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P-Y BERTIN Jefferson Laboratory and Université BLAISE PASCAL- IN2P3/CNRS for the DVCS HALL A collaboration 0 exclusive electro production Q 2 =2.3 GeV 2, X Bj =.36

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LH 2 / LD 2 target Polarized Electron Beam Scattered Electron Left HRS Electromagnetic Calorimeter DVCS events are identified with M X 2 Beam energy = 5.75 GeV Beam polarization = 75% Beam current = ~ 2 and 4 μA Luminosity = 1 and 4. 10 37 cm -2.s -1 nucleon -1 - >6.3° - Čerenkov based Electromagnetic Calorimeter -Specific Scattering Chamber -Customized Electronics & Data Acquisition Two-arm experiment : spectrometer and calorimeter Exclusivity

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Raw data Simulation X cross sections p(e,e 0 ) (M p +m ) 2 Photon detection threshol correction q 1 being the smallest enegy photon of Th= 1.00 GeV = 1.15 GeV = 1.25 GeV

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p At Q 2 =2.3 GeV 2 and x bj =.36 The continuum is significant compared to the p(e,e 0 )p The resonances are washed out into the continuum. As seen also in DIS Q 2 =2.0 GeV 2 Q 2 =0 GeV 2

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(M p +m ) 2 (M p +2m ) 2 (M p +3m ) 2 Br=8.5%

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-t GeV 2 0.1 0.2 0.3 0.01 0.02 0.03 b=-2 GeV -2 From Hall B

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(M p +m ) 2 (M p +2m ) 2 (M p +3m ) 2 Br=8.5% Br=100% Br=85% in our cut ~1% 0 p But we detect only e 0 => all the process interfere =>sum of the amplitudes What I am doing ? Semi inclusive DIS and Duality ??

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For each exclusive 0 event selected in the cut on M x 2 The physic variable are determined exactly Q 2, x bj with the spectrometer t with the position in the calorimeter ~2-3 mrd) Photons in the inner calorimeter (99 Block from 132) Window coincidence +/- 3 ns Accidentals e substracted Window 105<m MeV

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Analyze in the formalism of on photon exchange

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Chew, Goldberger, Low and Nambu Decomposition in CGLM amplitude : 0 cm angle ~ -(t-t min ) 6 amplitudes complex rTrT rLrL r TT r TL Reduced response functions r s

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All the trivial kinematics dependence, photon flux,, sin,…. Taken in account in the, Monte Carlo with radiative corrections, detector resolution,…. Use the same extraction method that the used for DVCS which take in account the bin migration by a global linear fit on 10(t)x24(f) experimental bins r TL ~5x(r T + r L )

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PRELIMINARY Q 2 =2.3 GeV 2 x Bj =0.36 =0.64 Corrected for real+virtual RC Corrected for efficiency Corrected for acceptance Corrected for resolution effects Systematic errors include : trigger threshold stability ( 1 to 1.2 GeV) missing mass cut (.9 to 1.15 GeV 2 ) extrapolation at fixed Q 2 and fixed X Bj. spectrometer, luminosity…… Extrapoled at fixed:

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+ coupling to n, with re-scattering Regge trajectory Exchange (, and B1) J. M. Lagets Prediction underestimates the cross-sections by a factor 5 JML x5 JML x5 JML x5 JML x5

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and = ??? Next experiment 2009 will allow a full Rosenbluth separation s Factorization hold only for longitudinal amplitude

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L Longitudinal part Prediction from model (VGG) based on Hand bag model and GPD VGG M Vanderhaeghen P. Guichon and M Gidal VGG x 5

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1 +-0 +- 2 +-0 But quid of two photons exchange? How to check validity of the one photon exchange ??

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END

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Photon electroproduction

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Analysis – Exclusivity check using Proton Array and MC Normalized (e,p, ) triple coincidence events Using extra recoil Proton-detector, we have checked the missing mass spectrum of double-coincidence events with those of a triple -coincidence. Monte-Carlo (e, )X – (e,p, ) 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%.

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Experimental observables linked to GPDs Experimentally, DVCS is undistinguishable with Bethe-Heitler However, we know FF at low t and BH is fully calculable Interference term allows access to linear amplitude Using a polarized beam on an unpolarized target 2 observables can be measured: At JLab energies, |T DVCS | 2 was supposed small Kroll, Guichon, Diehl, Pire, …

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Tests of scaling 1. Twist-2 terms should dominate and All coefficients have Q 2 dependence which can be tested!

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

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Q 2 dependence and test of scaling =0.26 GeV 2, =0.36 No Q 2 dependence: strong indication for scaling behavior and handbag dominance Twist-2 Twist-3

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Total cross-section Corrected for real+virtual RC Corrected for efficiency Corrected for acceptance Corrected for resolution effects Extracted Twist-3 contribution small ! PRL97, 262002 (2006) And it is impossible to disentangle DVCS 2 from the interference term large

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We have proposed to use different beam energies (different BH) to : ( experiment approved and planned to run end 2009 ) 1. Isolate the BH-DVCS interference term from the pure DVCS 2 Contribution (as a function of Q 2 ) Extraction of both linear and bilinear combinaton of GPDs Additional test of DVCS scaling ( unpolarized cross section) 2. Measure 5 response functions of the deep virtual 0 channel First test of factorization in ep ep 0 using L If test is positive, valuable complementary ( flavor) information in GPDs

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This proposal: assuming DVCS 2 =20

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On the deuterium

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Deuterium=proton+neutron+deuteron - Hydrogen=proton = neutron + deuteron Mn2Mn2 M n 2 +t/2 Missing mass assuming a proton target Helicity Asymmetry

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n-DVCS d-DVCS PRELIMINARY Deuteron contribution compatible with zero at large -t F. Cano & B. Pire calculation Eur. Phys. J. A19, 423(2004). PRELIMINARY Neutron contribution is small and compatible with zero Results can constrain GPD models (and therefore GPD E n ) VGG Code : M. Vanderhaeghen, P. Guichon and M. Guidal PRELIMINARY

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the Scaling test is positive Transverse -Transverse large. Description in terms of Quarks GPD and Hadronic description ( Regge exchange) miss by a factor 5~15 the data. VGG model misses by 30% DVCS 2 must be taken into account Agree with F. Cano B. Pire model To summarize

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New data taking in 2009 using 2 beam energies: Full extraction of linear terms and bilinear terms of GPDs Full separation of T and L for 0 electro production At Q 2 =1.5, 1.9, 2.3 GeV 2 We have demonstrated that : high precision DVCS measurements are doable using a high resolution spectrometer and a calorimeter Full DVCS program in Hall A (up to Q 2 =9 GeV 2 ) already approved with the 12 GeV upgrade

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END

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100 150 MeV 0 Invariant mass FWHM=21 MeV (M p +m ) 2 p(e,e 0 ) Raw data Simulation X cross sections Photon detection threshol correction q 1 being the smallest enegy photon of

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DVCS Analysis Normalized (e,p, ) triple coincidence events Check of the missing mass spectrum of double-coincidence events with the a triple -coicidence using a Auxilliary Proton array Monte-Carlo (e, )X – (e,p, ) 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%. Raw Raw – 0 (e,p, )

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After : -Normalizing H 2 and D 2 data to the same luminosity -Adding Fermi momentum to H 2 data 2 principle sources of systematic errors : -The contamination of π 0 electroproduction on the neutron (and deuteron). - The uncertainty on the relative calibration between H 2 and D 2 data A. Belitsky,D Muller A Kirchner Compton form factor :

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Deuteron contribution compatible with zero at large -t F. Cano & B. Pire calculation Eur. Phys. J. A19, 423(2004). PRELIMINAY Neutron contribution is small and compatible with zero Results can constrain GPD models (and therefore GPD E n )

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Analysis – Exclusivity check using Proton Array and MC Normalized (e,p, ) triple coincidence events Using extra recoil Proton-detector, we have check the missing mass spectrum of double-coincidence events with the a triple -coicidence. Monte-Carlo (e, )X – (e,p, ) 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%.

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p At Q 2 =2.3 GeV 2 and x bj =.36 The continuum is significant compared to the p(e,e 0 )p The resonances are washed out into the continuum. As seen also in DIS Q 2 =2.0 GeV 2 Q 2 =0 GeV 2

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After : -Normalizing H 2 and D 2 data to the same luminosity -Adding Fermi momentum to H 2 data 2 principle sources of systematic errors : -The contamination of π 0 electroproduction on the neutron (and deuteron). - The uncertainty on the relative calibration between H 2 and D 2 data Helicity Asymety Mx2Mx2 M x 2 +t/2 Missing masse assuming a proton target Deuterium=proton+neutron+Deuton - Hydrogen=proton = neutron + deuton

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Chew, Goldberger, Low and Nambu ==> CGLM Decomposition in CGLM amplitude : 0 cm angle ~ -(t-t min )

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k k q=k-k q

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