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Title: Deep Virtual Compton Scattering Results from Hall A at Jlab nuclex0607029, submitted to PRL


1
Deep Virtual Compton Scattering Results from
Hall A at Jlabnucl-ex/0607029, submitted to PRL
APS Division of Nuclear Physics Annual Meeting,
October 25-28, 2006 Nashville, TN
  • 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
2
DVCS Kinematics and Dynamics
q?? (k-k)? Q2 ?q2 W2 (qp)2 xBj Q2
/(2p?q) t ?2 (q-q)2 ?? xBj /(2- xBj)
  • Q2 Resolution of probe.

3
From DVCS to Generalized Parton Distributions
(GPDs)
x? initial/final momentum fractions
DVCS
g
g
  • The GPDs enter the DVCS amplitude as an integral
    over x
  • GPDs appear in the real part through a
    Principal-value integral over x
  • GPDs appear in the imaginary part along the
    line xx

but its not so simple
Collins, Freund
4
Experimental observables linked to GPDs
q k-k y (q?k)/(k?p) ?? q-q
Using a polarized beam on an unpolarized target,
two observables can be measured
5
Into the harmonic structure of DVCS
TBH2
Interference term
BH propagators j dependence
Belitsky, Mueller, Kirchner
6
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 are
    incorporated into analysis.
  • 3. Angular Harmonic terms ci, si, are
    Q2-independent in leading twist (except for QCD
    evolution).

7
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
H(e,e?)N?,N??,
H(e,e?)p
resonant or not
ep?e?0p H(e,e?) ?p
8
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 for
    calibration of MC

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
9
Digital trigger on calorimeter and fast
digitizing-electronics
1GHz Analog Ring Sampler (ARS)
t (ns)
FPGA Virtual Calorimeter
PbF2 blocks
Fast Digital Trigger
10
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
11
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
  • Maintain Resolution during ? 1043/cm2 integrated
    luminosity on H2
  • Optimal timing resolution
  • 101 TrueAccidental ratio at L1037/(cm2 s)
    unshielded calorimeter

2ns beam structure
12
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
13
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
14
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, Normalized to H(e,e?)
H(e,e?p) simulation, Normalized to data
lt2 in estimate of H(e,e?)N? below threshold
MX2lt(Mm)2
15
Analysis Extraction of observables
Re-stating the problem (difference of
cross-section)
Observable
Kinematic factors
GPD !!!
16
d? Helicity Difference Extraction of
observables
Averaged over t lt-tgt0.23 GeV2, ltxBgt0.36
N?N?
17
Analysis Difference of counts two of 4 bins
in t
lttgt (GeV2) ?0.23
Q22.3 GeV2, xBj0.36
?0.37
  • Twist-3 contribution is small
  • po contribution is small
  • po is Twist-3 (d?LT)

18
Cross Section Differences
lttgt?0.33 GeV2
lttgt?0.28 GeV2
lttgt?0.23 GeV2
lttgt?0.17 GeV2
ImCI(F)exp ImCI(Feff)
  • Model independent cross section results.
  • ImCI(F)exp BHImDVCS ??s1?ImDVCSDVCS.
  • Bilinear DVCS term is Twist-3 with no BH
    enhancement
  • ??s1? ? 0.01

19
Cross SectionSum
lttgt-0.33 GeV2
Corrected for real and virtual radiation
C, ?C can include ? ???DVCS2 term ??? ? 0.05
lttgt-0.27 GeV2 lttgt-0.23
GeV2 lttgt-0.17 GeV2
20
Results t-dependence, Twist-2

Consistent with Twist-2 dominance
ImCI
?ReCI?CI
ReCI
21
Q2-dependence averaged over t lttgt-0.23 GeV2
ImCI(F ) sin? term
ImCI VGG
Hq(x,0,t) x??t q(x) ?-dependence DD, b1 Eq
0
Average of three Q2 points.
ImCI 10 bound on Twist-4 d?LT(DVCS2)
terms
Twist-3
22
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 Asymmetries contain interference and
    bilinear DVCS terms in denominator.

23
DVCS at 11 GeV (Approved by PAC30)
HALL A H(e,e?) (no proton detection) 3,4,5
pass beam k 6.6, 8.8, 11 GeV Spectrometer
HRS k4.3 GeV Calorimeter 1.5 x larger, 1.5 to
3.0 m from target Similar MX2 resolution at each
setup. 1.0 GHz Digitizer for PbF2 Calorimeter
trigger upgrade ( better p0
subtraction) Luminosity x Calo acceptance/block
4x larger. Same statistic (250K)/setup
100 Days
CLAS12 Relative Asymmetries H(e,e?p) 80 days
Long.Pol. NH3(e,e?p) 120 days
24
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
CLAS12 Relative Asymmetries
25
Hall A Projected Statistics Q29.0 GeV2, xBj
0.60
5 bins in t for 0.1lttmin-tlt0.9 GeV2 ?t 0.050.4
GeV2
250K exclusive DVCS events total, in each of 11
Q2 xBj bins.
26
Conclusions
  • Precision measurement of H(e,e?)p exclusivity
  • Precision measurement of H(e,e?)p cross sections
  • ?-dependent cross sections
  • Twist-2 cos(?) and sin(?? terms
  • Twist-3 cos(2?) and sin(2?) terms small
  • Re Im parts of BHDVCS Interference

Full Program Approved In Hall A at 11 GeV
  • cos(?) term may contain substantial
    contributions of both ReBHDVCS and Bilinear
    DVCS terms.
  • Future separation of Interference and Cross
    section terms via Generalized Rosenbluth

ImCI
?ReCI?CI
ReCI
nucl-ex/0607029, submitted to PRL
27
Answers to Questions
28
Interference Bilinear DVCS Terms
???0.05 Acceptance average of kinematic weight
of Twist-2 Bilinear DVCS term in P1P2-1cos?
BHDVCS term Model of VGG BHDVCS
ReCI BHDVCS
29
Q2-dependence averaged over t lttgt-0.23 GeV2
ImCI 10 bound on Twist-4 Twist-3
d?LT(DVCS2) terms
ImCI VGG
ImCIeff Twist-3 suppression in (tmin-t)/Q2
kinematic coefficient, not in magnitude of ltqGqgt
matrix element
30
?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!

31
Relative Asymmetry
Fit VGG Laget
Statistical errors ? 0.1 per point.
32
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)
33
Bethe-Heitler and ?0 Contributions Q22.3 GeV2
lttgt ?0.33 GeV2
lttgt ?0.28 GeV2
Data
Fit
BH
?0 H(e,e?) ?X
lttgt ?0.17 GeV2
lttgt ?0.23 GeV2
34
Analysis Calorimeter acceptance
The t-acceptance of the calorimeter is uniform at
low tmin-t
5 bins in t
Min Max Avg
Large-t j dependence
35
Total cross section and GPDs

Interesting ! Only depends on H and E
36
Q2-dependence of Twist-3 term averaged over t
lttgt-0.23 GeV2
ImCI(F eff ) sin2? term
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