Overview on hard exclusive production at HERMES

1. Overview on hard exclusive production at HERMES Cynthia Hadjidakis on behalf of the HERMES collaboration Conference on Elastic and Diffractive Scattering Blois, 15-20 May, 2005 • Generalized Parton Distributions • Compton scattering (DVCS) • Exclusive mesons production • Summary and perspectives

2. Vector mesons (r, w, f): H E Pseudoscalar mesons (p, h): H E DVCS (g) depends on H, E, H, E Quantum number of final state selects different GPDs: ~ ~ ~ ~ Meson: valid only for longitudinal photon Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives Generalized Parton Distributions (GPDs) - Műller(1994) - - Ji & Radyushkin(1996) - Q2 Q2>>, t<< t

3. ~ → H, H ~ ~ → H, E Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives How to access GPDs? DVCS vector mesons pseudo-scalar mesons pion pairs → H → H, E → H, E

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

5. e+/e- 27.5 GeV Target: polarized H, D / unpolarized H, D, Ne, Kr, Xe 1H→<|Pt|> ~ 85 % 2H→<|Pt|> ~ 85 % 1H↑ <|Pt|> ~ 75 % Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives HERMES spectrometer 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 gdetected 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

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

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

8. Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives →H Beam charge asymmetry t-dependence GPD calculation: different parameterization for H • Vanderhaeghen(2002) – ACsensitive to GPD-models tiny e-p sample (L~10 pb-1) HERA: 2004-2005 e- beam (x5)

9. 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 TSAsensitive to GPD E (Jq) unexpected large sin 2f: from qgq correlations twist-3 GPDs?

10. - Goeke, Polyakov & Vanderhaeghen(2001) - interference between E and H sS: |ST| sin (f-fS) E H Erelated to Jq TSA sensitive to Jq Introduction| DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives →H,E r0 transverse target spin asymmetry 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. stot = sT + esL L/T separation not possible … but: sT suppressed by 1/Q2 Hermes kinematics: 0.80 < e < 0.96 → at large Q2, sL dominates Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives →E, H ~ ~ p+cross section measurement Comparison with GPD based model • Vanderhaeghen, Guichon & Guidal (1999) - Q2 dependence is in general agreement with the theoretical expectation Power corrections (k┴ and soft overlap) calculations overestimate the data

12. asymptotically for fixed xB and t Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives →E, H ~ ~ Factorization theorem prediction fit: 1/Qp p=1.9±0.5 p=1.7±0.6 p=1.5±1.0 ~ ~ interference between E and Hleads to a large transverse TSA asymmetry longitudinal TSA published [PLB535,2002] Coming soon: transverse TSA

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

15. Recoil detector nov. 2005 for 2 years Detection of the recoiling proton Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives Future analysis: recoil detector 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. Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives Summary and outlook GPDs probed by hardexclusivephoton 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