Deep Virtual Compton Scattering at Jlab Hall A

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Deep Virtual Compton Scattering at Jlab Hall A
Second Workshop on the QCD Structure of the
Nucleon 12-16 June 2006 Villa Mondragone, Italy

• Charles E. Hyde-Wright
• Old Dominion University, Norfolk VA
• chyde_at_odu.edu

Based on the work of A. Camsonne the DVCS Hall A
Ph.D. students M. Mazouz C. Munoz Camacho
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QCD, Confinement, and the Origin of Mass

• We have a good understanding of the strong
  interaction at extreme short distance with
  perturbative QCD
• We understand the long distance properties of the
  strong interaction in terms of Chiral
  Perturbation Theory
• Confinement and the origin of ordinary mass
  (baryon mass) occurs at an intermediate distance
  scale.
• Lattice QCD and many semi-phenomenological models
  give us a great deal of insight into the
  structure of hadrons at the confinement scale.
• Nuclear binding (e.g. Bdeuteron2.2 MeV,
  r-process nuclei) are 1 effects or smaller of
  the confinement scale 300 MeV/c.
• We need experimental observables of the
  fundamental quark and gluon degrees of freedom of
  QCD, in coordinate space.
• Forward parton distributions do not resolve the
  partons in space.
• Elastic Electro-Weak Form Factors measure spatial
  distributions, but the resolution cannot be
  selected independent of momentum transfer.
• Generalized Parton Distributions (GPD)!
• x,? momentum fraction variables
• t?2. ?? Fourier Conjugate to impact parameter
  of quark or gluon.
• Q2 Resolution of probe.

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Experimental observables linked to GPDs
q k-k Q2 ?q2gt0 ?q-q t??2 s
(kp)2 xBj Q2/(2pq) W2 (qp)2 Using a
polarized beam on an unpolarized target, 2
(actually 6) observables can be measured
At JLab energies, TDVCS2 is small TDVCS2 /
TBH2 -t xBj2 s2 / Q6
M. Diehl, yesterday
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Into the harmonic structure of DVCS
TBH2
Interference term
BH propagators j dependence
Belitsky, Mueller, Kirchner
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Tests of the handbag dominance
?V??d?T(DVCS) ??d?TT(DVCS) cos(2?)
?V????????????d?LT(DVCS) sin?

• Twist-2 terms should dominate s and Ds
• Subject to reasonableness of Twist-3 Matrix
  Elements
• 2. All coefficients have known Q2-dependence
  (Powers of -t/Q2 or (tmin-t)/Q2) which can be
  incorporated into analysis.
• 3. Angular Harmonic terms ci, si, are
  Q2-independent in leading twist (except for QCD
  evolution).

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Designing a DVCS experiment
Measuring cross-sections differential in 4
variables requires

• Good identification of the experimental process,
  i.e. exclusivity

With perfect experimental resolution
H(e,e?)X
resonant or not
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Hall A DVCS philosophy

• Precision measurement of kinematics
• Precision knowledge of the acceptance
• High Resolution Spectrometer (HRS) for electron
• Simple, high performance 11x13 element
  (3x3x19cm3) PbF2 Calorimeter
• Waveform digitizing
• Low resolution detection of proton direction

e p ? e (p) g
Scattered electron The HRS acceptance is well
known
Emitted photon The calorimeter has a
simple rectangular acceptance
R-function cut
g
Acceptance matching by design ! Virtual photon
 acceptance  placed at center of calorimeter
g
Simply t radius j phase
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Digital trigger on calorimeter and fast
digitizing-electronics
1. HRS Trigger
2. ARS Stop
In
1GHz Analog Ring Sampler (ARS)
t (ns)
4. Validate or Fast Clear (500ns)
3. SH 60ns gate
FPGA Virtual Calorimeter
PbF2 blocks
Zgtgt50?
Fast Digital Trigger
4. Find 2x2 clustersgt1GeV
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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

• 11x12 block PbF2 electromagnetic calorimeter

• 15cm LH2 target
• Left Hall A HRS with electron package

• 75 polarized 2.5uA electron beam

• 5x20 block plastic scintillator array

Dt (ns) for 9-block around predicted  DVCS 
block
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ARS system in a high-rate environment

• 5-20 of events require a 2-pulse fit
• Maintain Energy Position Resolution
  independent of pile-up events
• Optimal timing resolution
• 101 TrueAccidental ratio at L1037/(cm2 s)
  unshielded calorimeter

2ns beam structure
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E00-110 kinematics
The calorimeter is centered on the virtual photon
direction. Acceptance ????lt 150 mrad
50 days of beam time in the fall 2004, at 2.5mA
intensity
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Analysis Looking for DVCS events
HRS Cerenkov, vertex, flat-acceptance cut with
R-functions). Calo 1 cluster in coincidence in
the calorimeter above 1.2GeV. Coincidence
subtract accidentals, build missing mass of
H(e,g)X system. Generate estimate of ?0
H(e,e???Y events from measured H(e,e??)Y events.
H(e,e?)X MX2 kin3
Exclusive DVCS events
H(e, e ???N ? Threshold
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H(e,e?) Exclusivity
H(e,e?)X - H(e,e?)?Y Missing Mass2
H(e,e??p
H(e,e???
H(e,e?p) sample
H(e,e?p) simulation, Normalized to data
lt2 in estimate of H(e,e?)N? below threshold
MX2lt(Mm)2
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Analysis Extraction of observables
Re-stating the problem (difference of
cross-section)
Observable
Kinematic factors
GPD !!!
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Analysis Calorimeter acceptance
The t-acceptance of the calorimeter is uniform at
low tmin-t
5 bins in t
Min Max Avg
-0.40 -0.35 -0.37
-0.35 -0.30 -0.33
-0.30 -0.26 -0.28
-0.26 -0.21 -0.23
-0.21 -0.12 -0.17
Large-t j dependence
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d? Difference Extraction of observables
Averaged over t lt-tgt0.23 GeV2, ltxBgt0.36
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Analysis Difference of counts 2 of 4 bins in t

• Twist-3 contribution is small
• po contribution is small
• po is Twist-3 (d?LT)

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Total cross section and GPDs

Interesting ! Only depends on H and E
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Conclusion at 6 GeV

• High luminosity (gt1037) measurements of DVCS
  cross sections are feasible using trigger
  sampling system
• Tests of scaling yield positive results
• No Q2 dependence of CT2 and CT3
• Twist-3 contributions in both Ds and s are small
• Note DIS has small scaling violation in same x,
  Q2 range.
• In cross-section difference, accurate extraction
  of Twist-2 interference term
• High statistics extraction of cross-section sum.
• Models must calculate ReBHDVCSDVCS2
• ? d?(h) d?(h-) ? BH2
• Relative Asymmetry contains DVCS terms in
  denominator.

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Hall A at 11 GeV (in preparation for PAC30
HALL A H(e,e?) 3,4,5 pass beam k 6.6,
8.8, 11 GeV Spectrometer HRS k4.3
GeV Calorimeter 1.5 x larger Similar MX2
resolution at each setup. Same 1.0 GHz Digitizer
for PbF2 Calorimeter trigger improved
( better p0 subtraction) Luminosity x Calo
acceptance/block 2x larger. Same statistic
(250K)/setup
100 Days
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JLab12 Hall A with 3, 4, 5 pass beam
Absolute measurements d?(?e1) 250K events/setup
H(e,e?)p
Twist 2 Twist 3 separation. ImDVCSBH?DVCS2
ReDVCSBH ?DVCS2
100 days
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Projected Statistics Q29.0 GeV2, xBj 0.60
250K exclusive DVCS events total
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What systematic errors?

• At this day (June 2006)
• 3 HRSPbF2 acceptance luminosity
  target
• 3 H(e,eg)Xg p0 background
• 2 Inclusive H(e,eg)Np
• 2 Radiative Corrections
• 2 Beam polarization measurement

2 X
1 X
1 X
Total (quadratic sum) 5.1 (5.6)
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DVCS on the neutron and the deuteron - Preliminary
Q2 1.9 GeV2 lttgt -0.3 GeV2
Mx2 upper cut
It is clear that there are two contributions with
different sign DVCS on the neutron and DVCS on
the deuteron
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?0 Electroproduction Background Subtraction
H(e, e ???)X

M??

• Minimum angle in lab 4.4 (E00110)

• Asymmetric decay One high energy forward
  cluster mimics DVCS MX2!