deeply virtual compton jlab franck sabati saclay spin06 - kyoto october 6 th 2006 from gpds to dvcs,...

Deeply Virtual Compton Scattering @ JLabFranck SabatiéSaclay

SPIN’06 - KyotoOctober 6th 2006

From GPDs to DVCS, to GPDs backOnto the DVCS harmonic structureE00-110 experiment in Hall AScaling tests & GPD measurementE1-DVCS experiment at CLAS in Hall BSummary

Collins, Freund

GPDs from Theory to Experiment

Theory

x+ x-

t

GPDs

Handbag Diagram

Physical process

Experiment

Factorization theorem states:In the suitable asymptotic limit, the handbag diagram is the leading contribution to DVCS.

Q2 and largeat xB and t fixed

but it’s not so simple…

1. Needs to be checked !!!1

1

1

1

( , , ) +

( , , - i + ( , ) , )

DVCS

GPD x t

GPD x tT dxx

GPD x tdx

i

P x

2. 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=

Experimental observables linked to GPDs

3. Experimentally, DVCS is undistinguishable with Bethe-Heitler

However, we know FF at low t and BH is fully calculable

Using a polarized beam on an unpolarized target, 2 observables can be measured:

4 2

2

4 4

2

2

m2 I

ReBH BH D

DVC

B

BH

C

S

V S

B

d T Tdx dQ dtd

d d Tdx dQ d

T

tdT

At JLab energies,|TDVCS|2 should be small

4 2 2

2

4 4 2 2

2

2

Im2

Re DVCBH BH DVCS

B

BH DVCS DVCSDVCS

B

Sd T T Tdx dQ dtd

d d T T Tdx dQ

T

tT

d d

Kroll, Guichon, Diehl, Pire, …

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

1

1

1

1

( , , ) +

( , , - i + ( , ) , )

DVCS

GPD x t

GPD x tT dxx

GPD x tdx

i

P x

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

Observables and their relationship to GPDs

e-’

pe-*

hadronic plane

leptonic plane

0 1

42

1

2 3

1

10 1 22

2

2

1 22

4

21 2

24

2

1 ( , , ) cos cos 2

1 (

( ) ( )

( ) (, , ) cos cos 2 cos3

( , , ) sin sin 2

)

( ) ( )

BH BH BHB

B

B

B

B

I I I I

I I

c c c c

d x Q t c c cdx dQ dtd

x Q t

x Q td ddx

sdQ d

sdt

Into the harmonic structure of DVCS|TBH|2

Interference term

1 2( ) (1

)

BH propagators dependenceBelitsky, Mueller, Kirchner

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

d x Q t c c cdx dQ dtd

x Q t

x Q td ddx 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!

Analysis – Extraction of observables

Re-stating the problem (difference of cross-section):

24 4

2 1 21 2

sin sin 2( ) ( )

( , , ) IB I

B

x Q td ddx dQ d d

st

s

1 8 I ( )) m(2 IIs y y CK F

1 1 2 22()2 4

( )B

I BC x tF F Fx

F FM

2 ( , , ) ( , )I ,m q qq

q

e H t H t

GPD !!!

What we measure

Special case of the asymmetry

4 42

4 4 0 1

1 2

0 1

sin sin 2( , , )( ) cos ...

A B BH BH

I I

I I

d d x Q tc cd

s scd c

The asymmetry can be written as:

Pros: easier experimentally, smaller RC

Cons: - extraction of GPDs model-dependent (denominator complicated and not well known) - Large effects of the BH propagators in the denominator

Asymmetries are largely used in CLAS and HERMES measurements, where acceptance and systematics are more difficult to estimate.

E00-110 experimental setup and performances

• 75% polarized 2.5uA electron beam• 15cm LH2 target• Left Hall A HRS with electron package• 11x12 block PbF2 electromagnetic calorimeter• 5x20 block plastic scintillator array• 11x12 block PbF2 electromagnetic calorimeter

• 15cm LH2 target• Left Hall A HRS with electron package

• 75% polarized 2.5uA electron beam

Pbeam=75.32% ± 0.07% (stat)Vertex resolution 1.2mm

• 5x20 block plastic scintillator array

at

2.7%

4.2

E

EGeV

2.5x y mm t (ns) for 9-blockaround predicted« DVCS » block

E00-110 kinematics

The calorimeter is centeredon the virtual photon direction

50 days of beam time in the fall 2004, at 2.5A intensity113294 fbLu dt

Analysis – Looking for DVCS events

HRS: Cerenkov, vertex, flat-acceptance cut with R-functions

Calo: 1 cluster in coincidence in the calorimeter above 1 GeV

With both: subtract accidentals, build missing mass of (e,) system

Analysis – o subtraction effect on missing mass spectrum

Using 0→2 events in the calorimeter,the 0 contribution is subtracted bin by bin

After0 subtraction

Analysis – Exclusivity check using Proton Array and MC

Normalized (e,p,)triple coincidence events

Using Proton-Array, we compare the missing mass spectrum of the triple and double-coincidence events.

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%.

Analysis – Extraction of observables

Difference of cross-sections

2 22.3 GeV

0.36B

Q

x

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

Twist-2Twist-3

Extracted Twist-3contribution small !

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

Twist 4+ contributions are smaller than 10%

Total cross-section2 22.3 GeV

0.36B

Q

x

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

Extracted Twist-3contribution small !

E1-DVCS @ CLAS : a dedicated DVCS experiment

~50 cm

Inner Calorimeter+ Moller shielding solenoid

Beam Polarization: 75-85%Integ. Luminosity: 45 fb-1

M(GeV2)

E1-DVCS kinematical coverage and binning

W2 > 4 GeV2

Q2 > 1 GeV2

E1-DVCS exclusive DVCS selection

Remaining 0 contamination up to 20%, subtracted bin by binusing 0 events and MC estimation of 0(1) to 0(2) acceptance ratio

3-particle final state

E1-DVCS raw asymmetries

Very Preliminary

Integrated over t

ALU

<-t> = 0.18 GeV2 <-t> = 0.30 GeV2 <-t> = 0.49 GeV2 <-t> = 0.76 GeV2

ALU(90°)

Hall A dataOld CLAS data

E1-DVCS corrected ALU(90°)

Very Preliminary

SummaryCross-section difference (Hall A): High statistics test of scaling: Strong support for twist-2 dominance

First model-independent extraction of GPD linear combination from DVCS data in the twist-3 approximation

Upper limit set on twist-4+ effects in the cross-section difference:twist>3 contribution is smaller than 10%

Total cross-section (Hall A):

Bethe-Heitler is not dominant everywhere

|DVCS|2 terms might be sizeable but almost impossible to extract using only total cross-section: e+/e- or +/- beams seem necessary

Despite this, we performed a measurement of 2 different GPD integrals

BSA (CLAS):

Preliminary data in large kinematic range and good statistics !

Outlook

2 experiments to run in Hall B in ~2008

1 experiment to run in Hall A in ~2009

Extension of all the current experiments already proposed and approved for12 GeV running

Many theoretical progress expected in the meantime:

- Radiative corrections (P. Guichon)- Global analysis with adequate model (D. Mueller and others)- …

Backup

Comparison with models

Q2, x t,

Designing a DVCS experiment

Measuring cross-sections differential in 4 variables requires:

The high precision measurement of all 4 kinematical variables

Q2, x

Scattered electrondetected in the Hall A HRS:High precision determination

of the * 4-vector

Emitted photondetected in a high resolution

Electromagnetic Calorimeter:High precision determinationof the real photon directionq

q

t,

Designing a DVCS experiment

Measuring cross-sections differential in 4 variables requires:

Designing a DVCS experiment

Measuring cross-sections differential in 4 variables requires:

A good knowledge of the acceptance

Scattered electronThe HRS acceptance

is well known

Emitted photonThe calorimeter has a simple

rectangular acceptance

e p → e (p)

Perfect acceptancematching by design !Virtual photon « acceptance »placed at center of calorimeter

R-functioncut

*

Simply:t: radius: phase

Measuring cross-sections differential in 4 variables requires:

Good identification of the experimental process, i.e. exclusivity

Designing a DVCS experiment

ep epWithout experimental resolution

oep e

p

p

ep e

Designing a DVCS experiment

Good identification of the experimental process, i.e. exclusivity

Measuring cross-sections differential in 4 variables requires:

Without experimental resolution

o

ep

ep ep

e

ep ep

N

resonant or not

Designing a DVCS experiment

Good identification of the experimental process, i.e. exclusivity

Measuring cross-sections differential in 4 variables requires:

Without experimental resolution

If the Missing Mass resolution is good enough, with a tight cut, one get rids of the associated pion channels, but o electroproduction needs to be subtracted no matter what.

The baby steps towards the full nucleon wave function

After understanding the basic properties of the nucleon,physicists tried to understand its structure:

-By Elastic Scattering, we discovered the proton is not a point-like particle and we infered its charge and current distributions by measuring theForm Factors F1 and F2.

-By Deep Inelastic Scattering, we discovered quarks inside the nucleon and after 30 years of research, have a rather complete mapping of theQuark Momentum and Spin Distributions q(x), q(x).

Since the late 90’s, a new tool was developed, linking these representations of current/charge and momentum/spin distributions inside the nucleon, offering correlation information between different states of the nucleon in terms of partons. The study of Generalized Parton Distributions through Deep Exclusive Scattering will allow for a more complete description of the nucleon than ever before.

Mueller, Radyushkin, Ji

E00-110 custom electronics and DAQ scheme1. Electron trigger starts the game

2. Calorimeter trigger (350ns):

- selects clusters- does a fast energy reconstruction- gives a read-out list of the modules which enter clusters over a certain threshold- gives the signal to read-out and record all the experiment electronics channels

3. Each selected electronics channel is digitized on 128ns by ARS boards

t (ns)

4. Offline, a waveform analysis allows to extractreliable information from pile-up events

ARS system in a high-rate environment

- 5-20% of events require a 2-pulse fit - Energy resolution improved by a factor from 1.5 to 2.5 !- Optimal timing resolution

t (ns)

HRS-Calocoincidence

t=0.6 ns

Analysis – Calorimeter acceptance

The t-acceptance of the calorimeter is complicated at high-t:

5 bins in t:

-0.40 -0.35 -0.37

-0.35 -0.30 -0.33

-0.30 -0.26 -0.28

-0.26 -0.21 -0.23

-0.21 -0.12 -0.17

Min Max Avg

Xcalo (cm)

Ycalo (cm)

Calorimeter

Large-t dependence

Analysis – o contamination

Symmetric decay: minimum angle in lab of 4.4° at max o energy

Asymmetric decay: sometimes one high energy cluster… mimicks DVCS!

Analysis – o subtraction using data

1. Select o events in the calorimeter using 2 clusters in the calorimeter

2. For each o event, randomize the decay in 2-photons and select events for which only one cluster is detected (by MC)

3. Using appropriate normalization, subtract this number to the total number of 1-cluster (e,) events

Note: this not only suppressed o from electroproduction but also part of the o from associated processes

Invariant Massof 2-cluster events

135.5 MeV

9 MeVM

M

Summary plot

1

1

- ( i, ( , , ) , ) + DVCS GP GPD x tD xPT t dxx

Total cross-section and GPDs

4 2

0 1 2 32

22

32

2

0

2

2

1

1

1 ( , , ) cos cos 2 cos3

(2 )8 Re (1 )(2 )(2 )( ) ( ) (Re1

8 (2 ) R

)

e ( )

( ) ( )I I I I

I I

BHB

B

I

I I

I

BC

d T x Q tdx dQ dtd

y tK y y xy

F C F C F

C

c c c

K

c

c

c

Q

Fy

with

1 1 2 22

1 2

(( )2 4

( ) ( )

)

(2 2

)

B

B

B B

B

I

I

B

C F

C F

x tF F F Fx M

x xF Fx x

Interesting !Only depends on H and

E

CLAS: Phys.Rev.Lett.87:182002, 2001 HERMES: Phys.Rev.Lett.87:182001, 2001

DVCS Results : CLAS 4.2 and 4.8 GeV and HERMES

2 2

2

1.25 GeV0.19

0.19 GeVB

Qx

t

Preliminary CLAS analysiswith 4.8GeV data (G. Gavalian)

Preliminary

1-cluster eventscoming from all o

1-cluster eventscoming from o

electroproduction

MM2 cut

MeX (GeV2)

Analysis – missing mass of « subtracted » o

This method gives the number of o for all experimental bins

<-t>=-0.28 GeV2, 100°< <120°

CLAS 6 GeV

DVCSProton

ep→epπo/η

Hall A 6 GeV

DVCSprotonneutron

ep→epπo

CLAS 5.75 GeV

DVCS

DDVCS

ΔDVCS

D2VCS

PolarizedDVCS

ep→epρL

ep→epωL

ep→epπ0/η

ep→enπ+

ep→epΦ

HERMES 27 GeV

DVCS – BSA + BCA

+ nucleid-BSAd-BCA

ep→epρσL + DSA

ep→enπ+

+ ….

HERA27.5-900 GeV

DVCS

CLAS 4-5 GeV

DVCSBSA

HERMES

DVCS

BSA+BCA

With recoil detector

COMPASS

DVCS

+BCA

With recoil detector

Published ….. Preliminary results 2004 2005 ……… ….. 2009 ? … 2010

JLab@

12GeV

Deep Exclusive experimentsEV

ERY

THIN

G, w

ith more statistics than ever before

Deeply Exclusive Scattering program in the near future

2006-2007 HERMES2010-… COMPASS2010-… JLab@12GeV

Q2 and t dependence with 4.8 GeV data

Preliminary Preliminary

No clear dependence seen. Comparison with models necessary.And more accurate data clearly needed!

0.15 < xB< 0.41.50 < Q2 < 4.5 GeV2

-t < 0.5 GeV2

DVCS Beam Spin Asymmetry

PRELIMINARYH. Avakian & L. Elouadrhiri

GPD based predictions

0 are « suppressed » due to analysis cuts (only low t),but no subtraction or correction were done

PRELIMINARY

Once again, exclusivity and high statistics & precision data is the key !

The next generation DES experiments: a challenge

We need: •Resolution and exclusivity (3-particle final state)•High luminosity and/or acceptance (low cross-sections)• High transfers (factorization)

ep epX MAMI

850 MeV

ep epX Hall A

4 GeV

ep eγX HERMES

28 GeV

N+πN

Missing mass MX2

ep epX CLAS

4.2 GeV

πoγB

eam energy

MX2 [MeV2]

MX2 [MeV2]

Dependence of asymmetryand total cross-section asa function of xB, t, Q2 , bins

Projected results (sample)

Fast Digital Trigger

PbF2 block

Plastic ScintillatorProton Array

ARS system

Fast-digitizing electronics and smart calorimeter trigger

Addition of a charged particle vetoin front of the plastic scintillator array

The veto counter consists in 2 layers of2cm-thick scintillator paddles

Use of a deuterium targetProton DVCS is veto-ed by new detector

Neutron DVCS in Hall A at 6 GeV – E03-106 (Nov. 2004)P. Bertin, C.E. Hyde-Wright,F. Sabatié and E. Voutier

1 1 2 22

sin

( ) ( ) ( )4

( )2

B

B

xF t H F t F

A

tA F t EM

t Hx

Main contributionto the neutron