Deeply Virtual Compton Scattering on the neutron

Malek MAZOUZ

LPSC Grenoble

EINN 2005 September 23rd 2005

GPDs properties, link to DIS and elastic form factors

),,(~

,~

, , txEHEH qqqq Generalized Parton distributions

Link to DIS at =t=0

)()()0,0,(~

)()()0,0,(

xqxqxH

xqxqxHq

q

Link to form factors (sum rules)1

1

1

1

2

1

1 1

1 1

( , , ) ( )

( , , ) ( )

( , , ) ( ) , ( , , ) ( )

q q

q q

q q q qA A

dxH x t F t

dxE x t F t

dx H x t g t dx E x t h t

1

1

1 1( , ,0) ( , ,0)

2 2q q

q q qJ L xdx H x E x

Access to quark angular momentum (Ji’s sum rule)Quark correlations !

Brief overview of the theory

222

2222

)'(

)'(

Qppt

MkkqQ

k k’ q’

GPDsp p’

Simplest hard exclusive process involving GPDs

pQCD factorization theorem

Perturbative description

(High Q² virtual photon)

Non perturbative description by

Generalized Parton Distributions

Bjorken regime

X. Ji, Phys. Rev. DS56 (1997) 5511A. Radyushkin, Phys. Lett. B380 (1996) 417

x

2

22

22

B

B

B

x

x

M

Q

pq

Qx

fraction of longitudinal momentum

),,( txGPDix

dxDVCS

amplitude

What can be done at JLab Hall A

(DVCS)BHσdσd Im.55 �

Using a polarized electron beam:Asymmetry appears in Φ

K. Goeke, M.V. Polyakov and M. Vanderhaeghen

Direct handle on the imaginary part of the DVCS amplitudeEnhanced by the full magnitude of the BH amplitude

Purely real

BHDVCSBHd ).Re(.225

-High luminosity

-High precision measurement

cross-section difference in the handbag dominance

22 2

2

( , , , ) sin sin 2

with / 2 , / , ' ,

and the angle between the leptonic and photonic planes

BB B

B

d dx y A B

dx dyd d dx dyd d

x Q p q y q p k p p p

��

Γ contains BH propagators and some kinematics

B contains higher twist terms

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

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

2 4B

B

x tA F t F t F t F t

x M

H H E

0.1 1.34 0.81 0.38 0.04

0.3 0.82 0.56 0.24 0.06

0.5 0.54 0.42 0.17 0.07

0.7 0.38 0.33 0.13 0.07

Proton Target

Proton

2 ( )pF t 1 ( )pF t 1 2( ) ( ) /(2 )p pB BF t F t x x 2

2( / 4 ) ( )pt M F t t

t=-0.3

Target

Proton 1.13 0.70 0.98

H H E

Goeke, Polyakov and Vanderhaeghen

Model:

2 22 GeV

0.3

0.3B

Q

x

t

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

2 4B

B

x tA F t F t F t F t

x M

H H E

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

2 4

0.64 0.17 + 0.06

B

B

x tA F t F t F t F t

x M

A

H H E

0.1 -1.46 -0.01 -0.26 -0.04

0.3 -0.91 -0.04 -0.17 -0.06

0.5 -0.6 -0.05 -0.12 -0.08

0.7 -0.43 -0.06 -0.09 -0.08

Neutron Target

Neutron

2 ( )pF t 1 ( )pF t 1 2( ) ( ) /(2 )p pB BF t F t x x 2

2( / 4 ) ( )pt M F t t

t=-0.3

Target

Proton 0.81 -0.07 1.73

H H E

Goeke, Polyakov and Vanderhaeghen

Model:

2 22 GeV

0.3

0.3B

Q

x

t

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

2 4B

B

x tA F t F t F t F t

x M

H H E

1 2( ) ( ) !!!n nF t F t

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

2 4

0.03 0.01 0.12

B

B

x tA F t F t F t F t

x M

A

H H E

neutron

Experiment status

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

proton

neutron

xBj=0.364

Beam polarization was about 75.3% during the experiment

E00-110 (p-DVCS) was finished in November 2004 (started in September)

E03-106 (n-DVCS) was finished in December 2004 (started in November)

Ldt (fb-1)

Left High Resolution Spectrometer

LH2 or (LD2) target

Polarized beam

Electromagnetic Calorimeter (photon detection)

Scintillator Array(Proton Array)

Experimental method

(proton veto)Scintillating paddles

scattered electron

( )

e p e p

e D e n p

photon

Proton:(E00-110)

Neutron:(E03-106)

Only for Neutron experimentCheck of the recoil nucleon position

recoil nucleon

Reaction kinematics is fully defined

Calorimeter in the black box

(132 PbF2 blocks)

Proton Array

(100 blocks)

Proton Tagger

(57 paddles)

High luminosity measurement

2 13710L cm s

123710.4 scmLnucleon

Up to

At ~1 meter from target (Θγ*=18 degrees)

Requires good electronics

Low energy electromagnetic background

PMT

G=104

x10 electronics

Electronics

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.

Electronics

Calorimeter trigger

Not all the calorimeter channels are read for each event

Following HRS trigger, stop ARS.

30MHz trigger FADC digitizes all calorimeter signals in 85ns window.

- Compute all sums of 4 adjacent blocks.

- Look for at least 1 sum over threshold

- Validate or reject HRS trigger within 340 ns

Not all the Proton Array channels are read for each event

Analysis Status - Preliminary

Xeep 0Invariant mass of 2 photons in the calorimeter

Sigma=9.5 MeV

Good way to control the calorimeter calibration

ep e XMissing mass2 with LH2 target

Analysis Status – Very preliminary

φ

φ

φ

α (

N+ -

N- )

α (

N+ -

N- )

α (

N+ -

N- )

LH2 target

LD2 target

LD2 – LH2

Possible neutron signal !

0.5 GeV2 < missing mass 2 < 1.5 GeV2

0.5 GeV2 < missing mass 2 < 1.5 GeV2

Absolute cross sections necessary to extract helicity

dependence of neutron

Analysis Status – Very preliminary

φ

φ

φ

α (

N+ -

N- )

α (

N+ -

N- )

α (

N+ -

N- )

0.5 GeV2 < missing mass 2 < 1.5 GeV2

1.5 GeV2 < missing mass 2 < 2.5 GeV2

2.5 GeV2 < missing mass 2 < 3.5 GeV2

No signal

Signal

Conclusion

123710.4 scmLnucleon

With High Resolution spectrometer and a good calorimeter, we are able to measure:

•Helicity dependence of the neutron using LD2 and LH2 target.

Work at precisely defined kinematics: Q2 , s and xBj

Polarized cross sections to extract GPD E

Relative asymmetry considering Proton Array and Tagger.

Work at a luminosity up to

Proton preliminary results tomorrow morning

Coming soon:

Analysis status – preliminary

Sigma = 0.6ns

2 ns beam structure

Time difference between the electron arm and the detected photon

Selection of events in the coincidence peak

Determination of the missing particle (assuming DVCS kinematics)

Check the presence of the missing particle in the predicted block (or region) of the Proton Array

Sigma = 0.9ns

Time spectrum in the predicted block (LH2 target)

Analysis – preliminary

Triple coincidence

Missing mass2 of H(e,e’γ)x for triple coincidence events

Background subtraction with non predicted blocks

Proton Array and Proton Veto are used to check the exclusivity and reduce the background

π0 electroproduction - preliminary

Invariant mass of 2 photons in the calorimeter

Good way to control calorimeter calibration

Missing mass2 of epeπ0x

2 possible reactions:

epeπ0p

epenρ+ , ρ+ π0 π+

2π production threshold

Sigma = 0.160 GeV2

Sigma = 9.5 MeV

Missing mass2 with LD2 target

Time spectrum in the tagger (no Proton Array cuts)