1. Experimental overview on exclusive processes
Delia Hasch
1st Duality Workshop
June 06-08, 2005; LNF-INFN, Frascati (Italy)

2. hard exclusive processes
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
hard exclusive processes Q2 t
Q2>>, t<<
high luminosity  s~1/Q4, 1/Q6
high Q2  hard regime
high resolution  exclusivity
Quantum number of final state selects different GPDs:
Vector mesons (r, w, f): H E
Pseudoscalar mesons (p, h): H E
DVCS (g) depends on H, E, H, E ~

3. prerequisites H1, ZEUS, HERMES: H1, ZEUS 27.5 GeV e+, e-
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
prerequisites
H1, ZEUS, HERMES:
27.5 GeV e+, e-
H1, ZEUS
 distinct signature in 4p det.
Compass:
GeV m+
Jlab: HallA,B
up to 6 GeV e-
HERMES, COMPASS, JLab:
 missing mass/energy
technique:
CLAS: DMX ~ 30 MeV
HERMES: DMX ~ 400 MeV
CLAS
HERMES
H1, ZEUS: p (820 GeV)
HERMES: p , d
unpol. H, D, 4He, 14N, 20Ne, Kr, Xe
Compass: d , p
Hall A: p , n Hall B: p, d

4. Deeply virtual Compton Scattering
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
Deeply virtual Compton Scattering
Bethe-Heitler
HallA,B HERMES H1/ZEUS
DVCS BH
 NLO analysis of H1, ZEUS cross sections

5. 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

6. DVCS asymmetries DsLU ~ ~ ~ ~ beam target
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
DVCS asymmetries
DVCS-BH interference leads to
non-zero azimuthal asymmetry
DsLU
beam target
 separating GPDs through polarisation:
DsLU ~ sinf∙Im{F1H + x(F1+F2)H +kF2E} ~ ~
H, H, E
DsUL ~ sinf∙Im{F1H + x(F1+F2)(H + …} ~ ~
H, H
DsUT ~ sinf∙Im{k(F2H - F1E) + … }
H, E
kinematically suppressed
x = xB/(2-xB ),k = t/4M2

7. beam spin asymmetry CLAS ~  H H E 0.15 < xB< 0.4
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
beam spin asymmetry
[PRL87,2001]
CLAS
[PRL87,2001]
PRELIMINARY
(not for circulation)
0.15 < xB< 0.4
1.5 < Q2 < 4.5 GeV2
-t < 0.5 GeV2
Vanderhaeghen -

8. BSA on nuclear targets heavier targets : 4He, 14N, 20Ne, Kr, Xe
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
BSA on nuclear targets
heavier targets : 4He, 14N, 20Ne, Kr, Xe
… study dependence of coherent production on nuclei

9. target spin asymmetry CLAS ~  H H prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H ~
target spin asymmetry
CLAS
PRELIMINARY
(not for circulation)

10. target spin asymmetry transverse TSA: sensitivity to GPD E (Jq) ~
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
target spin asymmetry
Vanderhaeghen -
transverse TSA: sensitivity to GPD E (Jq)

11. beam charge asymmetry (only @ HERA) ~  H H E
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
beam charge asymmetry
HERA)

12. beam charge asymmetry (only @ HERA)
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
beam charge asymmetry
HERA)
GPD calculations: different parametrisations for H
Vanderhaeghen (1999/02) –
AC: high sensitivity to GPD
models
(tiny e-p sample ~10pb-1)
HERA: 2004/05 e- beam (5x)
… H1/ZEUS ?

13. prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
 H E ~
 H E
DVCS  DVMS
DVCS theoretically cleanest process to study GPDs
- azimuthal asymmetries from interference term
relate directly to amplitude
hard exclusive meson production: flavour separation ρ0
2u+d, 9g/4 ω
2u-d, 3g/4 f
s, g ρ+
u-d
! factorisation theorem for
sL only !

14. s vs W in bins of Q2 r f J/Y s ~ Wd with d=0.7 no dependence on Q2
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
s vs W in bins of Q2
 H E
J/Y r f
s ~ Wd with d=0.7
no dependence on Q2
 general transition to hard behaviour at
high Q2+M2

15. sL separation from angular distribution
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H E
sL separation from angular distribution
[Vanderhaegen et.al. (1999)]
--- 2-gluon exchange
--- quark exchange
if SCHC holds
(VM retains g* helicity):  r0
2002
< 10%
CLAS: analysis of sL for r0 (r+) from
6 GeV data (higher W !) going on …

16. - Goeke, Polyakov & Vanderhaeghen (2001) -
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H E
r0 transverse TSA
interference of H and E:
sS ~ |ST| sin (f-fS) E∙H
- Goeke, Polyakov & Vanderhaeghen (2001) -
E related to Jq  TSA sensitive to Jq
 same x-dependence behaviour as
GPD calculations

17. p+ cross section sT suppressed by 1/Q2 Comparison with
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H E ~
p+ cross section
Comparison with
GPD model for sL:
[Vanderhaegen, Guichon, Guidal (1999)]
Q2 dependence:
sT suppressed by 1/Q2
Q2 dependence: in general agreement with the theoretical expectation
LO calculations too small
power corrections (k┴ ,soft overlap) overestimate data

18. cross section ratios p0 production: no p-pole ~ x measure at
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H E ~
cross section ratios
p0 production: no p-pole
 H dominates
measure at
 HERMES
 Jlab: ~
[Mankiewicz,Piller & Radyushkin (1999)] x
PRELIMINARY
(not for circulation)

19. p+ transverse TSA ~ ~ sS ~ |ST| sin (f-fS) E∙H ~ ~ ~
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H E ~
p+ transverse TSA
- Frankfurt, Pobylitsa, Polyakov & Strikman (1999) -
- Frankfurt, Polyakov, Strikman & Vanderhaeghen (2000) - ~ ~
- Belitsky & Müller (2001) -
interference of H and E:
sS ~ |ST| sin (f-fS) E∙H
g*L p → p+ n ~ ~
TARGET SPIN ASYMMETRY
 gives access to relative size of H and E ~
soon from HERMES
Compass, Jlab …

20. dedicated experiments
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
dedicated experiments
DVCS with recoil: 2006/07
background “free” DVCS:
semi-inclusive bg: 5%  <<1%
associated bg: 10%  1%

21. JLab dedicated DVCS experiments in 2004 - 2005
M. Garcon
JLab/Hall A
JLab/CLAS
Inner Calorimetrer and supraconducting magnet within CLAS torus
Dedicated, high statistics, DVCS experiments
→ Detection of 3 particles e, p and γ in final state
→ Firmly establish scaling laws (up to Q2 ~ 5 GeV2),
if observed, or deviations thereof understood,
first significant measurement of GPDs.
→ Large kinematical coverage in xB and t
leads to femto-tomography of the nucleon
→ Opens the way for an ambitious program
with (CLAS12 and other)
Intro p e’ e γ

22. conclusions first exciting results on DVCS and DVMS available
prerequisities perspectives
DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm
 H H E ~
 H E ~
 H E
conclusions
first exciting results on DVCS and DVMS available
 polarisation provides new observables sensitive
to different (combinations) GPDS
dedicated experiments for DVCS measurements going
on at JLab or starting soon at HERMES
new facilities (high luminosity/energy/resolution) needed
to map GPDs in an global analysis of all hard exclusive processes

23. backup slides t – dependence of BSA for photon and pion production:
PRELIMINARY
(not for circulation)

24. factorisation theorem prediction
for fixed xB and t
asymptotically
fit: 1/Qp
p=1.9±0.5
p=1.7±0.6
p=1.5±1.0