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Deep Virtual Compton Scattering at Jlab Hall A

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### Deep Virtual Compton Scattering at Jlab Hall A

QCD Structure of the Nucleon

12-16 June 2006

Villa Mondragone, Italy

Charles E. Hyde-Wright

Old Dominion University, Norfolk VA

chyde@odu.edu

Based on the work of A. Camsonne

the DVCS Hall A Ph.D. students: M. Mazouz

C. Munoz Camacho

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. Bdeuteron=2.2 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.

QCD, Confinement, and the Origin of Massk extreme short distance with perturbative QCD

k’

q’

p’

p

Experimental observables linked to GPDsq = k-k’ Q2 = q2>0 =q-q’ t=2

s = (k+p)2xBj = Q2/(2p·q) W2 = (q+p)2

Using a polarized beam on an unpolarized target,

2 (actually 6) observables can be measured:

At JLab energies, |TDVCS|2 is small:

|TDVCS|2 / |TBH|2 ≈ -t xBj2 s2 / Q6

M. Diehl, yesterday

k extreme short distance with perturbative QCD

k’

q’

p’

p

hadronic plane

j

g

e-’

g*

e-

p

leptonic plane

Into the harmonic structure of DVCS|TBH|2

Interference term

BH propagators j dependence

Belitsky, Mueller, Kirchner

Tests of the handbag dominance extreme short distance with perturbative QCD

+ VdT(DVCS) + dTT(DVCS) cos(2)

+ VdLT’(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).

If the Missing Mass resolution is good enough, a tight cut removes the associated pion channels, but deep virtual po electroproduction still must be be subtracted with a statistical sample.

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

Hall A DVCS philosophy removes the associated pion channels, but deep virtual

- 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

5. Digitize Waveform removes the associated pion channels, but deep virtual

6. Pulse fit

Digital trigger on calorimeter and fast digitizing-electronics1. HRS Trigger

2. ARS Stop

In

1GHz Analog Ring Sampler

(ARS)

t (ns)

4. Validate or Fast Clear (500ns)

3. S&H60ns gate

FPGA Virtual Calorimeter

PbF2 blocks

Z>>50

Fast Digital Trigger

4. Find 2x2 clusters>1GeV

Vertex resolution removes the associated pion channels, but deep virtual

1.2mm

Pbeam=75.32% ± 0.07% (stat)

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

HRS-Calo removes the associated pion channels, but deep virtual

coincidence

st=0.6 ns

Dt (ns)

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
- 10:1 True:Accidental ratio at L=1037/(cm2 s) unshielded calorimeter

2ns beam structure

E00-110 kinematics removes the associated pion channels, but deep virtual

The calorimeter is centered

on the virtual photon direction.

Acceptance: < 150 mrad

50 days of beam time in the fall 2004, at 2.5mA intensity

H(e,e’ removes the associated pion channels, but deep virtual ) Y

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,eY 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’ removes the associated pion channels, but deep virtual ) 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

<2% in estimate of

H(e,e)N…

below threshold MX2<(M+m)2

Analysis – Extraction of observables removes the associated pion channels, but deep virtual

Re-stating the problem (difference of cross-section):

Observable

Kinematic

factors

GPD !!!

Y removes the associated pion channels, but deep virtual calo (cm)

Calorimeter

Xcalo (cm)

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

d removes the associated pion channels, but deep virtual Difference: Extraction of observables

Averaged over t

<-t>=0.23 GeV2, <xB>=0.36

Acceptance removes the associated pion channels, but deep virtual

effects

included in fit

Analysis – Difference of counts – 2 of 4 bins in t

- Twist-3 contribution is small
- po contribution is small
- po is Twist-3 (dLT’)

with removes the associated pion channels, but deep virtual

Total cross section and GPDs

| |

Interesting !

Only depends on H and E

Conclusion at 6 GeV removes the associated pion channels, but deep virtual

- High luminosity (>1037) 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 Re[BH*DVCS]+|DVCS|2
- = [d(h=+) + d(h=-) ] ≠ |BH|2
- Relative Asymmetry contains DVCS terms in denominator.

Hall A at 11 GeV (in preparation for PAC30 removes the associated pion channels, but deep virtual

HALL A: H(e,e’)

3,4,5 pass beam: k = 6.6, 8.8, 11 GeV

Spectrometer: HRS: k’≤4.3 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

Unphysical removes the associated pion channels, but deep virtual

JLab12: Hall A with 3, 4, 5 pass beamAbsolute measurements: d(e=±1)

250K events/setup

H(e,e’)p

Twist 2 & Twist 3 separation.

Im{DVCS*BH}+DVCS2

Re{DVCS*BH} +’DVCS2

100 days

Projected Statistics: Q removes the associated pion channels, but deep virtual 2=9.0 GeV2, xBj = 0.60

250K exclusive DVCS events total

For the future experiments removes the associated pion channels, but deep virtual

3.6 - 3.7 %

What systematic errors?

- At this day (June 2006):
- 3% HRS+PbF2 acceptance +luminosity + target
- 3% H(e,e’g)Xgp0 background
- 2% Inclusive H(e,e’g)Np
2%Radiative Corrections

- 2% Beam polarization measurement

2%X

1% X

1%X

Total (quadratic sum)= 5.1% (5.6%)

1 removes the associated pion channels, but deep virtual st cut

2nd cut

DVCS on the neutron and the deuteron - Preliminary

Q2= 1.9 GeV2

<t>= -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

removes the associated pion channels, but deep virtual 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!

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