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Michel Garçon – SPhN/Saclay – SIR2005 Workshop (Jefferson Lab , May 2005)

Generalized Parton Distributions: the present program at Jefferson Lab. Michel Garçon – SPhN/Saclay – SIR2005 Workshop (Jefferson Lab , May 2005). The specificities of JLab. High luminosity compensates relatively low energy

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Michel Garçon – SPhN/Saclay – SIR2005 Workshop (Jefferson Lab , May 2005)

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

  2. The specificities of JLab • High luminosity compensates relatively low energy (for exclusive measurements, same Q2 as high energy machines are achieved) • Large x range (.15 - .75) 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 W = 2.8 2.4 2 1.8 GeV

  3. GPD Reaction Obs. Expt Status ep→epγ (DVCS)BSA CLAS 4.2 GeV Published PRL CLAS 4.8 GeV Preliminary CLAS 5.75 GeV Preliminary (+ σ) Hall A 5.75 GeV Fall 04 CLAS 5.75 GeV Spring 05 ep→epγ (DVCS)TSA CLAS 5.65 GeV Preliminary e(n)→enγ (DVCS)BSA Hall A 5.75 GeV Fall 04 ed→edγ (DVCS)BSA CLAS 5.4 GeV under analysis ep→epe+e- (DDVCS)BSA CLAS 5.75 GeV under analysis ep→epρσL CLAS 4.2 GeV Published PLB CLAS 5.75 GeV under analysis ep→epω (σL) CLAS 5.75 GeV Accepted EPJA + other meson production channels π, η, Φ under analyses in the three Halls. From ep → epX Dedicated set-up

  4. 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:

  5. CLAS/DVCS at 4.2 and 4.8 GeV: ep→epγ from analysis of ep→epX spectra Preliminary γ π0 Published measurement at 4.2 GeV Phys.Rev.Lett.87:182002,2001 Preliminary CLAS analysis with 4.8 GeV data (G. Gavalian)

  6. 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 • Q2 up to 5.5 GeV2, • Integrated luminosity: 30 fb-1 • W up to 2.8 GeV W = 2.8 2.4 2 1.8 GeV

  7. CLAS/DVCS (ep → epX) at 5.75 GeV H. Avakian & L. Elouadrhiri PRELIMINARY (not for circulation) PRELIMINARY (not for circulation) 0.15 < xB< 0.4 1.50 < Q2 < 4.5 GeV2 -t < 0.5 GeV2 GPD based predictions (Vanderhaeghen) p0 are suppressed due to analysis cuts (only low t), residual contribution (π/γ ~ 5-15%) estimated from MC

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

  9. Exclusive 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→epπ0 contribution. * Remaining Φ-dependent π0 contribution (10-40%) extracted from MC. * π0 asymmetry measured 5.65 GeV run with NH3 longitudinally polarized target, Q2 up to 4.5 GeV2 PRELIMINARY (not for circulation) p0 asymmetry (two photons required) PRELIMINARY (not for circulation) S. Chen

  10. DDVCS (Double Deeply Virtual Compton Scattering) DDVCS-BH interference generates a beam spin asymmetry sensitive to e+ e- e- e- γ*T γ*T p p 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). M. Guidal & M. Vanderhaeghen, PRL 90 A. V. Belitsky & D. Müller, PRL 90

  11. 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) but… 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 Lepton pair squared invariant mass S. Morrow & M. Garçon

  12. Deeply virtual meson production Meson and Pomeron (or two-gluon) exchange … (Photoproduction) ω π, f2, P …or scattering at the quark level ? Flavor sensitivity of DVMP on the proton: ωL γ*L

  13. Exclusive ρ meson production: ep → epρ CLAS (4.2 GeV) CLAS (5.75 GeV) Regge (JML) GPD (MG-MVdh) Analysis in progress C. Hadjidakis et al., PLB 605 GPD formalism (beyond leading order) describes approximately data for xB<0.4, Q2 >1.5 GeV2 Two-pion invariant mass spectra

  14. Deeply virtual ω production at 5.75 GeV Analysis of cross sections from ep → epπ+X configurations L. Morand et al., hep-ex/0504057, to be published in EPJA Analysis of ω polarizationfrom ep → epπ+π-X configurations (for the first time for this channel above Q2 ~ 1 GeV2) • Cross sections larger than anticipated at high t (see J.-M. Laget, PRD 70, 054023) • SCHC does not seem to hold → not possible to extract σL • 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 Q2

  15. JLab dedicated DVCS experiments in 2004 - 2005 JLab/Hall A JLab/CLAS Calorimetrer and supraconducting magnet within CLAS torus Dedicated, high statistics, DVCS experiments → Detection of 3 particles e, p and γ in final state → Firmly establish scaling laws (up to Q2 ~ 5 GeV2), if observed, or deviations thereof understood, first significant measurement of GPDs. → Large kinematical coverage in xB and t leads to femto-tomography of the nucleon → Opens the way for an ambitious program with JLab@12GeV (CLAS12 and other) p e’ e γ

  16. Experiment completed (Sept-Nov. 2004) PbF2 electromagnetic calorimeter Scintillator array for proton detection Fast-digitizing electronics → analysis of double pulses (pile-up) (P. Bertin, C.E. Hyde-Wright, R. Ransome, F. Sabatié, et al. CEBAF/E-00-110) First double and triple coincidences ! (luminosity = 1037 cm-2s-1) HMS•calo HMS•calo•scint.

  17. 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-03-106) Veto detector added to the p-DVCS set-up → (within a model) Sensitivity to quark angular momentum J Experiment completed (Nov.-Dec. 2004)

  18. CLAS/DVCS Supraconducting solenoid 424 crystals, 16 mm long, pointing geometry, ~ 1.2 degree/crystal, APD readout Inner calorimeter (PbWO4) Calibration from π0→γγ σ = 7.5 MeV η Photon detection in IC and EC (view from target) Mγγ (GeV)

  19. Projected results (sample) Dependence of j asymmetry and total cross-section as a function of xB, t, Q2 , j (372 bins) E01-113 V. Burkert, L. Edouardrihi, M. Garçon, S. Stepanyan et al.Run March-May 2005

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

  21. 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.

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