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Generalized Parton Distributions studies @ JLab

Generalized Parton Distributions studies @ JLab. Franck Sabatié CEA Saclay. IWHSS’08 - Torino March 31 st , 2007. Introduction DVCS in JLab/Hall A DVCS in JLab/Hall B Comparison with models p 0 electroproduction in Halls A/B. Theory. Handbag Diagram. g *. g. GPDs. x+ x. x- x.

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Generalized Parton Distributions studies @ JLab

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  1. Generalized Parton Distributions studies @ JLab Franck Sabatié CEA Saclay IWHSS’08 - Torino March 31st, 2007 • Introduction • DVCS in JLab/Hall A • DVCS in JLab/Hall B • Comparison with models • p0 electroproduction in Halls A/B

  2. Theory Handbag Diagram g* g GPDs x+x x-x Factorization theorem states: In the suitable asymptotic limit, the handbag diagram is the leading contribution to DVCS. • 2. The GPDs enter the DVCS amplitude as an integral over x: • GPDs appear in the real part through a PP integral over x • GPDs appear in the imaginary part but at the line x=x Q2 and n large at xB and t fixed 1. Needs to be checked !!! Experiment Physical process g* g* g g t GPDs from Theory to Experiment but it’s not so simple… Collins, Freund

  3. Experimental observables linked to GPDs Experimentally, DVCS is undistinguishable with Bethe-Heitler However, we know FF at low t and BH is fully calculable Using a polarized beam on an unpolarized target, 2 observables can be measured: At low energy, |TDVCS|2 supposed small Ji, Kroll, Guichon, Diehl, Pire, …

  4. hadronic plane j g e-’ g* e- p leptonic plane Into the harmonic structure of DVCS |TBH|2 Interference term BH propagators j dependence Belitsky, Mueller, Kirchner

  5. Special case of the asymmetry The asymmetry can be written as: Pro: easier experimentally, smaller Radiative corr., smaller systematics Con: direct extraction of GPDs is model- (or hypothesis-) dependent (denominator complicated and unknown) It was naturally the first observable extracted from non-dedicated experiments… Potentially many more terms

  6. Dedicated DVCS experiment with CLAS proposed/accepted: E01-113, proton DVCS in Hall B Dedicated DVCS experiment in Hall A proposed/accepted: E03-106, neutron DVCS in Hall A First dedicated DVCS experiment proposed and accepted by a PAC: E00-110, proton DVCS in Hall A Jefferson Lab. Preliminary results from JLab experiments First experimental paper using H1-ZEUS non-dedicated data: Observation of DVCS at very low x Intense theoretical work begings: higher twist effects, corrections, models, interpretation of various F.T., full harmonic description, … The three JLab experiments take data within 1 year (2004-2005), about 4 years after the first proposition to the JLab PAC. HERMES and CLAS publish non-dedicated data on DVCS in the same PRL issue. They both show a large sine wave in the BSA, proving they indeed deal with the interference of DVCS and BH. Ji and Radyushkin show how the GPDs are interesting, and how to access them. Collins and Freund prove the factorization, Diehl, Pire and Gousset point out the rich harmonic structure of DVCS Invention of GPDs my Müller, who needed the theoretical object for his thesis work Timeline

  7. Published non-dedicated results on ALU and AUL JLab/Hall B - E1 & HERMES ALU JLab/Hall B - Eg1 AUL PRL 97, 072002 (2006) CLAS: PRL 87, 182002 (2001) HERMES: PRL 87, 182001 (2001) Both results show, with a limited statistics, a sin j behavior (necessary condition for handbag dominance) In the ALU result, DD models (VGG) tend to over-estimate the data

  8. E00-110 experimental setup • 75% polarized 2.5uA electron beam • 15cm LH2 target • Left Hall A HRS with electron package • 11x12 block PbF2 electromagnetic calorimeter • 5x20 block plastic scintillator array 50 days of beam time in the fall 2004, at 2.5mA

  9. Designing the Hall A DVCS experiment Measuring cross-sections differential in 4 variables requires: Measuring cross-sections differential in 4 variables requires: • A good knowledge of the acceptance e p →e (p) g Scattered electron The HRS acceptance is well known Emitted photon The calorimeter has a simple rectangular acceptance R-function cut g Perfect acceptance matching by design ! Virtual photon « acceptance » placed at center of calorimeter g* Simply: t: radius j: phase

  10. Analysis by missing mass (eg)X HRS: Cerenkov, vertex, flat-acceptance cut with R-functions Calo: 1 cluster in coincidence in the calorimeter above 1 GeV With both: subtract accidentals, build missing mass of (e,g) system

  11. Analysis – po subtraction effect on missing mass spectrum Using p0→2g events in the calorimeter, the p0 contribution is subtracted bin by bin After p0 subtraction

  12. Analysis – Exclusivity check using Proton Array and MC Using Proton-Array, we compare the missing mass spectrum of the triple and double-coincidence events. The missing mass spectrum using the Monte-Carlo gives the same position and width. Using the cut shown on the Fig.,the contamination from inelastic channels is estimated to be under 3%. Normalized (e,p,g) triple coincidence events Monte-Carlo (e,g)X – (e,p,g)

  13. Difference of cross-sections PRL97, 262002 (2006) Twist-2 Twist-3 Extracted Twist-3 contribution small ! Corrected for real+virtual RC Corrected for efficiency Corrected for acceptance Corrected for resolution effects Checked elastic cross-section @ ~1%

  14. Supposedly mostly H (~80%) Q2 dependence and test of scaling PRL97, 262002 (2006) <-t>=0.26 GeV2, <xB>=0.36 Twist-2 Twist-3 No Q2 dependence using BMK separation: strong indication for scaling behavior and handbag dominance Twist 4+ contributions are smaller than 10%

  15. Total cross-section PRL97, 262002 (2006) Extracted Twist-3 contribution small ! Corrected for real+virtual RC Corrected for efficiency Corrected for acceptance Corrected for resolution effects but impossible to disentangle DVCS2 from the interference term !

  16. DVCS on the neutron in JLab/Hall A: E03-106 LD2 target 24000 fb-1 xB=0.36, Q2=1.9 GeV2 MODEL-DEPENDENT Ju-Jd extraction

  17. E1-DVCS with CLAS : a dedicated DVCS experiment in Hall B Beam energy: ~5.8 GeV Beam Polarization: 75-85% Integ. Luminosity: 45 fb-1 Next half of data in 2008 Mgg (GeV2) Inner Calorimeter + Moller shielding solenoid ~50 cm

  18. E1-DVCS kinematical coverage and binning W2 > 4 GeV2 Q2 > 1 GeV2

  19. <-t> = 0.18 GeV2 <-t> = 0.30 GeV2 <-t> = 0.49 GeV2 <-t> = 0.76 GeV2 E1-DVCS : Asymmetry as a function of xB and Q2 Accurate data in a large kinematical domain Integrated over t

  20. VGG twist-2+3 VGG twist-2 JM Laget E1-DVCS : ALU(90°) as a function of -t + models Accepted by PRL

  21. E1-DVCS : Cross-sections over a wide kinematical range PhD Thesis H.S. Jo 0.2<-t<0.4 0.4<-t<0.6 0.09<-t<0.2 PRELIMINARY, Analysis in Progress 0.6<-t<1 1<-t<1.5 1.5<-t<2

  22. New developments in GPD modeling : minimal dual model arXiv:0803.1271v2 Polyakov, Vanderhaeghen Central quantities are t-dependent parton distributions q and e(x,t), obtained as x→0 limits of the nucleon GPDs: q and e are estimated using a dual representation (parton distributions as an infinite series of t-channel exchanges). Only the leading function is kept for simplicity. Only the GPD H is kept, supposedly dominant in DVCS. It is a simple one-parameter model with strong predictive content. If part of the data cannot be fit, it will mean higher twists/genuine non-forward effects need to be included: first step to understand DVCS data.

  23. Minimal dual model, confrontation with data Difference of cross sections (imaginary part of interference term) Data are perfectly described by minimal dual model DD model (VGG) Minimal dual model Data: Hall A polarized cross section data at Q2=2.3 GeV2

  24. Minimal dual model, confrontation with data (2) DD model (VGG) Minimal dual model Beam Spin Asymmetries (Hall B data) Total cross sections (Hall A data)

  25. p0 f e’ e leptonic plane p’ hadronic plane d4s d2s = G(Q2,xB) dQ2dxBdfdt dfdt 1 +√ 2e(e+1) ( cosf + e + e cos2f = d2s 2p dfdt sinf) +h√ 2e(e-1) dsL’T dsLT dsTT dsT dsL dt dt dt dt dt p0 electroproduction cross section • Q2= - (k-k’)2 • xB = Q2/2Mn n=Ee-Ee’ • t = (p-p’)2 • f (angle between leptonic and • hadronic plane) reduced cross section forg*p→pp0

  26. p0 cross sections in Hall A – a puzzle ! Submitted soon • Low-t cross-section largely • overshoots GPD & JML models !!! • TT term is large, suggesting a large transverse component. • But p0 cross-section ratio for proton and deuteron targets is found ~ 1 • More data needed ! Laget VGG

  27. p0 cross sections in Hall B S. Niccolai sT+esL vs. –t (GeV2/c2) PRELIMINARY, Analysis in Progress Arbitrary units Statistical errors only

  28. Summary Scaling test of DVCS cross section difference → DD models over-estimate it → Minimal dual model provide a good description Asymmetry measured in a large kinematical range, badly described by models. Cross section measured both accurately and (soon) in a large kinematical range, also badly described by models: suggest genuine off-forward contribution and/or higher twist at work. p0 cross sections (unseparated) measured: not well described by either Regge model or GPD model: a challenge for theory ! Like Stefano said: mode data are needed, but modelling is only in its first phase and shows interesting progress recently. JLab 6 & 12 GeV, HERMES recoil, COMPASS + upcoming Theory/Phenomenology work

  29. Experimental Setup and proposed experiments at 11 GeV Use of base CLAS12 equipment, including Inner Calorimeter (IC) Detection of the full (e,p,g) final state Perform 2 experiments for the extraction of the BSA and the TSA IC tungsten shielding Distance, target-IC: ~1.75m currently (note: this is NOT optimized for DVCS!)

  30. to 10% statistical error at high xB From 1% statistical error on extracted Twist-2 coefficient Beam Spin Asymmetry IC in standard position – 80 days – 10^35 Lum – VGG model

  31. Observables and their relationship to GPDs The cross-section difference accesses the imaginary part of DVCS and therefore GPDs at x = x The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x

  32. (low/medium energy) Deep Exclusive experiments Published or Finalized Published or Analysis in 2009 – 2001 close to be progress 2010 2010+ 2014? JLab@ 12GeV HERMES 27 GeV DVCS – BSA + BCA + nuclei BSA d-BCA + Polarized target DVCS ep→epρ σL + DSA CLAS 4-5 GeV DVCS BSA CLAS 5.75 GeV DVCS DDVCS ΔDVCS D2VCS Polarized DVCS ep→epρL ep→epωL ep→epπ0/η ep→enπ+ ep→epΦ Hall A 6 GeV DVCS proton neutron ep→epπo CLAS 6 GeV DVCS BSA Proton ep→epπo/η HERMES DVCS BSA+BCA With recoil detector COMPASS DVCS s+BCA With recoil detector JLab Hall A DVCS² CLAS BSA+TSA EVERYTHING, with more statistics than ever before In red: selected topics for JLab

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