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

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 ~

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

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 rotators @ HERMES for longitudinal beam polarization

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

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

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

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: 2004-2005 e- beam (x5)

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?

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)

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

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

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

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

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

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