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polarized positive + and negative - polarized positive + and negative - muon beams @ to perform DVCS measurements for GPD study for GPD study Nicole dHose, CEA-Saclay On behalf of the COMPASS collaboration workshop on positrons JLab 25 March 2009

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exclusive single-photon production μ p μ p d |T DVCS | 2 + |T BH | 2 + Interference Term d |T DVCS | 2 + |T BH | 2 + Interference Term DVCS small t p p BH GPD slow p small t At COMPASS or BH or DVCS can dominate the cross section

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COMPASS : domain 10 -2 < x < 10 -1 JLab COMPASS HERMES ZEUS H1 Limit due to luminosity ? Limit due to acceptance BH dominance

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|t |<1 GeV 2 x B =0.05 x B =0.1 x B =0.2 Q 2 =2 Q 2 =4 DVCS dominates BH important |t |<1 GeV 2 DVCS /2 BH/10 DVCS /2 BH/10 Comparison BH and DVCS at 160 GeV

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dσ (μp μp ) = dσ BH + dσ DVCS unpol + P μ dσ DVCS pol + e μ a BH Re A DVCS + e μ P μ a BH Im A DVCS Twist-3 M 01 DVCS + BH with + and - beam and unpolarized target Twist-2 M 11 Twist-2 gluon M -11 >> θ μ μ * p μ p μ p BH calculableDVCS + Belitsky,Müller,Kirchner )sin( DVCS pol s Qy e d 1 22 6 = 2P μ + 2e μ Beam Charge & spin difference D U,CS

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dσ (μp μp ) = dσ BH + dσ DVCS unpol + P μ dσ DVCS pol + e μ a BH Re A DVCS + e μ P μ a BH Im A DVCS Twist-3 M 01 DVCS + BH with + and - beam and unpolarized target Twist-2 M 11 Twist-2 gluon M -11 >> φ θ μ μ * p μ p μ p BH calculableDVCS + Belitsky,Müller,Kirchner )sin( DVCS pol s Qy e d 1 22 6 = 2P μ + 2e μ This is the good way to get rid of the large BH contribution if we are able to control the + and - fluxes and efficiencies we can extract c 0 Int and c 1 Int Beam Charge & spin difference D U,CS

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dσ (μp μp ) = dσ BH + dσ DVCS unpol + P μ dσ DVCS pol + e μ a BH Re A DVCS + e μ P μ a BH Im A DVCS Twist-3 M 01 Twist-2 M 11 Twist-2 gluon M -11 >> φ θ μ μ * p μ p μ p BH calculableDVCS + Belitsky,Müller,Kirchner = 2 + 2 + 2 e μ P μ Beam Charge & spin sum S U,CS DVCS + BH with + and - beam and unpolarized target

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dσ (μp μp ) = dσ BH + dσ DVCS unpol + P μ dσ DVCS pol + e μ a BH Re A DVCS + e μ P μ a BH Im A DVCS Twist-3 M 01 Twist-2 M 11 Twist-2 gluon M -11 >> φ θ μ μ * p μ p μ p BH calculableDVCS + Belitsky,Müller,Kirchner = 2 + 2 + 2 e μ P μ After integration over φ in all the acceptance and subtraction of BH contrib. contribution we can get DVCS contribution only, C 0 DVCS Beam Charge & spin sum S U,CS DVCS + BH with + and - beam and unpolarized target

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Using D U,CS / S U,CS : Beam Charge and Spin Asymmetry Comparison to different models from VGG statistical errors still under debate H(x,0,t) = q(x) e t = q(x) / x α t = α ln 1/x (α~0.1) H(x,ξ,t) ~ q(x) F(t)

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New predictions from Dieter Mueller BC&SA a Global Fit Dieter Mueller uses a Global Fit on H1/ZEUS, HERMES, CLAS and JLab Hall A and no JLab Hall A

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Using S U,CS : d DVCS / dt ~ exp(Bt) Projections for the t-slope measurement at COMPASS α =0.125 GeV -2 b 0 =5.83 0.5 GeV -2 x 0 =0.0012 (Q 2 =8,W=82) 1
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Still many open questions on FoM and Systematic effects: Efficiency to reject background that simulates exclusive μp μ p events Final accuracy of the + and - fluxes and detection efficiencies to combine both + and - measurements for D U,CS Final accuracy on the procedure to subtract the known BH contribution correctly renormalized to the measured yield for S U,CS Goal of a DVCS pilot run in 2009: - do an absolute measurement of BH (dominant at small x) FoM + - record and identify a few DVCS events very important for - collect μp μX or μ + ( -) ( °) X μp μ p ( °) ( )

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In 2008 In 2008 with 1.5 days of beam of low intensity done in a rush before the shut down (LHC accident) we have observed ~100 Single Eclusive photons identified to BH due to their distribution in analysis to be released tomorrow

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bins for BH with minimal DVCS contamination bins for BH with minimal DVCS contamination to study the phi dependence to study the phi dependence to determine the FoM to determine the FoM 6 % 1
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first insight for DVCS first insight for DVCS Experimental determination of the DVCS yield to control the credibility of estimation in 2009 15 Perform accurate control of: - the + and - fluxes - the halos flux - the detection efficiencies collect with μ+ and μ- inclusive, semi-inclusive collect with μ+ and μ- inclusive, semi-inclusive and exclusive events: and exclusive events: μp μX or μ + ( -) ( °) X or μ p ( °) ( ) μp μX or μ + ( -) ( °) X or μ p ( °) ( )

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roadmap - Letter of Intent CERN-SPSC-2009-003 a complete proposal for june 2009 - Demonstrate the feasibility of such an experiement - 1rst phase in ~2011/12 DVCS with unpolarised proton target - 2 nd phase in ~2013/14 DVCS with polarised proton target new equipments needed

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d /dt impact parameter b Proposal to study GPDs @ COMPASS in 2 phases Phase 1: DVCS experiment in ~2011/12 to constrain GPD H with +, - beam + unpolarized long LH2 (proton) target + recoil detector + ECAL1,2 (possibly 0) + all COMPASS equipment Phase 2: DVCS experiment in ~2013/14 to constrain GPD E with + and transversely polarized NH3 (proton) target + recoil detector + ECAL1,2 (possibly 0) + all COMPASS equipment d ( +, ) + d ( -, ) Im( F 1 H) sin d ( +, ) - d ( -, ) Re( F 1 H) cos d (, S ) - d (, S +π) Im( F 2 H – F 1 E) sin ( - S ) cos

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