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Physics Motivations Sensitivity to physics observables using COMPASS

Possibilities to perform DVCS measurement at COMPASS E. Burtin CEA-Saclay Irfu/SPhN On Behalf of the COMPASS Collaboration DIS 2009 - Madrid - 29 April, 2009. Physics Motivations Sensitivity to physics observables using COMPASS

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Physics Motivations Sensitivity to physics observables using COMPASS

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  1. Possibilities to perform DVCS measurementat COMPASSE. Burtin CEA-Saclay Irfu/SPhNOn Behalf of the COMPASS CollaborationDIS 2009 - Madrid - 29 April, 2009 PhysicsMotivations Sensitivityto physics observables using COMPASS Observation of exclusive photon production in 2008 test run

  2. g* g,p,r hard x+ x- soft GPDs P P’ t for quarks : 4 functions H,E,H,E(x,,t) ~~ Generalized Parton Distributions Factorisation: Q2 large, -t<1 GeV2 contains pdf H(x,0,0) = q(x) measured in DIS Generalized Parton Distributions contains form factors measured in elastic scattering contains information on the nucleon spin : Ji’s sum rule :

  3. *  Q² x+ x- p GPDs t mp=0.87 GeV Towards a 3-D nucleon picture (Px,ry,z) Lattice calculation (unquenched QCD): Negele et al., NP B128 (2004) 170 Göckeler et al., NP B140 (2005) 399  small valence quark core  widely spread sea q and gluons Hard Exclusive Scattering Deeply Virtual Compton Scattering ep ep p z x P r y x boost Chiral dynamics: Strikman et al., PRD69 (2004) 054012 gluon density generated by the  cloud increase of the N transverse size for xBj < mπ/mp=0.14 GPDs : H( x,,t ) Fourier access to correlations : ( Px, ry,z) Burkardt,Belitsky,Müller,Ralston,Pire

  4. E=190, 100GeV What makes Compass a special place ? Kinematical domain : Hera Iuminosity increase • CERN High energymuonbeam • 100 / 190 GeV • 80% Polarisation • μ+andμ-available • Opposite polarization • Targets • liquidHydrogen • Polarisedtarget Valence + sea + gluons

  5. GPD program : experimentalset-up DVCSμp μ’p’ μ’ 2.5m cryogenictarget to bedesigned and built ECal1 + ECal2 10° • Phase 1: long H2 target • - Phase 2: transverselypolarized μ upgrade ECAL2 new ECAL0 p’ Nμ=4.6 108/SPScycle (duration 9.6s,every 48s) 4m long Recoil proton detector to insureexclusivity to bedesigned and built

  6.  Known expression μ’ * θ μ p φ Harmonicdecomposition of the cross-section from Belitsky, Kirchner, Müller : polarized beam off unpolarized target dσ(μpμp) = dσBH + dσDVCSunpol + PμdσDVCSpol + eμ aBHRe ADVCS + eμ PμaBHIm ADVCS Pμ eμ eμPμ Twist-2 M11 >> Twist-3 M01 Twist-2 gluon M-11

  7. μ’ * θ μ p φ Harmonic decomposition : COMPASS case Case of COMPASS :μ+(P=-0.8) and μ-(P=+0.8) DU,CS : dσμ+- dσμ-= 2 PμdσDVCSpol+ eμaBHRe ADVCS SU,CS :dσμ++ dσμ-= 2(dσBH+dσDVCSunpol) + 2eμ PμaBHIm ADVCS

  8. μ’ * θ μ p φ Sensitivity of Beam Charge & Spin Asymmetry BCSA = DU,CS /SU,CS 160 GeVmuon beam 2.5m LH2 target 140 days L = 1222 pb-1 εglobal = 10 % D.Mueller-hep-ph/0904.0458 fit on world data

  9. BCSA() over the kinematical domain 4 < Q2 < 8 Points: VGG predictionPhys. Rev. D60:094017,1999 Statisticalerrorsonly Curves: FSS predictionPhys. Rev. D59:119901,1999 160 GeVmuon beam 2.5m LH2 target 140 days L = 1222 pb-1 εglobal = 10 %  (deg) 2 < Q2< 4 1 < Q2 < 2 0.03< x < 0.07 0.02 < x < 0.03 0.01< x < 0.02 0.005< x < 0.01

  10. t-slope measurement Using SU,CS : dDVCS / dt ~ exp(Bt) Input :B(x) = b0 + 2 α’ ln(x0/x)α’ =0.125 GeV-2from J/ 1 < Q2 < 8 GeV2 160 GeVmuon beam 2.5m LH2 target 140 days L = 1222 pb-1 εglobal = 10 %

  11. 2008 DVCS beam test 2008 DVCS beam test Goal :Evaluate feasibility of DVCS detection in the Compass environment 160 GeVμ on a 40cm long Liquid H2 target & RPD - 2 days of beam (μ+ and μ-) Use COMPASS “hadron” set-up μ’ γ proton

  12. Ring B Ring A Liquid H2 Target & RPD

  13. Recoil Proton Detector Proton identification in RPD Elasticscattering (hadron beam)

  14. Selection of the events Selection of events : - one vertex with μ and μ’ - no other charged tracks - only 1 high energy photon - 1 proton in RPD with p < 1. GeV/c DVCS Bethe-Heitler Timing difference : tμ- tRPD

  15. Exclusivity cuts μ’+γ Δp =|Pμ’+γ|-|PRPD| Δpperp < 0.2 GeV γ μ’ Δϕ proton Transverse plane Δϕ=ϕmiss- ϕRPD Δϕ < 36 deg

  16. Kinematical consistency : ϑγ*γ μ’ With μ, μ’ and γ : μ γ* With μ, μ’ and proton : γ proton μ+p->μ’+γ+p Events rejected

  17. μ’ * θ μ p φ Exclusive photon production signal After cuts All Q2 Before cuts All Q2 A flat background contribution in φissuppressed The peakatφ=0 remains => identified as BH

  18. Bethe-Heitler signal Monte-Carlo simulation of BH (dominant) and DVCS Deep VCS Bethe-Heitler After all cuts, Q2>1GeV2

  19. Conclusions & perspectives • COMPASS has a great potential in GPDs physics – Proposal in preparation • Phase 1 2011/12 :study of the GPD H with a LH2 target • measurement ott-slopes – transverse size of the nucleon • measurement of Beam Charge and Spin differences & asymmetries • Phase 2 2013/14 : study of the GPD E with atransv. polarized target • measurement of Beam Charge and Spin differences & asymmetries • Equipements needed : • 4m long RPD, 2.5m LH2 target, Extended & improved calorimetry, • RPD with polarized Target • 2008 DVCS test Run preliminary analysis • Observation of Exclusive Photon Production • In agreement with Bethe-Heitler contribution • 2009 : 2 weeks of DVCS tests approved by SPSC • Goals : Observe DVCS and measure BH precisely • Evaluate background level

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