P-Y BERTIN

Jefferson Laboratory and Université BLAISE PASCAL- IN2P3/CNRS

for the

DVCS HALL A collaboration

0 exclusive electro productionQ2=2.3 GeV2, XBj=.36

LH2 / LD2 targetPolarized Electron Beam

Scattered Electron

Left HRS

Electromagnetic Calorimeter

DVCS events are identified with MX

2

Beam energy = 5.75 GeV

Beam polarization = 75%

Beam current = ~ 2 and 4 μA

Luminosity = 1 and 4. 1037 cm-2.s-1 nucleon-1

- >6.3°

- Čerenkov based Electromagnetic Calorimeter

-Specific Scattering Chamber

-Customized Electronics & Data Acquisition

Two-arm experiment : spectrometer and calorimeter

Exclusivity

Raw dataSimulation X cross sections

p(e,e’0)

(Mp+m)2

Photon detection threshol correction th

q

2

'1 1

q’1 being the smallest enegy photon of Th= 1.00 GeV = 1.15 GeV = 1.25 GeV

p

At Q2=2.3 GeV2 and xbj=.36

•The continuum is significant compared to the p(e,e’0)p•The resonances are washed out into the continuum.

As seen also in DIS

Q2=2.0 GeV2

Q2=0 GeV2

(Mp+m)2

(Mp+2m )2

(Mp+3m )2

0Br=8.5%

-t GeV2 0.1 0.2 0.3

0.01

0.02

0.03

0

0

RbepepTot .1)(

b=-2 GeV-2 From Hall B

(Mp+m)2

(Mp+2m )2

(Mp+3m )2

0Br=8.5%

0Br=100%

0Br=85%

in our cut ~1% 0p

But we detect only e’0 => all the process interfere =>sum of the amplitudes What I am doing ? Semi inclusive DIS and Duality ??

For each exclusive 0 event selected in the cut on Mx2

The physic variable are determined exactly

Q2, xbj with the spectrometer t with the position in the calorimeter ~2-3 mrd)

Photon’s in the inner calorimeter (99 Block from 132)Window coincidence +/- 3 nsAccidentals esubstractedWindow 105<mMeV

td

d

dtdxdQ

d

Bj

2

3

2coscos)1(2

dt

d

dt

d

dt

d

dt

d

dt

d TTLTLT

sin)1(2 '

dt

d TL

Analyze in the formalism of on photon exchange

)()Re(cos2 2*2

*1

*2

2

2

1 FFFFT

)]cosRe(2[sin2

1 *4

*33

*24

*1

2

4

2

3*2 FFFFFFFFTT

Chew, Goldberger, Low and Nambu

Decomposition in CGLM amplitude: 0 cm angle ~ -(t-tmin)

)Re(cos2 6*

5*2

6

2

5 FFFFL

])cos()cosRe[(sin2

16

**3

*4

*15

**4

*3

*2

* FFFFFFFFTL

])cos()cosIm[(sin 6**

3*

4*

15**

4*

3*

2*

' FFFFFFFFTL

6 amplitudes complexalLongitudinFF

TransverseFFFF

6,5

4321 ,,,

rT

rL

rTT

rTL

rTL’

Reduced response functions r‘s

All the trivial kinematics dependence , photon flux, , sin,…. Taken in account in the, Monte Carlo with radiative corrections , detector resolution,….

Use the same extraction method that the used for DVCSwhich take in account the bin migration by a global linear fit on 10(t)x24(f) experimental bins

rTL~5x(rT+rL )

PRELIMINARYQ2=2.3 GeV2

xBj=0.36

=0.64

Corrected for real+virtual RCCorrected for efficiencyCorrected for acceptanceCorrected for resolution effects

LT

TL

TT

'TL

Systematic errors include : trigger threshold stability ( 1 to 1.2 GeV) missing mass cut ( .9 to 1.15 GeV2 ) extrapolation at fixed Q2 and fixed XBj. spectrometer, luminosity……

Extrapoled at fixed:

+

coupling to n, with re-scattering

Regge trajectory Exchange ( , and B1)

J. M. Laget’s Prediction underestimates the cross-sections by a factor 5

JML x5

JML x5

JML x5

JML x5

and

L

T

= ???

Next experiment 2009 will allow a full

Rosenbluth separation

s

Factorization hold only forlongitudinal amplitude

L Longitudinal part Prediction from

model (VGG) based on Hand bag model and GPD

VGG M Vanderhaeghen P. Guichon and M Gidal

VGG x 5

1

+-0 +-

2

+-0

But quid of two photons exchange?

How to check validity of the one photon exchange ??

END

Photon electroproduction

Analysis – Exclusivity check using Proton Array and MC

Normalized (e,p,)triple coincidence events

Using extra recoil Proton-detector, we have checked the missing mass spectrum of double-coincidence events with those of a triple -coincidence.

Monte-Carlo(e,)X – (e,p,)

2 cutXM

The missing mass spectrum using the Monte-Carlo gives the same position and width. Using the cut shown on the Fig.,the contamination from inelastic channels is estimated to be under 3%.

Experimental observables linked to GPDs

Experimentally, DVCS is undistinguishable with Bethe-Heitler

However, we know FF at low t and BH is fully calculableInterference term allows access to linear amplitude Using a polarized beam on an unpolarized target 2 observables can be measured:

42

2

4 4

2

2

m2 I

ReBH BH D

DVC

B

BH

C

S

V S

B

dT T

dx dQ dtd

d dT

dx dQ d

T

tdT

At JLab energies,|TDVCS|2 was supposed small

42 2

2

4 42 2

2

2

Im2

Re DVCBH BH DVCS

B

BH DVCS DVCSDVCS

B

SdT T T

dx dQ dtd

d dT T T

dx dQ

T

tT

d d

Kroll, Guichon, Diehl, Pire, …

1

42

1 0 1 22

22

24 4

2

1 2

1 2

0 1 2 3

1 22

1( , , ) cos cos 2

1 ( , , ) cos cos 2 cos3

( ) ( )

( ) (

( , , )sin sin 2

)

( ) (

)

BH BH BHB

B

BI I I I

I IB

B

dx Q t c c c

dx dQ dtd

x Q t

x Q td d

dx dQ dtd

c c c c

s s

Tests of scaling

1. Twist-2 terms should dominate and All coefficients have Q2 dependence which can be tested!

Difference of cross-sections

2 22.3 GeV

0.36B

Q

x

Corrected for real+virtual RCCorrected for efficiencyCorrected for acceptanceCorrected for resolution effectsChecked elastic cross-section @ ~1%

Twist-2Twist-3

Extracted Twist-3contribution small !

PRL97, 262002 (2006)

New work by P. Guichon !

Q2 dependence and test of scaling

<-t>=0.26 GeV2, <xB>=0.36

No Q2 dependence: strong indication forscaling behavior and handbag dominance

Twist-2Twist-3

Total cross-section 2 22.3 GeV

0.36B

Q

x

Corrected for real+virtual RCCorrected for efficiencyCorrected for acceptanceCorrected for resolution effects

Extracted Twist-3contribution small !

PRL97, 262002 (2006)

And it is impossible to disentangle DVCS2 from the interference term

22

2

4

)Re(2 DVCSDVCSBHBH TTTT

dtddxdQ

d

large

We have proposed to use different beam energies (different BH) to :

( experiment approved and planned to run end 2009)

1. Isolate the BH-DVCS interference term from the pure DVCS2

Contribution (as a function of Q2)

• Extraction of both linear and bilinear combinaton of GPDs • Additional test of DVCS scaling ( unpolarized cross section)

2. Measure 5 response functions of the deep virtual 0 channel

•First test of factorization in ep ep0 using L

•If test is positive , valuable complementary ( flavor) information in GPDs

This proposal: assuming DVCS2=20

On the deuterium

Deuterium=proton+neutron+deuteron

- Hydrogen=proton

= neutron + deuteron

Mn2 Mn

2+t/2Missing mass assuming a proton target

2

0( ) ( )hS N N d N N d

Helicity Asymmetry

n-DVCS

d-DVCS

PRELIMINARY

PRELIMINARY

Deuteron contribution compatible with zero at large -t

F. Cano & B. Pire calculation

Eur. Phys. J. A19, 423(2004).

PRELIMINARY

Neutron contribution is small and compatible with zeronnn HHxE~

0.346.0)(

Results can constrain GPD models (and therefore GPD En)VGG Code : M. Vanderhaeghen, P. Guichon and M. Guidal

PRELIMINARY

peep

DeeD

neen

peep

)',(

)',(

)',(

)',(

0

the Scaling test is positive

Transverse -Transverse large. Description in terms of Quarks GPD and Hadronic description ( Regge exchange) miss by a factor 5~15 the data .

K

K VGG model misses by 30%

DVCS2 must be taken into account

0 KBKM

nnn HHxE ]~

0.346.0)([

0 KAgree with F. Cano B. Pire model

To summarize

New data taking in 2009 using 2 beam energies:

Full extraction of linear terms and bilinear terms of GPDs

Full separation of T and L for 0 electro production

At Q2=1.5, 1.9, 2.3 GeV2

We have demonstrated that : high precision DVCS measurements are doable using a high resolution spectrometer and a calorimeter

Full DVCS program in Hall A (up to Q2=9 GeV2) already approved with the 12 GeV upgrade

END

100 150 MeV

0 Invariant mass

FWHM=21 MeV

(Mp+m)2

p(e,e’0)

Raw dataSimulation X cross sections

Photon detection threshol correction th

q

2

'1 1

q’1 being the smallest enegy photon of

DVCS Analysis

Normalized (e,p,)triple coincidence events

Check of the missing mass spectrum of double-coincidence events with the a triple -coicidence using a Auxilliary Proton array

Monte-Carlo(e,)X – (e,p,)

2 cutXM

The missing mass spectrum using the Monte-Carlo gives the same position and width. Using the cut shown on the Fig.,the contamination from inelastic channels is estimated to be under 3%.

Raw

Raw –0

(e,p,)

After :

-Normalizing H2 and D2 data to the same luminosity

-Adding Fermi momentum to H2 data

2 principle sources of systematic errors :

-The contamination of π0 electroproduction on the neutron (and deuteron).

- The uncertainty on the relative calibration between H2 and D2 data

A. Belitsky,D Muller A Kirchner Compton form factor :

nn

Bj

Bjnunp

I EtFM

tHtFtF

x

xHFC )(

4

~))()((

2 2

2

211

Deuteron contribution compatible with zero at large -t

F. Cano & B. Pire calculation

Eur. Phys. J. A19, 423(2004).

PRELIMINAY

Neutron contribution is small and compatible with zero

Results can constrain GPD models (and therefore GPD En)

Analysis – Exclusivity check using Proton Array and MC

Normalized (e,p,)triple coincidence events

Using extra recoil Proton-detector, we have check the missing mass spectrum of double-coincidence events with the a triple -coicidence .

Monte-Carlo(e,)X – (e,p,)

2 cutXM

The missing mass spectrum using the Monte-Carlo gives the same position and width. Using the cut shown on the Fig.,the contamination from inelastic channels is estimated to be under 3%.

p

At Q2=2.3 GeV2 and xbj=.36

•The continuum is significant compared to the p(e,e’0)p•The resonances are washed out into the continuum.

As seen also in DIS

Q2=2.0 GeV2

Q2=0 GeV2

After :

-Normalizing H2 and D2 data to the same luminosity

-Adding Fermi momentum to H2 data

2 principle sources of systematic errors :

-The contamination of π0 electroproduction on the neutron (and deuteron).

- The uncertainty on the relative calibration between H2 and D2 data

2

0( ) ( )hS N N d N N d

Helicity Asymety

Mx2 Mx

2+t/2Missing masse assuming a proton target

Deuterium=proton+neutron+Deuton

- Hydrogen=proton

= neutron + deuton

2 (1 ) cos cos2L TT L TTddt

d dd

ddt tt

ddtd

)()(cos2 2*

2*

1*2

2

2

1 FFFFT

)]cos(2[sin2

1 *4

*33

*24

*1

2

4

2

3*2 FFFFFFFFTT

Chew, Goldberger, Low and Nambu ==> CGLM

Decomposition in CGLM amplitude:

0 cm angle ~ -(t-tmin)

)(2sin *2 LT

TT

k

k’

q=k-k’

q’