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Deep Virtual Compton Scattering : experimental status and perspectives

Deep Virtual Compton Scattering : experimental status and perspectives. Nucleon'05, 14/10/05. M. Guidal, IPN Orsay. 1/ Generalities about GPDs. 2/ Review of experimental data. 3/ Perspectives. Leading order/twist (handbag diagram) accessible at moderate Q 2 (precocious scaling),.

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Deep Virtual Compton Scattering : experimental status and perspectives

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  1. Deep Virtual Compton Scattering : experimental status and perspectives Nucleon'05, 14/10/05 M. Guidal, IPN Orsay

  2. 1/ Generalities about GPDs 2/ Review of experimental data 3/ Perspectives

  3. Leading order/twist (handbag diagram) accessible at moderate Q2 (precocious scaling), Interference with Bethe-Heitler process, Bethe-Heitler e’ H,E(x,x,t) H,E(x,x,t) DVCS: ~ ~ e g g e’ e’ g* e g e g* g* p p’ p p’ p p’ GPDs DVCS : Golden process to access GPDs g* t g -2x x+x x-x p’(=p+D) p

  4. x /2 2 t=(p-p ’) x= B 1-x /2 B x = xB ! ds 1 1 2 q q 2 2 H (x,x,t,Q ) E (x,x,t,Q ) dx dx +…. ~ A +B 2 dQ d x dt x-x+ie x-x+ie B -1 -1 Deconvolution needed ! x : mute variable ~ ~ H,E,H,E Hq(x,x,t) but only x and t accessible experimentally g* t g,M,... x~xB x p p’

  5. GPD and DVCS (at leading order:) Beam or target spin asymmetry contain only ImT, therefore GPDs at x = x and -x Cross-section measurement and beam charge asymmetry (ReT) integrate GPDs over x (M. Vanderhaeghen)

  6. Ds 2s ep epg s+ - s- s+ + s- A = = x = xB/(2-xB) k = -t/4M2 Global analysis of polarized and unpolarized data needed for GPD separation Polarized beam, unpolarized target: ~ ~ DsLU~ sinf{F1H+ x(F1+F2)H+kF2E}df H(x,x,t),H(x,x,t), E(x,x,t) (BSA) Kinematically suppressed Unpolarized beam, longitudinal target: ~ ~ H, H DsUL~ sinf{F1H+x(F1+F2)(H+ … }df (l)TSA Unpolarized beam, transverse target: H, E DsUT~ sinf{k(F2H – F1E) + …..}df (t)TSA

  7. The actors

  8. The present :(dedicatedexperiments) JLab Hall A Ds, BSA CLAS(5.75 GeV)Ds, BSA The future : HERMES (recoil detector) COMPASS (recoil detector) JLab(12 GeV) DVCS timeline The past :(evidence for handbag mechanism in non-dedicatedexperiments) HERA s HERMES BSA HERMES BCA, (l,t)TSA(preliminary) CLAS(4.2 GeV)BSA CLAS(4.8 GeV)BSA, (l)TSA(preliminary)

  9. The past(non-dedicated experiments)

  10. Deep Exclusive reactions: an experimental challenge Missing mass MX2 ep  epX MAMI 850 MeV ep  epX Hall A 4 GeV • Resolution • Exclusivity • Luminosity γ π0 ep  epX CLAS 4.2 GeV are the key issues for this physics! N N+π ep  eγX HERMES 28 GeV

  11. Beam Spin Asymmetry (BSA) (ep epg) (ep epg) S

  12. First experimental signatures First observations of DVCS beam asymmetries in 2000 CLAS HERMES Q2 = 2.6 GeV2 , xB = 0.11, -t = 0.27 GeV2 Q2 = 1.25 GeV2 , xB = 0.19, -t = 0.19 GeV2 Phys.Rev.Lett.87:182002,2001 Phys.Rev.Lett.87:182001,2001 Vdh, Guichon, Guidal (1999) twist-2 + twist-3 : Kivel, Polyakov, Vdh (2000)

  13. 0.15 < xB< 0.4 1.50 < Q2 < 4.5 GeV2 -t < 0.5 GeV2 PRELIMINARY PRELIMINARY Vdh, Guichon, Guidal (1999) calculation 5.75 GeV data(H. Avakian & L. Elhouadrhiri) twist-2 + twist-3 Kivel, Polyakov, Vdh (2000) CLAS/DVCS at 4.8 and 5.75 GeV PRELIMINARY GPD based predictions (BMK) 4.8 GeV data(G. Gavalian) :

  14. Charge Spin Asymmetry (BCA) (e-p epg) (e+p epg) S

  15. Vdh, Guichon, Guidal (1999) : calculation Guidal, Polyakov, Radyushkin, VdH (2005) Beam charge asymmetry t-dependence tiny e-p sample (L~10 pb-1) HERA: 2004-2005 e- beam (x5)

  16. (longitudinal) Target Spin Asymmetry (l TSA) (ep epg) (ep epg) S

  17. Longitudinal target spin asymmetry HERMES Preliminary target spin asymmetries on p and d sin f in agreement with GPD models Preliminary HERMES data but unexpected large sin 2f Preliminary CLAS data Experimental Studies with CLAS Data were collected as a by-product during the eg1 2000run: 5.75 GeV with NH3 longitudinally polarized target, <Q2> ~ 1.8 GeV2

  18. (transverse) Target Spin Asymmetry (t TSA) (ep epg) (ep epg) S

  19. Transverse target spin asymmetry

  20. b x (fm) Guidal, Polyakov, Radyushkin, VdH (2005) y z x

  21. orbital angular momentum carried by quarks evaluated at μ2 = 2.5 GeV2

  22. The present (dedicated experiments)

  23. JLab dedicated DVCS experiments in 2004 - 2005 High statistics and unambiguous epg final state determination JLab/Hall A JLab/CLAS Calorimeter and supraconducting magnet within CLAS torus p e’ e γ

  24. A typical event in CLAS (Hall B, JLab) e’ g Add an EM calorimeter at forward angles p 420 PbWO4 crystals : ~10x10 mm2, l=160 mm Read-out : APDs +preamps JLab/ITEP/ Orsay/Saclay collaboration 1/ DVCS (Hall B) epa epg Add a “Moller shield” solenoid around the target

  25. Data taking :MarchtoMay 2005 CLAS(preliminary analysis of a 2 hours run) All (eγp) events Calibration from π0→γγ σ = 7.5 MeV (eγp) events after kinematical cuts η Mγγ (GeV)

  26. Expected Kinematical Dependencies About 380 bins in f, xB, t

  27. 2/DVCS in JLab/Hall A Experiment completed (Nov.-Dec. 2004) • High Resolution Hall A spectrometer for electron detection • 100-channel scintillator array for proton detection • 132-block PbF2 electromagnetic calorimeter for photon detection • Detection of all • 3 final-state particles • ensures exclusivity

  28. DVCS on the neutron DVCS-BH interference generates a beam spin cross section difference Main contribution for the proton Main contribution for the neutron → (within a model) Sensitivity to quark angular momentum J Veto detector added to the p-DVCS set-up

  29. New CLAS experiment : Longitudinal Target Spin Asymmetry ~ DsUL~ sinfIm{F1H+x(F1+F2)(H+.. } • CLAS eg1 (preliminary) • CLAS (eg1+IC) projected 6 GeV run with NH3 longitudinally polarized target (CLAS + IC) 60 days of beam time Approved at the latest JLab PAC

  30. The future

  31. Recoil detector nov. 2005 for 2 years Detection of the recoiling proton clean reaction identification improve statistical precision (unpolarised data with high density target)

  32. (f (x), g (x), F (t), G (t), F(z), pion cloud, Jq…) 1 1 1 A Summary The most complete information on the structure of the nucleon : GPDs EXPERIMENT : Up to 2005 : first experimental signatures in different Kinematics, in different observables, are very encouraging We enter a new era with high-statistics, high resolution dedicated experiments: definitely sign the validity of the approach (factorization, scaling,...) and we are on our way to extract/strongly constraint the GPDs and extract the physics

  33. DVCS with a polarized target: New CLAS experiment ~ DsLL~ CBH+ cosfRe{F1H+x(F1+F2)(H+xE/2)+.. } 5.7 GeV run with NH3longitudinally polarized target+IC Projections for 60 days The double spin asymmetry in DVCS with longitudinally polarized target will provide access to the real part of Compton form factors

  34. DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm prerequisities perspectives [A.Freund, M. McDermott EPJC23(2002)]  Hg Hq DVCS cross section • first measurement of t –slope: ds/dt = ds/dt|t=0∙exp(-bt) b= 6.02±0.35±0.39 GeV-2 absolute normalisation! • comparison to NLO QCD: band width given by b measurement

  35. Status of GPDs Studies at Jefferson Lab GPD Reaction Obs. Expt Status ep→epγ (DVCS)BSA CLAS 4.2 GeV Published PRL CLAS 4.8- 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 From ep → epX Dedicated set-up Dedicated set-up

  36. 0.15 < xB< 0.4 1.50 < Q2 < 4.5 GeV2 -t < 0.5 GeV2 PRELIMINARY PRELIMINARY 5.75 GeV data(H. Avakian & L. Elhouadrhiri) CLAS/DVCS at 4.8 and 5.75 GeV PRELIMINARY GPD based predictions (BMK) 4.8 GeV data(G. Gavalian)

  37. Beam Spin Asymmetry Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives →H Beam spin and charge asymmetry [PRL87,2001]

  38. 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

  39. Energy dependence BH DVCS Calculation (M.G.&M.Vanderhaeghen)

  40. DDVCS: first observation of ep → epe+e- * 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

  41. Compton Scattering “DVCS” (Deep Virtual Compton Scattering)

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