Deeply Virtual Compton Scattering from the Proton and the Neutron

(E00-110 & E03-106: DVCS/nDVCS)

Hall A Collaboration Meeting December 6, 2005

Spokespersons: Pierre-Yves Bertin, Charles Hyde-Wright, Ron Ransome, Franck Sabatie, and Eric Voutier

Elena Kuchina

Rutgers, The State University of New Jersey

Hall A DVCS at 6 GeV:Methods and Goals

E00-110:

Measure the absolute cross section of DVCS on proton E00-110 (3 Q² values: 1.4, 1.9, 2.3 GeV²) and on neutron E03-106 (Q²=1.9 GeV²)

E03-106: Simplest access to the least known of GPDs: E First constraint of nucleon orbital angular momentum through model of E

Check Handbag dominance & Test factorization of DVCS Deduce Q² dependence and relative importance of leading twist and higher twists in helicity dependent cross-section Measure linear combination of GPD’s

…including Re (DVCS)

Unique characteristics:

High luminosity (up to 4·1037 cm-2s-1) Well defined kinematics (high resolution detection of e- and γ)

Method: Measure weighted cross-section differences as function of φ angle Deduce Q2 dependence & relative importance of twist-2 and higher twists

Goal:

DVCS experimental setup

contains BH propagators and some kinematics

B contains twist-3 terms

A is a linear combination of three GPDs evaluated at x=ξ

)Γ( 2 ,,y,xBj

2sinsin),,,( 2

22BAyx

ddyddx

d

ddyddx

dBj

BjBj

�

planes photonic and leptonic ebetween th angle the-

and , , , 2

with 2

pppk

pqy

qp

QxBj

E)(4

H~

))()((2

H)( 22211

tFM

ttFtF

x

xtFA

Bj

Bj

Cross-section difference assuming handbag dominance

s (GeV²)

Q² (GeV²)

Pe (Gev/c)

Θe (deg)

-Θγ* (deg)

4.94 2.32 2.35 23.91 14.80 5832

4.22 1.91 2.95 19.32 18.25 4365

3.5 1.5 3.55 15.58 22.29 3097

4.22 1.91 2.95 19.32 18.25 24000

E00-110p-DVCS9-11 ‘04

E03-106n-DVCS11-12 ‘04

xBj=0.364

Kinematics is fully defined

Beam polarization was about 75.3% during the experiment

Ldt (fb-1)

5.7572 GeVpolarized

110 cm

LH2 (LD2) e' (LHRS)

p'(n')

Experiment kinematic settings

Analysis Method

ee

NNY e iii )(exp

MC includes radiative corrections (both external and internal).

2sin),(),,,(sin),(),,,( 32

322

2 BjBjBjBj xtCtQxxtCtQx

ee xx

eMC CCLY

ii

i .Acc2sin .Accsin )( 3322

3

2

2

2

2

)(

)()(

C

CYY

e eExp

eMC

eExp

i i

ii

Calorimeter acceptance

Acceptance in φis very dependent on t

for large -t

Preliminary results on p-DVCS•Difference if Yields N+-N- :

•3 bins in Q2 x 5 bins in t (x 50 bins in φ)

• 0.5< Mx² < 1.2 GeV2

• relatively small contribution of the twist-3 term

• Q2 dependence of the twist-2 and twist-3 terms:

• Twist-2 term (high precision) ~ 12% variation

• Twist-3 term (20-30% uncertainty) is ~ 0

• handbag dominance test

•Twist-2 absolute value & t-dependence is well reproduced by VGG model.

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

2 4B

B

x tA F t F t F t F t

x M

H H E

φ

LD2 target – LH2 target

Possible neutron signal !

0.5 GeV2 < missing mass 2 < 1.5 GeV2

Absolute cross sections necessary to extract helicity dependence of neutron

n-DVCS – Very preliminary

1.5 GeV2 < missing mass 2 < 2.5 GeV2

2.5 GeV2 < missing mass 2 < 3.5 GeV2 NO signal

To do Acceptance checks

Virtual radiative corrections

Contribution from πº

Total cross-section: access to real part of DVCS and other cross-section coefficients

Polarized cross sections to extract GPD E

Relative asymmetry considering Proton Array and Tagger

Summary and Conclusions

Very exciting times where first high precision data is coming out.Test of handbag dominance t-dependence and access to linear combinations of GPDs

DVCS is a golden probe. New exclusive experiments in different kinematic domains and with different observables (BSA, BCA,TSA…) are needed in order to fully map out GPDs.