1. Nucleon'05, 14/10/05 M. Guidal, IPN Orsay
Deep Virtual Compton Scattering : experimental status and perspectives
Nucleon'05, 14/10/05
M. Guidal, IPN Orsay

2. 1/ Generalities about GPDs
2/ Review of experimental data
3/ Perspectives

3. DVCS : Golden process to access GPDs
-2x
x+x
x-x
H,E(x,x,t) ~
p’(=p+D) p
Leading order/twist (handbag diagram) accessible at moderate Q2 (precocious scaling),
DVCS: e g* p e’ p’ g
Bethe-Heitler
GPDs
Interference with Bethe-Heitler process,

4. x = xB ! Deconvolution needed !
Hq(x,x,t) but only x and t accessible experimentally g* t
g,M,...
x~xB x= x B
1-x /2
t=(p-p ’) 2
x = xB ! /2 x
H,E,H,E ~ p p’ ds
dQ d x dt 2 B ~ A
H (x,x,t,Q ) q
x-x+ie dx +B
E (x,x,t,Q )
dx +…. 1 -1
Deconvolution needed !
x : mute variable

5. GPD and DVCS Beam or target spin asymmetry Cross-section measurement
(at leading order:)
Cross-section measurement
and beam charge asymmetry (ReT)
integrate GPDs over x
Beam or target spin asymmetry
contain only ImT,
therefore GPDs at x = x and -x
Intro
(M. Vanderhaeghen)

6. ep epg ~ ~ ~ ~ H(x,x,t), H(x,x,t), E(x,x,t) (BSA) H, H (l)TSA H, E
Global analysis of polarized and unpolarized data needed for GPD separation
ep epg
x = xB/(2-xB)
k = -t/4M2
A = Ds 2s
s+ - s-
s+ + s- =
Polarized beam, unpolarized target: ~ ~
DsLU ~ sinf{F1H + x(F1+F2)H +kF2E}df
H(x,x,t), H(x,x,t), E(x,x,t)
(BSA)
Kinematically suppressed
Unpolarized beam, longitudinal target: ~ ~
H, H
DsUL ~ sinf{F1H+x(F1+F2)(H + … }df
(l)TSA
Unpolarized beam, transverse target:
H, E
DsUT ~ sinf{k(F2H – F1E) + ….. }df
(t)TSA

7. The actors + theory DESY CERN JLab HERMES ZEUS/H1 COMPASS Hall A
Vector & PS mesons
DVCS
CERN
COMPASS
Vector mesons
DVCS
JLab
Hall A
Hall B
Hall C
p-DVCS
n-DVCS
Vector mesons
d-DVCS
Pseudoscalar mesons
+ theory
(almost) everywhere

8. DVCS timeline
The past : (evidence for handbag mechanism in non-dedicated experiments)
HERA s
HERMES BSA
HERMES BCA, (l,t)TSA (preliminary)
CLAS(4.2 GeV) BSA
CLAS(4.8 GeV) BSA, (l)TSA (preliminary)
The present : (dedicated experiments)
JLab Hall A Ds, BSA
CLAS(5.75 GeV) Ds, BSA
The future :
HERMES (recoil detector)
COMPASS (recoil detector)
JLab(12 GeV)

9. The past (non-dedicated experiments)

10. Deep Exclusive reactions: an experimental challenge
Missing mass MX2
ep  epX MAMI 850 MeV
ep  epX Hall A GeV
Resolution
Exclusivity
Luminosity γ π0
ep  epX CLAS GeV
are the key issues
for this physics!
Intro N
N+π
ep  eγX HERMES GeV

11. Beam Spin Asymmetry (BSA)
(ep epg)
(ep epg) S

12. First experimental signatures
First observations of DVCS beam asymmetries in 2000
CLAS
HERMES
Q2 = 2.6 GeV2 ,
xB = 0.11,
-t = 0.27 GeV2
Q2 = 1.25 GeV2 ,
xB = 0.19,
-t = 0.19 GeV2
Phys.Rev.Lett.87:182002,2001
Phys.Rev.Lett.87:182001,2001
Vdh, Guichon, Guidal (1999)
twist-2 + twist-3 :
Kivel, Polyakov, Vdh (2000)

13. CLAS/DVCS at 4.8 and 5.75 GeV PRELIMINARY PRELIMINARY :
0.15 < xB< 0.4
1.50 < Q2 < 4.5 GeV2
-t < 0.5 GeV2
PRELIMINARY
5.75 GeV data (H. Avakian &
L. Elhouadrhiri)
PRELIMINARY
Vdh, Guichon, Guidal (1999)
Kivel, Polyakov, Vdh (2000)
calculation
twist-2 + twist-3
GPD based predictions
(BMK)
4.8 GeV data (G. Gavalian) :

14. Charge Spin Asymmetry (BCA)
(e-p epg)
(e+p epg) S

15. Beam charge asymmetry t-dependence
Vdh, Guichon, Guidal (1999)
calculation :
Guidal, Polyakov, Radyushkin, VdH (2005)
tiny e-p sample (L~10 pb-1)
HERA: e- beam (x5)

16. (longitudinal) Target Spin Asymmetry (l TSA)
(ep epg)
(ep epg) S

17. Longitudinal target spin asymmetry
Preliminary CLAS data
Experimental Studies with CLAS
Data were collected as a by-product during the eg run: GeV
with NH3 longitudinally polarized target, <Q2> ~ 1.8 GeV2
HERMES
Preliminary target
spin asymmetries
on p and d
but unexpected
large sin 2f
sin f in agreement
with GPD models
Preliminary HERMES data

18. (transverse) Target Spin Asymmetry (t TSA)
(ep epg)
(ep epg) S

19. Transverse target spin asymmetry

20. Guidal, Polyakov, Radyushkin, VdH (2005)x b
(fm) y z x

21. orbital angular momentum carried by quarks
evaluated at μ2 = 2.5 GeV2
PROTON
2 Jq
(GPRV 05) Δq
HERMES
(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

22. The present (dedicated experiments)

23. JLab dedicated DVCS experiments in 2004 - 2005
High statistics and unambiguous epg final state determination
JLab/Hall A
JLab/CLAS
Calorimeter and supraconducting magnet within CLAS torus
Intro p e’ e γ

24. Add an EM calorimeter at forward angles
A typical
event in
CLAS
(Hall B, JLab) p
1/ DVCS (Hall B)
epa epg g
Add an EM calorimeter at forward angles
420 PbWO4 crystals : ~10x10 mm2, l=160 mm
Read-out : APDs +preamps
JLab/ITEP/
Orsay/Saclay
collaboration
Add a “Moller shield”
solenoid around the target

25. 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

26. Expected Kinematical Dependencies
About 380 bins in
f, xB, t

27. 2/DVCS in JLab/Hall A
Experiment completed
(Nov.-Dec. 2004)
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

28. Sensitivity to quark angular momentum J
DVCS on the neutron
DVCS-BH interference generates
a beam spin cross section difference
Main contribution
for the proton
Main contribution
for the neutron
Intro
→ (within a model)
Sensitivity to quark angular momentum J
Veto detector added to the p-DVCS set-up

29. ~ New CLAS experiment : Longitudinal Target Spin Asymmetry
DsUL ~ sinfIm{F1H+x(F1+F2)(H +.. }
CLAS eg1 (preliminary)
CLAS (eg1+IC) projected
6 GeV run with NH3 longitudinally polarized
target (CLAS + IC)
60 days of beam time
Approved at the latest
JLab PAC

30. The future

31. precision (unpolarised data with high density target)
Recoil detector
nov. 2005
for 2 years
Detection of the
recoiling proton
clean reaction
identification
improve statistical
precision (unpolarised
data with high density target)

34. Summary EXPERIMENT : The most complete information on the structure
of the nucleon : GPDs
(f (x), g (x), F (t), G (t), F(z), pion cloud, Jq…) 1 A
EXPERIMENT :
Up to 2005 : first experimental signatures in different
Kinematics, in different observables, are very encouraging
We enter a new era with high-statistics, high resolution
dedicated experiments: definitely sign the validity of the
approach (factorization, scaling,...) and we are on our
way to extract/strongly constraint the GPDs and
extract the physics

35. DVCS with a polarized target: New CLAS experiment~
DsLL ~ CBH+ cosfRe{F1H+x(F1+F2)(H +xE/2)+.. }
5.7 GeV run with NH3 longitudinally polarized target+IC
Projections for 60 days
The double spin asymmetry in DVCS with longitudinally polarized target will provide access to the real part of Compton form factors

36. DVCS cross section first measurement of t –slope:
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 Hg Hq
DVCS cross section
first measurement of
t –slope:
ds/dt = ds/dt|t=0∙exp(-b t)
b = 6.02±0.35±0.39 GeV-2
absolute normalisation!
[A.Freund, M. McDermott EPJC23(2002)]
comparison to NLO QCD:
band width given by b
measurement

37. Status of GPDs Studies at Jefferson Lab
GPD Reaction Obs. Expt Status
ep→epγ (DVCS) BSA CLAS 4.2 GeV Published PRL
CLAS 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
From
ep → epX
Dedicated set-up
Dedicated set-up
Intro

38. CLAS/DVCS at 4.8 and 5.75 GeV PRELIMINARY PRELIMINARY
0.15 < xB< 0.4
1.50 < Q2 < 4.5 GeV2
-t < 0.5 GeV2
PRELIMINARY
5.75 GeV data (H. Avakian &
L. Elhouadrhiri)
PRELIMINARY
GPD based predictions
(BMK)
4.8 GeV data (G. Gavalian)

39. Beam spin and charge asymmetry
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ H
Beam spin and charge asymmetry
Beam Spin Asymmetry
[PRL87,2001]

40. GPD Reaction Obs. Expt Status
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.
From
ep → epX
Dedicated set-up
Intro

41. Energy dependence BH
DVCS
Calculation (M.G.&M.Vanderhaeghen)

42. 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)
but…
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
Intro
Lepton pair squared invariant mass

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