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Overview on hard exclusive production at HERMES

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

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Overview on hard exclusive production at HERMES

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  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

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