1. Overview on hard exclusive production at HERMES
Cynthia Hadjidakis
on behalf of the HERMES collaboration
Conference on Elastic and Diffractive Scattering
Blois, May, 2005
Generalized Parton Distributions
Compton scattering (DVCS)
Exclusive mesons production
Summary and perspectives

2. Generalized Parton Distributions (GPDs)
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
Generalized Parton Distributions (GPDs)
- Műller (1994) -
- Ji & Radyushkin (1996) - Q2
Q2>>, t<<
Meson: valid only for longitudinal photon t
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. How to access GPDs? DVCS → H vector mesons → H, H pseudo-scalar mesons
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
How to access GPDs?
DVCS
vector mesons
pseudo-scalar mesons
pion pairs
→ H ~
→ H, H
→ H, E
→ H, E ~
→ H, E

4. HERMES at DESY e-beam: e+/e-, Ee=27.5 GeV, PB= 55%
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
HERMES at DESY
e-beam: e+/e-, Ee=27.5 GeV, PB= 55%
spin HERMES for longitudinal beam polarization

5. HERMES spectrometer Tracking system: dP/P = 2 %, dq < 1 mrad
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
HERMES spectrometer
e+/e-
27.5 GeV
Tracking system: dP/P = 2 %, dq < 1 mrad
(charged) Particle Identification: RICH, TRD, preshower, calorimeter
Photon: calorimeter: dP/P = 5 % for high energy photon
no recoil detection
e+ p → e+ g (p) only e+ and g detected
Exclusive reaction signed via the missing mass technique
MX = ( e + p – e’ – g )
Exclusive reaction selected with a cut on MX
Background contamination estimated with non-exclusive MC
1H→ <|Pt|> ~ 85 %
2H→ <|Pt|> ~ 85 %
1H↑ <|Pt|> ~ 75 %
Target: polarized H, D / unpolarized H, D, Ne, Kr, Xe

6. Deep Virtual Compton Scattering: e p → e’ p’ g
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
Deep Virtual Compton Scattering: e p → e’ p’ g
DVCS
Bethe-Heitler
for HERMES kinematics:
DVCS << Bethe-Heitler
DVCS-BH interference leads to non-zero azimuthal asymmetry

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

8. Beam charge asymmetry t-dependence
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ H
Beam charge asymmetry t-dependence
GPD calculation:
different parameterization for H
Vanderhaeghen (2002) –
AC sensitive to GPD-models
tiny e-p sample (L~10 pb-1)
HERA: e- beam (x5)

9. Longitudinal target spin asymmetry measurement
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ H,H ~
Longitudinal target spin asymmetry measurement
sin f in agreement with GPD models
Coming soon: transverse TSA sensitive to GPD E (Jq)
unexpected large sin 2f: from qgq correlations twist-3 GPDs?

10. r0 transverse target spin asymmetry
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ H,E
r0 transverse target spin asymmetry
interference between E and H
- Goeke, Polyakov & Vanderhaeghen (2001) -
E related to Jq  TSA sensitive to Jq
sS: |ST| sin (f-fS) E H xB x
Large negative asymmetry at low x and large t
Same x-dependence behaviour as GPDs calculations
(no direct theoretical comparison yet)
transverse polarized target: still running (end 2005)

11. p+ cross section measurement
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ E, H
~ ~
p+ cross section measurement
Comparison with GPD based model
Vanderhaeghen, Guichon & Guidal (1999) -
stot = sT + e sL
L/T separation not possible
… but:
sT suppressed by 1/Q2
Hermes kinematics:
0.80 < e < 0.96
→ at large Q2, sL dominates
Q2 dependence is in general agreement with the theoretical expectation
Power corrections (k┴ and soft overlap) calculations overestimate the data

12. Factorization theorem prediction
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ E, H
~ ~
Factorization 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
~ ~
interference between E and H leads to a large transverse TSA asymmetry
longitudinal TSA published [PLB535,2002]
Coming soon: transverse TSA

13. Pion pairs production: e p (d)→ e’ p (d) p+ p-
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ H, E
Pion pairs production: e p (d)→ e’ p (d) p+ p-
Legendre moment:
<P1> sensitive to the interference between different p+p- isospin states

14. Legendre Moment: Mpp dependence
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
→ H, E
Legendre Moment: Mpp dependence
interference between
S-wave and lower r0 tail
mpp < 0.6 GeV
[PLB599,2004]
minimum interference
between S-P waves
mpp ~ 0.77 GeV
indication of
r0 –f2 interference
mpp ~ 1.3 GeV
GPD model calculations:
■▲ quark exchange
― quark + 2-gluon exchange

15. Future analysis: recoil detector
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
Future analysis: recoil detector
Recoil detector
nov. 2005
for 2 years
Detection of the
recoiling proton
clean reaction identification
improve statistical precision (unpolarised data with high density target)
half time e- beam/e+ beam: e+/-p→ e’p’g
improve detector resolution: it will allow multi-dimensional binning in x and t

16. Combining all the measurements will allow to constrain GPD models
Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives
Summary and outlook
GPDs probed by hard exclusive photon and meson production
Measurements of exclusive processes at HERMES:
DVCS, vector mesons, p+, p+p-
winter 2005: recoil detector installation
HERMES dedicated to exclusive processes!
Combining all the measurements will allow to constrain GPD models