Michel Garçon – SPhN/Saclay – SIR2005 Workshop (Jefferson Lab, May 2005) Generalized Parton Distributions: the present program at Jefferson Lab Generalized Parton Distributions: the present program at Jefferson Lab

 High luminosity compensates relatively low energy (for exclusive measurements, same Q 2 as high energy machines are achieved) Large x range ( ) above resonance region into DIS  High resolution truly exclusive measurements  Experimental equipment from high resolution to large acceptance spectrometers, polarized targets, new electromagnetic calorimeters, highly polarized electron beam (80%) ☺ All this enhanced in the planned 12 GeV upgrade The specificities of JLab W = GeV

ep→epγ (DVCS) BSA CLAS4.2 GeVPublished PRL CLAS 4.8 GeVPreliminary CLAS5.75 GeVPreliminary (+ σ) Hall A5.75 GeVFall 04 CLAS5.75 GeVSpring 05 ep→epγ (DVCS) TSA CLAS5.65 GeVPreliminary e(n)→enγ (DVCS) BSA Hall A5.75 GeVFall 04 ed→edγ (DVCS) BSA CLAS5.4 GeVunder analysis ep→epe + e - (DDVCS) BSA CLAS5.75 GeVunder analysis ep→epρ σ L CLAS4.2 GeVPublished PLB CLAS5.75 GeVunder analysis ep→epω (σ L ) CLAS5.75 GeVAccepted EPJA + other meson production channels π, η, Φ under analyses in the three Halls. GPDReactionObs. ExptStatus From ep → epX Dedicated set-up

DVCS and GPDs : Beam and target spin asymmetries DVCS-BH interference generates a beam spin cross section difference: and a target spin cross section difference: Spin asymmetries:

CLAS/DVCS at 4.2 and 4.8 GeV: ep→epγ from analysis of ep→epX spectra CLAS/DVCS at 4.2 and 4.8 GeV: ep→epγ from analysis of ep→epX spectra Published measurement at 4.2 GeV Phys.Rev.Lett.87:182002,2001 Preliminary CLAS analysis with 4.8 GeV data (G. Gavalian) Preliminary γ π0π0

CLAS: high luminosity run at 5.75 GeV First JLab experiment with GPDs in mind (october 2001 – january 2002) - polarized electrons, E = 5.75 GeV - Q 2 up to 5.5 GeV 2, -Integrated luminosity: 30 fb -1 - W up to 2.8 GeV W = GeV

0.15 < x B < < Q 2 < 4.5 GeV 2 -t < 0.5 GeV 2 PRELIMINARY (not for circulation) H. Avakian & L. Elouadrhiri GPD based predictions (Vanderhaeghen)  0 are suppressed due to analysis cuts (only low t), residual contribution (π/γ ~ 5-15%) estimated from MC CLAS/DVCS (ep → epX) at 5.75 GeV PRELIMINARY (not for circulation)

CLAS/DVCS (ep → epX) at 5.75 GeV t – dependence of BSA for photon and pion production: PRELIMINARY (not for circulation)

 0 asymmetry (two photons required) Exclusive S. Chen 5.65 GeV run with NH 3 longitudinally polarized target, Q 2 up to 4.5 GeV 2 DVCS with a polarized target in CLAS * Detect all 3 particles in the final state (e,p,γ) to eliminate contribution from N (but calorimeter is at too large angles), * Apply kinematical cuts to suppress ep→epπ 0 contribution. * Remaining Φ-dependent π 0 contribution (10-40%) extracted from MC. * π 0 asymmetry measured PRELIMINARY (not for circulation) PRELIMINARY (not for circulation)

DDVCS (Double Deeply Virtual Compton Scattering) DDVCS (Double Deeply Virtual Compton Scattering) γ* T M. Guidal & M. Vanderhaeghen, PRL 90 A. V. Belitsky & D. Müller, PRL 90 The (continuously varying) virtuality of the outgoing photon allows to “tune” the kinematical point (x,ξ,t) at which the GPDs are sampled (with |x | < ξ). e-e- e+e+ e-e- p p e-e- DDVCS-BH interference generates a beam spin asymmetry sensitive to

DDVCS: first observation of ep → epe + e - in CLAS * Positrons identified among large background of positive pions * ep→epe + e - cleanly selected (mostly) through missing mass ep→epe + X * Φ distribution of outgoing γ* and beam spin asymmetry extracted (integrated over γ* virtuality) A problem for both experiment and theory: * 2 electrons in the final state → antisymmetrisation was not included in calculations, → define domain of validity for exchange diagram. * data analysis was performed assuming two different hypotheses either detected electron = scattered electron or detected electron belongs to lepton pair from γ* Hyp. 2 seems the most valid → quasi-real photoproduction of vector mesons but… Lepton pair squared invariant mass S. Morrow & M. Garçon

Deeply virtual meson production Meson and Pomeron (or two-gluon) exchange … … or scattering at the quark level ? π, f 2, P ρ0ρ0 (σ), f 2, P ω π, f 2, P Φ P Flavor sensitivity of DVMP on the proton: ω ρ0ρ0 2u+d, 9g/4 ω 2u-d, 3g/4 Φ s, g ρ+ρ+ u-d γ* L ωLωL (Photoproduction)

Exclusive ρ meson production: ep → epρ CLAS (4.2 GeV) Regge (JML) C. Hadjidakis et al., PLB 605 GPD formalism (beyond leading order) describes approximately data for x B 1.5 GeV 2 GPD (MG-MVdh) CLAS (5.75 GeV) Analysis in progress Two-pion invariant mass spectra

L. Morand et al., hep-ex/ , to be published in EPJA Analysis of ω polarization from ep → epπ + π - X configurations (for the first time for this channel above Q 2 ~ 1 GeV 2 ) Cross sections larger than anticipated at high t (see J.-M. Laget, PRD 70, ) SCHC does not seem to hold → not possible to extract σ L handbag diagram estimated to contribute only about 1/5 of measured cross sections → ω most challenging/difficult channel to access GPD Evidence for unnatural parity exchange   0 exchange very probable even at high Q 2 Deeply virtual ω production at 5.75 GeV Analysis of cross sections from ep → epπ + X configurations

JLab/Hall AJLab/CLAS Calorimetrer and supraconducting magnet within CLAS torus e e’ p γ Dedicated, high statistics, DVCS experiments → Detection of 3 particles e, p and γ in final state → Firmly establish scaling laws (up to Q 2 ~ 5 GeV 2 ), if observed, or deviations thereof understood, first significant measurement of GPDs. → Large kinematical coverage in x B and t leads to femto-tomography of the nucleon → Opens the way for an ambitious program with (CLAS12 and other) JLab dedicated DVCS experiments in

Experiment completed (Sept-Nov. 2004) PbF 2 electromagnetic calorimeter Fast-digitizing electronics → analysis of double pulses (pile-up) First double and triple coincidences ! (luminosity = cm -2 s -1 ) HMScalo HMScaloscint. Scintillator array for proton detection (P. Bertin, C.E. Hyde-Wright, R. Ransome, F. Sabatié, et al. CEBAF/E )

DVCS on the neutron DVCS-BH interference generates a beam spin cross section difference Main contribution for the proton Main contribution for the neutron (P. Bertin, C.E. Hyde-Wright, F. Sabatié, E. Voutier et al. CEBAF/E ) → (within a model) Sensitivity to quark angular momentum J Veto detector added to the p-DVCS set-up Experiment completed (Nov.-Dec. 2004)

Supraconducting solenoid Inner calorimeter (PbWO 4 ) CLAS/DVCS 424 crystals, 16 mm long, pointing geometry, ~ 1.2 degree/crystal, APD readout Photon detection in IC and EC (view from target) Calibration from π 0 →γγ σ = 7.5 MeV M γγ (GeV) η

Dependence of  asymmetry and total cross-section as a function of x B, t, Q 2,  bins  Projected results (sample) E V. Burkert, L. Edouardrihi, M. Garçon, S. Stepanyan et al. Run March-May 2005

Full exclusivity from 3-particle detection Hall A CLAS (from preliminary analysis of 2-hour run) All (eγp) events (eγp) events after kinematical cuts

Conclusions and outlook Jefferson Lab is playing a leading role in providing the experimental basis of the GPD concept. Once proper scaling laws are verified in DVCS (and possibly DVMP), first significant constraints on GPD models from dedicated experiments. A complete mapping and measurement of GPDs (from the quark sea to the valence region) will probably have to await the 12 GeV upgrade.