Deep Virtual Compton Scattering : Deep Virtual Compton Scattering : experimental status and perspectivesexperimental status and perspectives

1/ Generalities about GPDs1/ Generalities about GPDs

2/ Review of experimental data2/ Review of experimental data

3/ Perspectives3/ Perspectives

p p’(=p+)

H,E(x,,t)H,E(x,,t)

~~

x-

t

x+

Leading order/twist (handbag diagram) accessible at moderate Q2 (precocious scaling),

DVCS : Golden process to access GPDsDVCS : Golden process to access GPDs

DVCS:DVCS:

e

p

e’

p’

e

p

e’

p’

Bethe-HeitlerBethe-Heitler

e

p

e’

p’

GPDs

Interference with Bethe-Heitler process,

p p’

H,E,H,E~ ~

x

t

Deconvolution needed !Deconvolution needed !x : mute variable

x

Hq(x,,t) but only and t accessible experimentally

d

dQ d dt2

B

~ AH (x,,t,Q )2q

x-idx +BE (x,,t,Q )2

q

x-idx +….

1 1

-1 -1

2

= xB1-x /2B t=(p-p ’)2

x = xB !

/2

GPD and DVCSGPD and DVCS

1

1

1

1

),,(),,(

~),,(

~ tHidxx

txHPdx

ix

txHT DVCS

Cross-section measurementand beam charge asymmetry (ReT)

integrate GPDs over x

Beam or target spin asymmetrycontain only ImT,

therefore GPDs at x = and

(at leading order:)(M

. V

and

erh

aeg

he

n)

LU~ sin{F1H + (F1+F2)H +kF2E}d~

Polarized beam, unpolarized target:

Unpolarized beam, longitudinal target:

UL~ sin{F1H+(F1+F2)(H + … }d~

Unpolarized beam, transverse target:

UT~ sin{k(F2H – F1E) + …. }d

= xB/(2-xB)

k = -t/4M2

H(,t), H(,,t), E(,,t)

Kinematically suppressed

H, H~

H, E

A =

=

~

ep ep

Global analysis of polarized and unpolarized data needed for GPD separation

(BSA)

(l)TSA

(t)TSA

The actorsThe actors

JLab

Hall A Hall B Hall C

p-DVCS

n-DVCS

Vector mesons

p-DVCS

d-DVCS

Pseudoscalar mesons

DESYHERMES ZEUS/H1

Vector & PS mesons

DVCS

CERNCOMPASS

Vector mesons

DVCS

+ theory (almost) everywhere

DVCS timelineDVCS timeline

The past : (evidence for handbag mechanism in non-dedicated experiments)

HERA HERMES BSAHERMES BCA, (l,t)TSA (preliminary)CLAS(4.2 GeV) BSACLAS(4.8 GeV) BSA, (l)TSA (preliminary)

The present : (dedicated experiments)

JLab Hall A , BSACLAS(5.75 GeV) , BSA

The future :HERMES (recoil detector)COMPASS (recoil detector)JLab(12 GeV)

The past (non-dedicated experiments)

• Resolution• Exclusivity• Luminosity

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

π0γ

Deep Exclusive reactions: an experimental challengeDeep Exclusive reactions: an experimental challenge

are the key issues for this physics!

Beam Spin Asymmetry (BSA)

(ep ep (ep ep

First experimental signaturesFirst experimental signatures

First observations of DVCS beam asymmetries in 2000

CLAS HERMES

Phys.Rev.Lett.87:182002,2001

Q2 = 1.25 GeV2 ,

xB = 0.19,

-t = 0.19 GeV2

Q2 = 2.6 GeV2 ,

xB = 0.11,

-t = 0.27 GeV2

Phys.Rev.Lett.87:182001,2001

twist-2 + twist-3 :

Vdh, Guichon, Guidal (1999) Kivel, Polyakov, Vdh (2000)

4.8 GeV data (G. Gavalian)

PRELIMINARY

0.15 < xB< 0.41.50 < Q2 < 4.5 GeV2

-t < 0.5 GeV2

PRELIMINARY

PRELIMINARY

5.75 GeV data (H. Avakian &L. Elhouadrhiri)

CLAS/DVCS at 4.8 and 5.75 GeVCLAS/DVCS at 4.8 and 5.75 GeV

: Vdh, Guichon, Guidal (1999) Kivel, Polyakov, Vdh (2000)

calculation twist-2 + twist-3

Charge Spin Asymmetry (BCA)

(e-p ep (e+p ep

Beam charge asymmetry t-dependence

tiny e-p sample (L~10 pb-1)HERA: 2004-2005 e- beam (x5)

γpepe ''/-

Vdh, Guichon, Guidal (1999) calculation : Guidal, Polyakov, Radyushkin, VdH (2005)

(longitudinal) Target Spin Asymmetry (l TSA)

(ep ep (ep ep

Experimental Studies with CLASData were collected as a by-product during the eg1 2000 run: 5.75 GeV with NH3 longitudinally polarized target, <Q2> ~ 1.8 GeV2

Preliminary CLAS data

Longitudinal target spin asymmetry

HERMESPreliminary target spin asymmetrieson p and d

but unexpected large sin 2

sin in agreement with GPD models

Preliminary HERMES data

(transverse) Target Spin Asymmetry (t TSA)

(ep ep (ep ep

Transverse target spin asymmetry

Guidal, Polyakov, Radyushkin, VdH (2005)

x

b (fm)

y

xpz

xz

b

PROTON 2 Jq

(GPRV 05)

ΔqHERMES

(1999)

2 Lq

u 0.63 0.57 ± 0.04 0.06 ± 0.04

d -0.06 -0.25 ± 0.08 0.19 ± 0.08

s 0.03 -0.01 ± 0.05 0.04 ± 0.05

u + d + s

0.60 0.30 ± 0.10 0.30 ± 0.10

orbital angular momentum orbital angular momentum carried by carried by quarksquarks

evaluated at μ2 = 2.5 GeV2

The present (dedicated experiments)

JLab/Hall A JLab/CLAS

Calorimeter and supraconducting magnet within CLAS torus

e

e’p

γ

JLab dedicated DVCS experiments in 2004 - 2005JLab dedicated DVCS experiments in 2004 - 2005

High statistics and unambiguous ep final state determination

Add a “Moller shield”solenoid around the target

e’A typical event in CLAS (Hall B, JLab)

p

Add an EM calorimeter at forward angles

1/ DVCS 1/ DVCS (Hall B)(Hall B)

ep ep

420 PbWO4 crystals : ~10x10 mm2, l=160 mm Read-out : APDs +preamps

JLab/ITEP/Orsay/Saclaycollaboration

Data taking : March to May 2005

Calibration from π0→γγ

σ = 7.5 MeV

Mγγ (GeV)

η

CLAS (preliminary analysis of a 2 hours run)

All (eγp) events(eγp) events after kinematical cuts

About 380 bins in, xB, t

Expected Kinematical Dependencies

•High Resolution Hall A spectrometer for electron detection•100-channel scintillator array for proton detection•132-block PbF2 electromagnetic calorimeter for photon detection

Detection of all 3 final-state particles ensures exclusivity

2/DVCS in JLab/Hall A2/DVCS in JLab/Hall A

Experiment completed

(Nov.-Dec. 2004)

EHH

)(4

~)()(

2)( 22211 tF

M

ttFtF

x

xtFA

B

B

DVCS on the neutronDVCS on the neutron

DVCS-BH interference generatesa beam spin cross section difference

Main contribution for the proton Main contribution

for the neutron

→ (within a model) Sensitivity to quark angular momentum J

sinA

Veto detector added to the p-DVCS set-up

New CLAS experiment : Longitudinal Target Spin Asymmetry

CLAS eg1 (preliminary)

• CLAS (eg1+IC) projected6 GeV run with NH3 longitudinally polarized

target (CLAS + IC)

60 days of beam time

Approved at the latest

JLab PAC

UL~ sinIm{F1H+(F1+F2)(H +.. }~

The future

Recoil detector nov. 2005 for 2 years

Detection of the recoiling proton

clean reaction identification

improve statistical precision (unpolarised data with high density target)

The most complete information on the structureof the nucleon : GPDs

SummarySummary

(f (x), g (x), F (t), G (t), (z), pion cloud, Jq…)11 1111 AA

Up to 2005 : first experimental signatures in different Kinematics, in different observables, are very encouraging

We enter a new era with high-statistics, high resolutiondedicated experiments: definitely sign the validity of the approach (factorization, scaling,...) and we are on ourway to extract/strongly constraint the GPDs and extract the physics

EXPERIMENT :

5.7 GeV run with NH3

longitudinally polarized target+IC

DVCS with a polarized target: New CLAS experimentDVCS with a polarized target: New CLAS experiment

Projections for 60 days

LL~ CBH+ cosRe{F1H+(F1+F2)(H +xE/2)+.. }~

The double spin asymmetry in DVCS with longitudinally polarized target will provide access to the real part of Compton form factors

DVCS cross section DVCS cross section first measurement oft –slope:d/dt = d/dt|t=0∙exp(-b t)

b 6.02±0.350.39 GeV2

|e)(),,( b|tq xqtxH |e)(),,( b|tg xgxtxH

[A.Freund, M. McDermott EPJC23(2002)]

absolute normalisation!

comparison to NLO QCD:

band width given by b measurement

prerequisities perspectives

DVCS asymm | VECTOR MESONS asymm | PS MESONS asymm

Hg Hq

ep→epγ (DVCS) BSA CLAS 4.2 GeV Published PRL

CLAS 4.8- 5.75 GeV Preliminary

(+ σ) Hall A 5.75 GeV Fall 04

CLAS 5.75 GeV Spring 05

ep→epγ (DVCS) TSA CLAS 5.65 GeV Preliminary

e(n)→enγ (DVCS) BSA Hall A 5.75 GeV Fall 04

ed→edγ (DVCS) BSA CLAS 5.4 GeV under analysis

ep→epe+e- (DDVCS) BSA CLAS 5.75 GeV under analysis

GPD Reaction Obs. Expt Status

),,( tH From

ep → epX

),,(~

tH ),,( tE

)( du

Status of GPDs Studies at Jefferson Lab

Dedicated set-up

Dedicated set-up

4.8 GeV data (G. Gavalian)

PRELIMINARY

0.15 < xB< 0.41.50 < Q2 < 4.5 GeV2

-t < 0.5 GeV2

PRELIMINARY

PRELIMINARY

5.75 GeV data (H. Avakian &L. Elhouadrhiri)

CLAS/DVCS at 4.8 and 5.75 GeVCLAS/DVCS at 4.8 and 5.75 GeV

Beam spin and charge asymmetryγpepe ''

SBeam Spin Asymmetry

Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives → H

[PRL87,2001]

ep→epγ (DVCS) BSA CLAS 4.2 GeV Published PRL

CLAS 4.8 GeV Preliminary

CLAS 5.75 GeV Preliminary

(+ σ) Hall A 5.75 GeV Fall 04

CLAS 5.75 GeV Spring 05

ep→epγ (DVCS) TSA CLAS 5.65 GeV Preliminary

e(n)→enγ (DVCS) BSA Hall A 5.75 GeV Fall 04

ed→edγ (DVCS) BSA CLAS 5.4 GeV under analysis

ep→epe+e- (DDVCS) BSA CLAS 5.75 GeV under analysis

ep→epρ σL CLAS 4.2 GeV Published PLB

CLAS 5.75 GeV under analysis

ep→epω (σL) CLAS 5.75 GeV Accepted EPJA

+ other meson production channels π, η, Φ under analyses in the three Halls.

GPD Reaction Obs. Expt Status

),,( tH From

ep → epX

Dedicated set-up

),,(~

tH ),,( tE

),,( txH

x

duEH )(,

x

duEH )2(,

)( du

...

2

1''

5

dt

d

dt

d

dtddkd

d LTV

ee

1

1V

Vete

B ECxQ 1,2

Energy dependenceEnergy dependence

BH

DVCS

Calculation (M.G.&M.Vanderhaeghen)

DDVCS: first observation of ep → epe+e-DDVCS: first observation of ep → epe+e-

* Positrons identified among large background of positive pions

* ep→epe+e- cleanly selected (mostly) through missing mass ep→epe+X

* Φ distribution of outgoing γ* and beam spin asymmetry extracted(integrated over γ* virtuality)

A problem for both experiment and theory:

* 2 electrons in the final state → antisymmetrisation was not included in calculations,

→ define domain of validity for exchange diagram.

* data analysis was performed assuming two different hypotheses

either detected electron = scattered electron

or detected electron belongs to lepton pair from γ*

Hyp. 2 seems the most valid

→ quasi-real photoproduction of vector mesons

but…

Lepton pair squared invariant mass

Compton ScatteringCompton Scattering

“ “DVCS” (Deep Virtual Compton Scattering)DVCS” (Deep Virtual Compton Scattering)