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Deeply Virtual Compton Scattering on the neutron. Malek MAZOUZ. LPSC Grenoble. Hall A. EINN 2005. September 23 rd 2005. Link to form factors (sum rules). Generalized Parton distributions. Link to DIS at x =t=0. Access to quark angular momentum (Ji’s sum rule). Quark correlations !.

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Deeply virtual compton scattering on the neutron
Deeply Virtual Compton Scattering on the neutron


LPSC Grenoble

Hall A

EINN 2005

September 23rd 2005

Link to form factors (sum rules)

Generalized Parton distributions

Link to DIS at x=t=0

Access to quark angular momentum (Ji’s sum rule)

Quark correlations !

GPDs properties, link to DIS and elastic form factors

Brief overview of the theory







Brief overview of the theory

X. Ji, Phys. Rev. DS56 (1997) 5511

A. Radyushkin, Phys. Lett. B380 (1996) 417

Simplest hard exclusive process involving GPDs

Bjorken regime

pQCD factorization theorem

Perturbative description

(High Q² virtual photon)

fraction of longitudinal


Non perturbative description by

Generalized Parton Distributions


What can be done at jlab hall a

Purely real

What can be done at JLab Hall A

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 amplitude

Enhanced by the full magnitude of the BH amplitude

-High luminosity

-High precision measurement

Cross section difference in the handbag dominance
cross-sectiondifference in the handbag dominance

Γcontains BH propagators and some kinematics

B contains higher twist terms

A is a linear combination of three GPDs evaluated at x=ξ

Proton target
Proton Target



Goeke, Polyakov and Vanderhaeghen


Neutron target
Neutron Target




Goeke, Polyakov and Vanderhaeghen


Experiment status

E00-110 (p-DVCS) was finished in November 2004 (started in September)

E03-106 (n-DVCS) was finished in December 2004 (started in November)





Beam polarization was about 75.3% during the experiment

Experimental method
Experimental method



Left High Resolution Spectrometer

scattered electron



LH2 or (LD2) target

Polarized beam

Reaction kinematics is fully defined


Scintillating paddles

recoil nucleon

(proton veto)

Check of the recoil nucleon position

Only for Neutron experiment

Scintillator Array

Electromagnetic Calorimeter (photon detection)

(Proton Array)

Proton Array

(100 blocks)

Calorimeter in the black box

(132 PbF2 blocks)

Proton Tagger

(57 paddles)





High luminosity measurement

Up to

At ~1 meter from target (Θγ*=18 degrees)

Low energy electromagnetic background

Requires good 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.


Not all the calorimeter channels are read for each event

Calorimeter trigger

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

Sigma=9.5 MeV

Invariant mass of 2 photons in the calorimeter

Good way to control the calorimeter calibration

Missing mass2 with LH2 target

Analysis Status – Very preliminary

LH2 target

0.5 GeV2 < missing mass 2 < 1.5 GeV2

α (N+ - N-)


LD2 target

α (N+ - N-)

0.5 GeV2 < missing mass 2 < 1.5 GeV2


LD2 – LH2

Possible neutron signal !

α (N+ - N-)

Absolute cross sections necessary to extract helicity dependence of neutron


Analysis Status – Very preliminary

0.5 GeV2 < missing mass 2 < 1.5 GeV2

α (N+ - N-)



α (N+ - N-)

1.5 GeV2 < missing mass 2 < 2.5 GeV2


No signal

α (N+ - N-)

2.5 GeV2 < missing mass 2 < 3.5 GeV2



  • 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

Work at a luminosity up to

Coming soon:

Polarized cross sections to extract GPD E

Relative asymmetry considering Proton Array and Tagger.

Proton preliminary results tomorrow morning

Analysis status – preliminary

Sigma = 0.6ns

Time difference between the electron arm and the detected photon

2 ns beam structure

Selection of events in the coincidence peak

Determination of the missing particle (assuming DVCS kinematics)

Time spectrum in the predicted block (LH2 target)

Sigma = 0.9ns

Check the presence of the missing particle in the predicted block (or region) of the Proton Array

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

0 electroproduction preliminary
π0 electroproduction - preliminary

Invariant mass of 2 photons in the calorimeter

Sigma = 9.5 MeV

Good way to control calorimeter calibration

Sigma = 0.160 GeV2

Missing mass2 of epeπ0x

2π production threshold

2 possible reactions:


epenρ+ , ρ+ π0 π+

Missing mass2 with LD2 target

Time spectrum in the tagger

(no Proton Array cuts)