.70 0.98
H H E
Goeke, Polyakov and Vanderhaeghen
Model:
2 22 GeV
0.3
0.3B
Q
x
t
1 1 2 22( ) ( ) ( ) ( )
2 4B
B
x tA F t F t F t F t
x M
H H E
1 1 2 22( ) ( ) ( ) ( )
2 4
0.64 0.17 + 0.06
B
B
x tA F t F t F t F t
x M
A
H H E
0.1 -1.46 -0.01 -0.26 -0.04
0.3 -0.91 -0.04 -0.17 -0.06
0.5 -0.6 -0.05 -0.12 -0.08
0.7 -0.43 -0.06 -0.09 -0.08
Neutron Target
Neutron
2 ( )pF t 1 ( )pF t 1 2( ) ( ) /(2 )p pB BF t F t x x 2
2( / 4 ) ( )pt M F t t
t=-0.3
Target
Proton 0.81 -0.07 1.73
H H E
Goeke, Polyakov and Vanderhaeghen
Model:
2 22 GeV
0.3
0.3B
Q
x
t
1 1 2 22( ) ( ) ( ) ( )
2 4B
B
x tA F t F t F t F t
x M
H H E
1 2( ) ( ) !!!n nF t F t
1 1 2 22( ) ( ) ( ) ( )
2 4
0.03 0.01 0.12
B
B
x tA F t F t F t F t
x M
A
H H E
neutron
Experiment status
s (GeV²)
Q² (GeV²)
Pe (Gev/c)
Θe (deg)
-Θγ* (deg)
4.94 2.32 2.35 23.91 14.80 5832
4.22 1.91 2.95 19.32 18.25 4365
3.5 1.5 3.55 15.58 22.29 3097
4.22 1.91 2.95 19.32 18.25 24000
proton
neutron
xBj=0.364
Beam polarization was about 75.3% during the experiment
E00-110 (p-DVCS) was finished in November 2004 (started in September)
E03-106 (n-DVCS) was finished in December 2004 (started in November)
Ldt (fb-1)
Left High Resolution Spectrometer
LH2 or (LD2) target
Polarized beam
Electromagnetic Calorimeter (photon detection)
Scintillator Array(Proton Array)
Experimental method
(proton veto)Scintillating paddles
scattered electron
( )
e p e p
e D e n p
photon
Proton:(E00-110)
Neutron:(E03-106)
Only for Neutron experimentCheck of the recoil nucleon position
recoil nucleon
Reaction kinematics is fully defined
Calorimeter in the black box
(132 PbF2 blocks)
Proton Array
(100 blocks)
Proton Tagger
(57 paddles)
High luminosity measurement
2 13710L cm s
123710.4 scmLnucleon
Up to
At ~1 meter from target (Θγ*=18 degrees)
Requires good electronics
Low energy electromagnetic background
PMT
G=104
x10 electronics
Electronics
1 GHz Analog Ring Sampler (ARS)
x 128 samples x 289 detector channels
Sample each PMT signal in 128 values (1 value/ns)
Extract signal properties (charge, time) with a wave form Analysis.
Allows to deal with pile-up events.
Electronics
Calorimeter trigger
Not all the calorimeter channels are read for each event
Following HRS trigger, stop ARS.
30MHz trigger FADC digitizes all calorimeter signals in 85ns window.
- Compute all sums of 4 adjacent blocks.
- Look for at least 1 sum over threshold
- Validate or reject HRS trigger within 340 ns
Not all the Proton Array channels are read for each event
Analysis Status - Preliminary
Xeep 0Invariant mass of 2 photons in the calorimeter
Sigma=9.5 MeV
Good way to control the calorimeter calibration
ep e XMissing mass2 with LH2 target
Analysis Status – Very preliminary
φ
φ
φ
α (
N+ -
N- )
α (
N+ -
N- )
α (
N+ -
N- )
LH2 target
LD2 target
LD2 – LH2
Possible neutron signal !
0.5 GeV2 < missing mass 2 < 1.5 GeV2
0.5 GeV2 < missing mass 2 < 1.5 GeV2
Absolute cross sections necessary to extract helicity
dependence of neutron
Analysis Status – Very preliminary
φ
φ
φ
α (
N+ -
N- )
α (
N+ -
N- )
α (
N+ -
N- )
0.5 GeV2 < missing mass 2 < 1.5 GeV2
1.5 GeV2 < missing mass 2 < 2.5 GeV2
2.5 GeV2 < missing mass 2 < 3.5 GeV2
No signal
Signal
Conclusion
123710.4 scmLnucleon
With High Resolution spectrometer and a good calorimeter, we are able to measure:
•Helicity dependence of the neutron using LD2 and LH2 target.
Work at precisely defined kinematics: Q2 , s and xBj
Polarized cross sections to extract GPD E
Relative asymmetry considering Proton Array and Tagger.
Work at a luminosity up to
Proton preliminary results tomorrow morning
Coming soon:
Analysis status – preliminary
Sigma = 0.6ns
2 ns beam structure
Time difference between the electron arm and the detected photon
Selection of events in the coincidence peak
Determination of the missing particle (assuming DVCS kinematics)
Check the presence of the missing particle in the predicted block (or region) of the Proton Array
Sigma = 0.9ns
Time spectrum in the predicted block (LH2 target)
Analysis – preliminary
Triple coincidence
Missing mass2 of H(e,e’γ)x for triple coincidence events
Background subtraction with non predicted blocks
Proton Array and Proton Veto are used to check the exclusivity and reduce the background
π0 electroproduction - preliminary
Invariant mass of 2 photons in the calorimeter
Good way to control calorimeter calibration
Missing mass2 of epeπ0x
2 possible reactions:
epeπ0p
epenρ+ , ρ+ π0 π+
2π production threshold
Sigma = 0.160 GeV2
Sigma = 9.5 MeV
Missing mass2 with LD2 target
Time spectrum in the tagger (no Proton Array cuts)