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Deeply Virtual Compton Scattering in JLAB Hall A

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Title: Brief overview of the theory Author: mazouz Last modified by: malek mazouz Created Date: 6/6/2005 1:18:54 PM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: Deeply Virtual Compton Scattering in JLAB Hall A


1
Deeply Virtual Compton Scattering in JLAB Hall A
Malek MAZOUZ
For JLab Hall A DVCS collaborations
Hall A
LPSC Grenoble mazzouz_at_lpsc.in2p3.fr
5th ICPHP
May 22nd 2006
2
Generalized parton distributions GPDs
Parton distribution via Deep inelastic scatering
Form Factors via Elastic scaterring
Generalized parton distribution via Deep
exclusive scaterring
Two independent informations about the nucleon
structure
Link
Mueller, Radyushkin, Ji
3
Generalized parton distributions GPDs
Probability ?(x)2 that a quark carries a
fraction x of the nucleon momentum
Parton distributions q(x), ?q(x) measured in
inclusive reactions (D.I.S.)
GPDs measure the Coherence ?(x?) ?(x-?) between
a initial state with a quark carrying a fraction
x? of the nucleon momentum and a final state
with a quark carrying a fraction x-?
Dependence in t new wealth of physics to explore
Mueller, Radyushkin, Ji
4
GPDs properties, link to DIS and elastic form
factors
5
How to access GPDs DVCS
Collins, Freund, Strikman
Simplest hard exclusive process involving GPDs
Bjorken regime
pQCD factorization theorem
Perturbative description (High Q² virtual photon)
fraction of longitudinal momentum
Non perturbative description by Generalized
Parton Distributions
6
Deeply Virtual Compton Scattering
7
What is done at JLab Hall A
But using a polarized electron beam Asymmetry
appears in F
The cross-section difference accesses the
Imaginary part of DVCS and therefore GPDs at x?
Purely real and fully calculable
Small at Jlab enegies
The total cross-section accesses the real part of
DVCS and therefore an integral of GPDs over x
Kroll, Guichon, Diehl, Pire
8
cross-section difference in the handbag dominance
Pire, Diehl, Ralston, Belitsky, Kirchner, Mueller
G contains BH propagators and some kinematics
9
Test of the handbag dominance
To achieve this goal, an experiment was initiated
at JLab Hall A on hydrogen target with high
luminosity (1037 cm-2 s-1) and exclusivity.
Another experiment on a deuterium target was
initiated to measure DVCS on the neutron. The
neutron contribution is very interesting since it
will provide a direct measure of GPD E (less
constrained!)
10
Neutron Target

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
Proton 0.81 -0.07 1.73
Model
neutron
Goeke, Polyakov and Vanderhaeghen
t-0.3
11
Experiment kinematics
E00-110 (p-DVCS) was finished in November 2004
(started in September)
E03-106 (n-DVCS) was finished in December 2004
(started in November)
xBj0.364
s (GeV²) Q² (GeV²) Pe (Gev/c) Te (deg) -T? (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
(fb-1)
proton
neutron
Beam polarization was about 75.3 during the
experiment
12
Experimental method
Proton (E00-110)
Left High Resolution Spectrometer
scattered electron
Neutron (E03-106)
LH2 or (LD2) target
Polarized beam
Reaction kinematics is fully defined
photon
Scintillating paddles
recoil nucleon
(proton veto)
Check of the recoil nucleon position
Only for Neutron experiment
Scintillator Array
Electromagnetic Calorimeter (photon detection)
(Proton Array)
13
Proton Array (100 blocks)
Calorimeter in the black box (132 PbF2 blocks)
Proton Tagger (57 paddles)
14
Analysis - Selection of DVCS events
Mp2
15
p0 contamination subtraction
One needs to do a p0 subtraction if the only
(e,?) system is used to select DVCS events.
Symmetric decay two distinct photons are
detected in the calorimeter ? No contamination
Asymmetric decay 1 photon carries most of the p0
energy ? contamination because DVCS-like event.
16
p0 contamination subtraction
17
Check of the exclusivity
One can predict for each (e,?) event the Proton
Array block where the missing proton is supposed
to be (assuming DVCS event).
18
Extraction of observables
Q2 independent
MC includes real radiative corrections
(externalinternal)
A B
19
Extraction of observables
  • p0 Contribution is small
  • twist-3 contribution is small

2 bins in (Q2,t) out of 15
Acceptance included in fit
20
Check of the handbag dominance
A (twist-2) and B (twist-3) for a full bin
lttgt0.25 GeV2
Strong indication for the validity of twist-3
approximation and the handbag dominance
21
DVCS on the neutron and the deuteron
Same exclusivity check as before
The number of detected p0 with hydrogen and
deuterium target (same kinematics) shows that
In our kinematics p0 come essentially from proton
in the deuterium
p0 asymmetry is small
No p0 subtraction needed for neutron and coherent
deuteron
22
DVCS on the neutron and the deuteron - Preliminary
Q2 1.9 GeV2 lttgt -0.3 GeV2
Mx2 upper cut
23
Conclusion
High statistics extraction of the total
cross-section (another linear combination of GPD!)
Analysis in progress to extract the neutron and
deuteron contribution
24
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25
Proton Target

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 1.13 0.70 0.98
Model
Goeke, Polyakov and Vanderhaeghen
t-0.3
26
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27
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.
28
Electronics
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
29
Calorimeter resolution and calibration
  • Time resolution lt 1ns for all detectors
  • Energy resolution of the calorimeter

- Photon position resolution in the calorimeter
2mm
Invariant mass of 2 photons in the calorimeter
s 9MeV
Detecting p0 in the calorimeter checks its
calibration
30
High luminosity measurement
Up to
At 1 meter from target (T?18 degrees)
Low energy electromagnetic background
Requires good electronics
31
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
32
Analysis preliminary
Triple coincidence
Missing mass2 of H(e,e?)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
33
p0 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?ep0x
2p production threshold
2 possible reactions ep?ep0p ep?en? , ?? p0 p
34
Missing mass2 with LD2 target
35
Time spectrum in the tagger
(no Proton Array cuts)
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