Diapositive 1

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p0 exclusive electro production
Q22.3 GeV2, XBj.36
P-Y BERTIN
Jefferson Laboratory and Université BLAISE
PASCAL- IN2P3/CNRS
for the
DVCS HALL A collaboration
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Two-arm experiment spectrometer and calorimeter

Beam energy 5.75 GeV Beam polarization
75 Beam current 2 and 4 µA Luminosity 1
and 4. 1037 cm-2.s-1 nucleon-1
Left HRS
Scattered Electron
DVCS events are identified with MX2
LH2 / LD2 target
Polarized Electron Beam
g

• Qggt6.3
• - Cerenkov based Electromagnetic Calorimeter
• Specific Scattering Chamber
• Customized Electronics Data Acquisition

Exclusivity
Electromagnetic Calorimeter
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Th 1.00 GeV 1.15 GeV 1.25 GeV
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• At Q22.3 GeV2 and xbj.36
• The continuum is significant compared to the
  p(e,ep0)p
• The resonances are washed out into the continuum.

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in our cut 1 p0p
But we detect only ep0 gt all the process
interfere gtsum of the amplitudes
What I am doing ? Semi
inclusive DIS and Duality ??

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Photons in the inner calorimeter (99 Block from
132) Window coincidence /- 3 ns Accidentals gg
, ep0 substracted Window 105ltmgglt165 MeV
For each exclusive p0 event selected in the
cut on Mx2 The physic variable are
determined exactly Q2, xbj
with the spectrometer t with the
position in the calorimeter ( qgg 2-3 mrd)
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Analyze in the formalism of on photon exchange
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Decomposition in CGLM amplitude
q p0 cm angle -(t-tmin)
Chew, Goldberger, Low and Nambu
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All the trivial kinematics dependence , photon
flux, e, sinq,. Taken in account in the, Monte
Carlo with radiative corrections , detector
resolution,.
Use the same extraction method that the used for
DVCS which take in account the bin migration by a
global linear fit on 10(t)x24(f) experimental
bins
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Extrapoled at fixed
PRELIMINARY
Q22.3 GeV2 xBj0.36 e0.64 c21.11
Corrected for realvirtual RC Corrected for
efficiency Corrected for acceptance Corrected for
resolution effects
Systematic errors include trigger
threshold stability ( 1 to 1.2 GeV)
missing
mass cut ( .9 to 1.15 GeV2 )

extrapolation at fixed Q2 and
fixed XBj.

spectrometer, luminosity
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Regge trajectory Exchange ( w, r and B1)

JML x5
JML x5
JML x5
JML x5
J. M. Lagets Prediction underestimates the
cross-sections by a
factor 5
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Factorization hold only for longitudinal
amplitude
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sL Longitudinal part Prediction from
model (VGG) based on Hand bag
model and GPD
VGG x 5
VGG ? M Vanderhaeghen P. Guichon and M Gidal
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But quid of two photons exchange?
How to check validity of the one photon exchange
??
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END
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Photon electroproduction
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Analysis Exclusivity check using Proton Array
and MC
Using extra recoil Proton-detector, we have
checked the missing mass spectrum of
double-coincidence events with those of a triple
-coincidence.
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.
Normalized (e,p,g) triple coincidence events
Monte-Carlo
(e,g)X (e,p,g)
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Experimental observables linked to GPDs
Experimentally, DVCS is undistinguishable with
Bethe-Heitler
However, we know FF at low t and BH is fully
calculable Interference term allows access to
linear amplitude Using a polarized beam on an
unpolarized target 2 observables can be measured
At JLab energies, TDVCS2 was supposed small
Kroll, Guichon, Diehl, Pire,
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Tests of scaling
1. Twist-2 terms should dominate s and Ds 2. All
coefficients have Q2 dependence which can be
tested!
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Difference of cross-sections
PRL97, 262002 (2006)
Twist-2 Twist-3
Extracted Twist-3 contribution small !
Corrected for realvirtual RC Corrected for
efficiency Corrected for acceptance Corrected for
resolution effects Checked elastic cross-section
_at_ 1
New work by P. Guichon !
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Q2 dependence and test of scaling
lt-tgt0.26 GeV2, ltxBgt0.36
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Total cross-section
PRL97, 262002 (2006)
Corrected for realvirtual RC Corrected for
efficiency Corrected for acceptance Corrected for
resolution effects
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We have proposed to use different beam energies
(different BH) to (
experiment approved and planned to run end 2009)
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On the deuterium
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Neutron contribution is small and compatible with
zero
Results can constrain GPD models (and therefore
GPD En)
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To summarize
the Scaling test is
positive
VGG model misses by 30 DVCS2 must be taken into
account
Agree with F. Cano B. Pire model
Transverse -Transverse large. Description in
terms of Quarks ?GPD and Hadronic description
( Regge exchange) miss by a
factor 515 the data .
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We have demonstrated that high
precision DVCS measurements are doable
using a high resolution spectrometer and a
calorimeter
Full DVCS program in Hall A (up to Q29 GeV2)
already approved with the 12 GeV upgrade
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END
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DVCS Analysis
Check of the missing mass spectrum of
double-coincidence events with the a triple
-coicidence using a Auxilliary Proton array
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.
Raw
Raw p0
Normalized (e,p,g) triple coincidence events
Monte-Carlo
(e,g)X (e,p,g)
(e,p,g)
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Neutron contribution is small and compatible with
zero Results can constrain GPD models (and
therefore GPD En)
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Analysis Exclusivity check using Proton Array
and MC
Using extra recoil Proton-detector, we have
check the missing mass spectrum of
double-coincidence events with the a triple
-coicidence .
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.
Normalized (e,p,g) triple coincidence events
Monte-Carlo
(e,g)X (e,p,g)
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• At Q22.3 GeV2 and xbj.36
• The continuum is significant compared to the
  p(e,ep0)p
• The resonances are washed out into the continuum.

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Missing masse assuming a proton target
Mx2t/2
Mx2
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