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M. Guidal, IPN Orsay

ICHEP, 21/07/2010. Extraction of Compton Form Factors from DVCS data. M. Guidal, IPN Orsay. General introduction to GPDs. From data to GPDs. General introduction to GPDs. From data to GPDs. Structure function in momentum coordinates. Operator in space coordinates. Process.

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M. Guidal, IPN Orsay

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  1. ICHEP, 21/07/2010 Extraction of Compton Form Factors from DVCS data M. Guidal, IPN Orsay

  2. General introduction to GPDs From data to GPDs

  3. General introduction to GPDs From data to GPDs

  4. Structure function in momentum coordinates Operator in space coordinates Process Diagramme ep a ep ep a epg ep a eX (restricting myself to LT-LO, chiral even, quark sector)

  5. Standard Parton Distributions Elastic Form Factors Ji’s sum rule 2Jq =  x(H+E)(x,ξ,0)dx x x : don’t appear in DIS : NEW INFORMATION (nucleon spin)  H(x,ξ,t)dx = F(t)( ξ) H(x,0,0) = q(x), H(x,0,0) = Δq(x) ~ t γ, π, ρ, ω… -2ξ x+ξ x-ξ ~ ~ H, H, E, E (x,ξ,t)

  6. Long.mom./trans.pos. correlations Pion cloud F (t), G (t) x,b 1,2 A,PS « D-term » 0 <x > GPDs F(z) DDs t=0 -1 <x > q(x),D q(x) 1 <x > J R (t),R (t) q A V

  7. x /2 2 t=(p-p ’) x= B 1-x /2 B x = xB! ds 1 1 2 q q H (x,x,t) E (x,x,t) dx dx +…. ~ A +B 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’

  8. 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

  9. General introduction to GPDs From data to GPDs

  10. The experimental actors DESY HERMES H1/ZEUS p-DVCS BSA,BCA, tTSA,lTSA p-DVCS X-sec,BCA JLab Hall A Hall B p-DVCS X-sec p-DVCS BSAs,lTSAs

  11. In general, 8 GPD quantities accessible (Compton Form Factors) DVCS : golden Channel Anticipated Leading Twist dominance already at low Q2

  12. Only 3 CFFs come out from the fit with finite error bars: HIm , HImand HRe ~ M.G. EPJA 37 (2008) 319 M.G. & H. Moutarde, EPJA 42 (2009) 71) M.G. PLB 689 (2010) 156 M.G. arXiv:1005.4922 [hep-ph] (acc.PLB) Given the well-established LT-LO DVCS+BH amplitude DVCS Bethe-Heitler GPDs Model-independent fit, at fixed xB, t and Q2, of DVCS observables with MINUIT + MINOS 7 unknowns (the CFFs), non-linear problem, strong correlations

  13. (model dependent Fit of D. Muller, K. Kumericki Hep-ph 0904.0458 HIm HRe JLab xB=0.36,Q2=2.3 As energy increases: * « Shrinkage » of HIm HIm HRe HERMES * HIm>HRe xB=0.09,Q2=2.5 *Different t-behavior for HIm&HRe

  14. xB dependence at fixed t of HIm VGG prediction

  15. Fitting the CLAS & HERMESlTSAs: xB-dependence at fixed t ~ of HIm HERMES JLab Fit with 7 CFFs (boundaries 5xVGG CFFs) Fit with 7 CFFs (boundaries 3xVGG CFFs) VGG prediction

  16. t-dependence at fixedxB ~ of HIm& HIm Axial charge more concentrated than electromagnetic charge ? Fit with 7 CFFs (boundaries 3xVGG CFFs) Fit with 7 CFFs (boundaries 5xVGG CFFs) ~ Fit with ONLYH and H VGG prediction

  17. Procedure tested by Monte-Carlo Procedure is working on real data; extraction of HIm and HReat JLab (cross sections) and HERMES (asymmetries) energies Relatively large uncertainties on extracted CFFs (due to lack of observables -and precision on data-) Introducing more theoretical input will reduce uncertainties (but model dependency) Large flow of new observables and data expected soon; will bring much more experimental constraints to extract CFFs with minimum theoretical input First CFFs model independent fits (leading-twist/leading order approximation); “Minimal theoretical input”

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