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Deep Virtual Compton Scattering : experimental status and perspectives

This document presents a comprehensive overview of the current experimental status and future perspectives of Deep Virtual Compton Scattering (DVCS). It discusses General Parton Distributions (GPDs), experimental data reviews, and key processes such as the handbag diagram and its interference with the Bethe-Heitler process. The analysis covers dedicated experiments at JLab and HERA, emphasizing the importance of beam and target spin asymmetries in extracting GPD information. The future direction of DVCS research, particularly in understanding the quark's orbital angular momentum, is also outlined.

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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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