A future n-DVCS experiment in Hall A

Malek MAZOUZ

October 15th 2007

What was done in E03-106

Motivations

Experimental upgrades

Projected Results

DVCS meeting

Faculté des Sciences de Monastir – Tunisia

Deeply Virtual Compton Scattering

1

1

( , , )DVCS GPD x tT dx

x i

1

1 ...

( , )( , , )

,i G

GPD x tP

xPDx x td

The GPDs enter the DVCS amplitude as an integral over x :

GPDs appear in the real part through a PP integral over x

GPDs appear in the imaginary part at the line x=±ξ

kk’

q’

GPDsp p’

factorization

Simplest hard exclusive process involving GPDs

Mueller, Radyushkin, JiCollins, Freund, Strikman

What could be done at JLab Hall A

2 25 5 Im2 ( .B DVCS DH DV VCSCST T )d σ d σ T T

����

2 25 Re2 . ( )BH BH D SS VCDVCTd T T T

The cross-section difference accesses the Imaginary part of DVCS and therefore GPDs at x=±ξ

The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x

Purely real and fully calculable

Twist-3 term

Kroll, Guichon, Diehl, Pire …

Twist-2 term Bilinear combinations of GPDs

Expression of the cross-section difference

Combination to be extracted from the data

I5 5

12 21 2

sin( )

1 1

2

m C( ) ( )

twist-3 term sin( , 2 )s

I

B e B e

d d

dQ dx dtd d dQ dx dtd d P P

��

1 1 2 22( ) ( ) ( ) ( )

4I t

C F t F t F t F tM

H H E

2 ( , , ) ( , )m ,qq

q

qE te E t E

GPDs

n-DVCS experiments provide different linear combinations of GPDs than p-DVCS experiments.

n-DVCS experiments have different flavor sensitivity than p-DVCS experiments.

A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).

E03-106 experimental apparatus and kinematics

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 = ~ 4 μA

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

2 2

2 2

2

1.9 GeV

0.36

4.22 GeV

0.1 0.5 GeV

B

Q

x

W

t

I.A. and DVCS on coherent deuteron

, ' , ' , ' , 'D e e X d e e d n e e n p e e p Impulse approximation :

With a Deuterium target one can have 3 different DVCS processes

p

nd

n

dd dd

p-DVCS d-DVCSn-DVCS(incoherent) (coherent)(incoherent)

Access deuteron GPDs

overlap < 3%

Final state interaction effects between a pn pair should be small

(GeV/c)p

prob

abili

ty

eD e X

p-DVCS and n-DVCS

accidentals

MN2

MN2 +t/2

d-DVCS

Analysis method

0e X XeD e Contamination by

Mx2 cut = (MN+Mπ)2

N + mesons(Resonnant or not)

Extraction of observables

2

exp exp

2 2

2 2

2

1

2

sin( , , , ) ( ,m s, , ) inmI In

B

B

e

B d

B

d e

e

e nx x

d d

d

C C

Q dx d d d dQ dx d d d

��

exp exp

( )

( ) .sin .sinm m

e e

e e

I In n

Expe i i

Mdi d

Ce x x i

N i N N

N i L Acc A cC cC

MC sampling MC samplingLuminosity

220 12 7 bins in , ,e Xi M t with

Under the MX2 cut :

A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).A. Kirchner, D. Muller, Eur. Phys. J. C32, 347 (2004).

, ' , ' , ' , 'D e e X H e e X n e e n d e e d

Fit of :

by :

What was done : E03-106 results

Deuteron moments compatible with zero at large |t|

Neutron moments are small and compatible with zero

Results can constrain GPD models (and therefore GPD E)

- Large systematic error due to the uncertainty on the relative calibration between the LH2 and the LD2 data.

- Large systematic error due to the uncertainty on the contamination of the DVCS-like π0 channel.

M. Mazouz et al., submitted to PRL.arXiv0709.0450

n-DVCS is sensitive to Jd

p-DVCS is sensitive to Ju

Complementarity between neutron and proton measurements

What was done : E03-106 results

Model dependent extraction of Ju and Jd

What is interesting to do now

kk’

q’

GPDsp p’

factorization

Already determined in E03-106

Sensitive to x=±±…

1

1

( , , )DVCS GPD x tT dx

x i

1

1 ...

( , )( , , )

,i G

GPD x tP

xPDx x td

2 25 Re2 . ( )BH BH D SS VCDVCTd T T T

Expression of the unpolarized cross section

5

2

0 1 221 2

1

( ) ( )

twi

e e

st-3 terms gluon terms

cos(

)

DVCS

B e

dBH

dQ dx dC C C

td d PC

P

1 1 2 22( ) ( ) ( ) ( )

4I t

C F t F t F t F tM

H H E

21 1 2 22( ) ( ) ( ) ( )

4I I t

C C F t F t F t F tM

H H E E

2

2 22 22 2

4(1 ) 2 e1

2 24 4

B BDVCS

B B B B

x x

C t tx x x x

M M

HH HH HE HE

EE EE

1

2

1

1 1e , , f

ffe dx

xH x

xt

H GPDs

In the nucleon case :

A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).

What was NOT done in E03-106 and why

5 exp2

0 1 221 2

2

0 1 21 2

e e

1cos( )

( ) ( )

1cos( )

( ) ( )e e

I I I DVCS

I I I DVCS

n n n n n nB

n n

d d d d d d d

e

d

dBH

dQ dx dtd d P P

BH

C C C C

C C C CP P

Cannot be separated with a φ analysis

Six twist-2 coefficients to be extracted from the data

The large systematic error (>50%) of E03-106 did not allow a significant extraction of unpolarized cross sections.

The DVCS2 and the interference terms have a similar φ dependence and therefore cannot be separated.

We propose to perform a new n-DVCS experiment at the same kinematics than E03-106 (Q2=1.9 GeV2 and xB=0.36) but with 2 different beam energies (4.82 GeV and 6 GeV) and with 5% systematic errors.

Interference and DVCS2 Separation

0 21 2

1( ) ( ) n nP P

The Γ factors depend on the beam energy while the GPD combinations do not.

Different beam energies allow to separate the DVCS2 and the interference terms

C. Muňoz Camacho et al., E07-007.

Accessible observables

Neutron GPDs integral

Determine twice as accurately. I In dandm C m C

Measure the polarized cross sections of exclusive π0 electroproduction on the neutron and coherent deuteron.

n nI

nI Iande C e C C Linear combinations :

Bilinear combinations :S

nDVCC

Deutron GPDs integral ?

d dI

dI Iande C e C C Linear combinations :

Bilinear combinations :S

dDVCC

Model Calculation for n-DVCS

DVCSnC

with GPD E

without GPD E

VGG model : Goeke et al., Prog. Part. Nucl. Phys 47 (2001), 401.M. Vanderhaeghen, P.A.M. Guichon, M. Guidal, Phys. Rev. D 60 (1999), 094017.

4.82 GeVk

6.00 GeVk

GPD Ě contribution ≈ 30%

In VGG model, the DVCS2 contribution is larger than the interference one.

DVCS2/BH2 depends on the beam energy

Experimental upgrades

We will use the experimental apparatus of the previous DVCS experiments with a few modifications and upgrades ( already planed for the approved p-DVCS experiment)

50% expanded calorimeter (13 x 16 blocks instead of 11 x 12 blocks) Improve the π0 contamination subtraction at high -t

No recoil detectors will be used Reduction of the experimental dead time

Faster electronics to improve the data record and transfer

New trigger logic (higher threshold + specific trigger for π0)Detection of enough π0 to subtract correctly the contamination and determine the π0 electro-production polarized cross sections.

Remove the systematic error due to the uncertainty on the contamination of DVCS-like π0 channel.

Calorimeter Calibration

Elastic calibration H(e,e’p) : will be performed periodically.

We have 2 independent methods to continuously check and correct the calorimeter calibration

1st method : missing mass of D(e,e’π-)X reaction2nd method : Invariant mass of 2 detected photons in the calorimeter (π0)

Calorimeter blocks with radiation damage will be cured with blue light.

Will help to keep a good resolution.

Frequent swap of LH2 and LD2 target

Remove the systematic error due to the uncertainty on the relative calibration between LH2 et LD2 data

Projected results

Target Beam energy (GeV)

LH2 4.82 5250

LH2 6.00 1750

LD2 4.82 13500

LD2 6.00 13500

Ldt (fb-1)

27000 fb-1

7000 fb-1

Projected results

previous systematics

new systematics

Ine C I I

n ne C C DVCSnC

Ide C I I

d de C C DVCSdC

The error bars are computed with the E03-106 experimental resolution

Requested kinematics and beam time

Target Beam energy (GeV)

k’ (GeV/c)

q’ (GeV)

W2 (GeV2)

Θe (deg)

-Θγ* (deg)

Requested beam time

(hours)

LH2 4.82 2.01 2.73 4.26 25.60 16.07 90 (approved)

LH2 6.00 3.19 2.73 4.26 18.13 18.45 30 (approved)

LD2 4.82 2.01 2.73 4.26 25.60 16.07 200LD2 6.00 3.19 2.73 4.26 18.13 18.45 200

400Total requested beam time

2 2 an1.9 GeV =0.3 d 6BQ x

Beam current = 4 μA

Summary

The results of the DVCS experiments in Hall A prove that, with our experimental apparatus, we are able to measure the DVCS polarized cross sections and extract GPDs from these quantities.

Unfortunately, this measurement was not very significant in E03-106 because of very large systematics… But we know how to remove these systematics in a future experiment.

n-DVCS experiments appear as a mandatory step towards a better knowledge of the nucleon structure.

The unpolarized n-DVCS cross section represents a valuable source of information about GPDs (GPDs integral)

A future n-DVCS experiment will also provide interesting results about d-DVCS and π0 electroproduction on the neutron and deuteron.

END Of the first part

Proposed kinematics and luminosity

Target Beam energy (GeV)

LH2 4.82 5250

LH2 6.00 1750

LD2 4.82 13500

LD2 6.00 13500

27000Total LD2 Luminosity

2 2 an1.9 GeV =0.3 d 6BQ x

Same kinematics than E03-106 but with 2 beam energies

Ldt (fb-1)

Extraction of observables

5 exp2

0 1 221 2

2

exp exp exp

0 1 21 2

exp exp exp

e e

1cos( )

( ) ( )

1cos( )

( ) ( )e e

n n n n n n n n

d d

I I I DVCS

I I I DVCS

B e

d d d d d d

dBH

dQ dx dtd d P P

BHP P

C C C C

C C C C

2

22

( ) ( )

( )e

Exp MCe e

Expi

e

N i N i

i

exp expI In d

exp expI I I In n d d

DVCSexp DVCSexpn d

e C , e C

e C C , e C C

C , C

Same extraction procedure than the previous experiment one:Fit of the experimental distribution by a Monte Carlo simulation within a global analysis involving a binning on

( )ExpeN i ( )MC

eN i2

e Xt M ki

Additional binning on the beam energy

Analog Ring Sampler

1 GHz Analog Ring Sampler (ARS)

x 128 samples x 289 detector channels

Sample each PMT signal in 128 values (1 value/ns)

Extract signal properties (charge, time) with a wave form Analysis.

Allows to deal with pile-up events.

Analysis method

Adjust the calibration and the resolution of H2 and D2 data relatively to each other.

Systematic error due to the relative calibration between H2 and D2 data

Add the Fermi motion to the target nucleon for H2 data

Nb

of

coun

ts

Invariant mass (GeV)

Variation of calibration coefficients during the experiment due to radiation damage.

Cal

ibra

tion

varia

tion

(%

)

Calorimeter block number

Solution : extrapolation of elastic coefficients assuming a linearity between the received radiation dose and the gain variation

By selecting n(e,e’π-)p events, one can predict the energy deposit in the calorimeter using only the cluster position.

a minimisation between the measured and the predicted energy gives a better calibration.

2

( , ' )D e e X

Analysis method

Add the Fermi motion to the target nucleon for H2 data

Subtract the π° contamination

Analysis method

Substract H2 data from D2 data

The π° contamination is treated as a systematic error

0

00.5

n n

p p

e e

e e

Exclusivity and helicity signal2

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

sin(φ) and sin(2φ) moments

Results are coherent with the fit of a single sin(φ) contribution

n-DVCS results from E03-106

d-DVCS results from E03-106

Prediction from F. Cano and B. Pire.

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

Experimental results

+E03-106

E03-106 results

π0 to subtract

π0 contamination subtraction

Mx2 cut =(Mp+Mπ)2

H2 data

Subtraction of 0 contamination (1 in the calorimeter) is obtained from a phase space simulation which weight is adjusted to the experimental 0 cross section (2 in the calorimeter).

Model Calculation for n-DVCS

4.82 GeV k

Model Calculation for n-DVCS

6.00 GeV k

Projected results

Projected results

Model Calculation for d-DVCS

Cano-Pire model : F. Cano and B. Pire, Eur. Phys. J. A 10 (2004), 423.

Ide C

I Id de C C DVCS

dC

Idm C

Model Calculation for d-DVCS

4.82 GeV k

Model Calculation for d-DVCS

6.00 GeV k

Projected results (KIN 1)

Projected results (KIN 1)

Projected results (KIN 3)

Projected results (KIN 3)

Projected results

Resolution effect

Calorimeter energy calibration

We have 2 independent methods to check and correct the calorimeter calibration

1st method : missing mass of D(e,e’π-)X reaction

Mp2

By selecting n(e,e’π-)p events, one can predict the energy deposit in the calorimeter using only the cluster position.

a minimisation between the measured and the predicted energy gives a better calibration.

2

Calorimeter energy calibration

2nd method : Invariant mass of 2 detected photons in the calorimeter (π0)

π0 invariant mass position check the quality of the previous calibration for each calorimeter region.

Corrections of the previous calibration are possible.

Nb

of

coun

ts

Invariant mass (GeV)

5 5

1I

DVCS

22 21 2

1

I sin( ) sin(2 )1 1

2 ( ) ( )

m C ( )

m C ( )

m C ( si , n( ) )

I I

B

eff

eff

e B e

DVCS

d d

dQ dx dtd d dQ dx dtdF

F F

dF

P P

��