Deeply Virtual Compton Scattering in JLAB Hall A

Malek MAZOUZ

LPSC Grenoble : mazzouz@lpsc.in2p3.fr5th ICPHP May 22nd 2006

For JLab Hall A & DVCS collaborations

Form Factors via Elastic scaterring

Parton distribution via Deep inelastic scatering

Generalized parton distributions: GPDs

Two independent informations about the nucleon structure

Generalized parton distribution via Deep exclusive scaterring

LinkMueller, Radyushkin, Ji

Generalized parton distributions: GPDs

xProbability |Ψ(x)|2 that a quark carries a fraction x of the nucleon momentum

Parton distributions q(x), Δq(x) measured in inclusive reactions (D.I.S.)

x+ x-

t

GPDs measure the Coherence Ψ*(x+ξ) Ψ(x-ξ) between a initial state with a quark carrying a fraction x+ξ of the nucleon momentum and a final state with a quark carrying a fraction x-ξ

4 GPDs : , , , ( , , )H H E E x tFor each quark flavor

Dependence in t : new wealth of physics to explore

Mueller, Radyushkin, Ji

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)

How to access GPDs: DVCS

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

2

22

22

B

B

B

x

x

M

Q

pq

Qx

fraction of longitudinal momentum

Collins, Freund, Strikman

Deeply Virtual Compton Scattering

1

1

( , , )DVCS GPD x tT dx

x i

1

1 ...( , ,

( , , ))

GPDx

x tP

xGPDd ix t

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 but at the line x=ξ

What is done at JLab Hall A

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

����

But using a polarized electron beam: Asymmetry appears in Φ

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

Small at Jlab enegies

Kroll, Guichon, Diehl, Pire …

cross-section difference in the handbag dominance

2 22 2 2 2

2

( , , , ). ( , , , ).

with / 2 and ' ,

and the angle between the leptonic and photonic

si

pl

n si

e

n

an s

2A B e B B eB e B e

B

d dx x

dQ dx d d d dQ dx d d d

x Q p q

A B

p p

��

Γ contains BH propagators and some kinematics

B contains twist 3 terms

A contains twist 2 terms and is a linear combination of three GPD imaginary part 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

Pire, Diehl, Ralston, Belitsky, Kirchner, Mueller

Test of the handbag dominance

-Twist-2 contribution(Γ.A.sinφ) dominate the total cross-section and cross-section difference.

-Twist-2 term (A) and twist-3 term (B) have only log(Q2 ) dependence.

Test of the handbag dominance

To achieve this goal, an experiment was initiated at JLab Hall A on hydrogen target with high luminosity (1037 cm-2 s-1) and exclusivity.

Another experiment on a deuterium target was initiated to measure DVCS on the neutron. The neutron contribution is very interesting since it will provide a direct measure of GPD E (less constrained!)

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 kinematics

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)

Analysis - Selection of DVCS events

ep e X

DVCS

Mp2

Mx2 cut = (Mp+Mπ)2

Contamination by0e p pep e N + mesons

(Resonnant or not)

π0 contamination subtraction

One needs to do a π0 subtraction if the only (e,γ) system is used to select DVCS events.

Symmetric decay: two distinct photons are detected in the calorimeter No contamination

Asymmetric decay: 1 photon carries most of the π0 energy contamination because DVCS-like event.

π0 to subtract

- accidentalsep e X

π0 contamination subtraction

Still to check the exclusivity under the missing mass cut !

Mx2 cut =(Mp+Mπ)2

Check of the exclusivityOne can predict for each (e,γ) event the Proton Array block where the missing proton is supposed to be (assuming DVCS event).

Missing mass2 with PA check

Contamination < 3%

Extraction of observables

22 2

22 2

( , , , sin sin) ( 2, , , )B e B e

A B e B B e

dx

d

dQ dx d d d dQ dx d d dBxA

��

Q2 independent

( )

( ) .sin .sin 2

e e

e e

Expe i i

MCAe x i x Bi

N i N N

N i L AcA c cB Ac

MC sampling MC sampling

MC includes real radiative corrections (external+internal)

2

22

( ) ( )

( )e

Exp MCe e

Expi

e

N i N i

i

A

B

Extraction of observables

Acceptance included in fit

2 bins in (Q2,t) out of 15

5.48 0.

6.46 6.9

41

4B

A

2.17 0.

3.13 0.8

16

6B

A

- π0 Contribution is small

- twist-3 contribution is small

Check of the handbag dominance

A (twist-2) and B (twist-3) for a full bin <t>=0.25 GeV2

Strong indication for the validity of twist-3 approximation and the handbag dominance

ed e X

ep e X

ed e X ep e X

By subtracting proton contribution from deuterium, one should access to the neutron and coherent deuteron contributions.

Q2= 1.9 GeV2

<t>= -0.3 GeV2

DVCS on the neutron and the deuteron

Same exclusivity check as before

The number of detected π0 with hydrogen and deuterium target (same kinematics) shows that:

0

0

( )1.0

( )

e e X

e e X

d

p

In our kinematics π0 come essentially from proton in the deuterium

No π0 subtraction needed for neutron and coherent deuteron

π0 asymmetry is small

DVCS on the neutron and the deuteron - Preliminary2 2( ) resolution effects

2X p

tM coherent M Q2= 1.9 GeV2

<t>= -0.3 GeV2

Mx2 upper cut

1st cut2nd cut

It is clear that there are two contributions with different sign : DVCS on the neutron and DVCS on the deuteron

The same extraction method (with an additional binning on the Mx2)

will be applied on this data to have at least the twist-2 terms.

Conclusion

With High Resolution spectrometer and a good calorimeter, we are able to measure the Helicity dependence of the nucleon.

Work at precisely defined kinematics: Q2 , s and xBj

Work at a high luminosity

All tests of Handbag dominance give positive results :

No Q2 dependence of twist-2 and twist-3 terms.

Twist-3 contribution is small.

Accurate extraction of a linear combination of GPDS (twist-2 terms)

High statistics extraction of the total cross-section (another linear combination of GPD!)

Analysis in progress to extract the neutron and deuteron contribution

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

0.34 0.17 + 0.06

B

B

x tA F t F t F t F t

x M

A

H H E

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

Calorimeter resolution and calibration

-Time resolution < 1ns for all detectors

-Energy resolution of the calorimeter :

2.5% at 4.2E

GeVE

- Photon position resolution in the calorimeter: 2mm

Invariant mass of 2 photons in the calorimeter

σ= 9MeV

Detecting π0 in the calorimeter checks its calibration

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

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)