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Deep Virtual Compton Scattering at Jlab Hall A


Elastic Electro-Weak Form Factors measure ... 75% polarized 2.5uA electron beam 15cm LH2 target Left Hall A HRS ... 2% Beam polarization ... – PowerPoint PPT presentation

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Title: Deep Virtual Compton Scattering at Jlab Hall A

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

Based on the work of A. Camsonne the DVCS Hall A
Ph.D. students M. Mazouz C. Munoz Camacho
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
  • 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.

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
Into the harmonic structure of DVCS
Interference term
BH propagators j dependence
Belitsky, Mueller, Kirchner
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
  • 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

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
resonant or not
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
Emitted photon The calorimeter has a
simple rectangular acceptance
R-function cut
Acceptance matching by design ! Virtual photon
 acceptance  placed at center of calorimeter
Simply t radius j phase
Digital trigger on calorimeter and fast
1. HRS Trigger
2. ARS Stop
1GHz Analog Ring Sampler (ARS)
t (ns)
4. Validate or Fast Clear (500ns)
3. SH 60ns gate
FPGA Virtual Calorimeter
PbF2 blocks
Fast Digital Trigger
4. Find 2x2 clustersgt1GeV
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 
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
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
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
H(e,e?) Exclusivity
H(e,e?)X - H(e,e?)?Y Missing Mass2
H(e,e?p) sample
H(e,e?p) simulation, Normalized to data
lt2 in estimate of H(e,e?)N? below threshold
Analysis Extraction of observables
Re-stating the problem (difference of
Kinematic factors
GPD !!!
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
d? Difference Extraction of observables
Averaged over t lt-tgt0.23 GeV2, ltxBgt0.36
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)

Total cross section and GPDs

Interesting ! Only depends on H and E
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

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
100 Days
JLab12 Hall A with 3, 4, 5 pass beam
Absolute measurements d?(?e1) 250K events/setup
Twist 2 Twist 3 separation. ImDVCSBH?DVCS2
100 days
Projected Statistics Q29.0 GeV2, xBj 0.60
250K exclusive DVCS events total
What systematic errors?
  • At this day (June 2006)
  • 3 HRSPbF2 acceptance luminosity
  • 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)
(No Transcript)
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
?0 Electroproduction Background Subtraction
H(e, e ???)X

  • Minimum angle in lab 4.4 (E00110)
  • Asymmetric decay One high energy forward
    cluster mimics DVCS MX2!