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Physics Motivations Sensitivity of observables Experimental issues Beam tests results

Prospects for GPD studies at COMPASS E. Burtin CEA-Saclay Irfu/SPhN On b ehalf of the COMPASS Collaboration CERN , March 4th, 2010. Physics Motivations Sensitivity of observables Experimental issues Beam tests results. g *. g , p,r. hard. x+ . x- . soft. GPDs. P. P’. t.

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Physics Motivations Sensitivity of observables Experimental issues Beam tests results

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  1. Prospects for GPD studiesat COMPASSE. Burtin CEA-Saclay Irfu/SPhNOnbehalfof the COMPASS CollaborationCERN , March 4th, 2010 Physics Motivations Sensitivity of observables Experimental issues Beam tests results

  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 3-Dpartonic structure of the nucleon (Pz,ry,z) Hard Exclusive Scattering Deeply Virtual Compton Scattering ep ep p z x P r y x boost GPDs : H( x,,t ) Fourier (=0) access to correlations : ( Px, ry,z) Burkardt,Belitsky,Müller,Ralston,Pire

  4. What makes Compass unique ? • CERN High energymuonbeam • 100 - 190 GeV • 80% Polarisation • μ+andμ-available • Opposite polarization Luminositylimit Foreseen program : DVCS and mesonproduction off a liquidH2 target (unpolarized) • Will explore the intermediate • xBjregion • Uncoveredregionbetween • ZEUS+H1 and HERMES+Jlab xBj Gluon, sea and valence quarks

  5. μ’ * θ μ p  Comparison of BH and DVCS at 160 GeV DVCS : Bethe-Heitler : x=0.01 x=0.04 x=0.1    BH dominates BH and DVCS at the samelevelDVCS dominates excellentaccessDVCS amplitude studyofdDVCS/dt referenceyieldthrough the interference

  6. Known expression μ’ * θ μ p φ Azimuthal angular dependence analysis 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 Angular dependence analysis Case of COMPASS :μ+(P=-0.8) and μ-(P=+0.8) unpolarized H2target DU,CS : dσμ+- dσμ-= 2 PμdσDVCSpol+ eμaBHRe ADVCS => Re(F1H) SU,CS :dσμ++ dσμ-= 2(dσBH+dσDVCSunpol) + 2eμ PμaBHIm ADVCS => dσ/dt => Im (F1H)

  8. FromSU,CS : t-slope measurement Using SU,CS : dDVCS / dt ~ exp(-Bt) B ~ ½ <r2> Assuming 3% systematicerror on BH Ansatzatsmall x :B(x) = b0 + 2 α’ ln(x0/x)α’ =0.125 GeV-2 (FFS) 160 GeVmuon beam 2.5m LH2 target 2 years L = 1222 pb-1 εglobal = 10 % 1 < Q2 < 8 GeV2 3 σ slope measurement for : α’ > 0.30 (ECAL 1+2 ) α’ > 0.16 (ECAL 0+1+2)

  9. μ’ * θ μ p φ Angular dependence analysis Case of COMPASS :μ+(P=-0.8) and μ-(P=+0.8) unpolarized H2target DU,CS : dσμ+- dσμ-= 2 PμdσDVCSpol+ eμaBHRe ADVCS => Re(F1H) SU,CS :dσμ++ dσμ-= 2(dσBH+dσDVCSunpol) + 2eμ PμaBHIm ADVCS => dσ/dt => Im (F1H)

  10. μ’ * θ μ p φ DU,CS : Beam Charge & Spin Difference DU,CS : dσμ+- dσμ-= 2 PμdσDVCSpol+ eμaBHRe ADVCS 160 GeVmuon beam 2.5m LH2 target 2 years L = 1222 pb-1 εglobal = 10 % Bj => Re(F1H)

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

  12. statistical precision of the cosϕ modulation BCSA = DU,CS /SU,CS = A0+ ACS,Ucosϕ+ A2 cos2ϕ B B B B B B

  13. Meson production : filter of GPDs • Cross section measurement : • Vectormeson : ρ,ω,…H & E • Pseudo-scalar : π,η… H & E ~ ~ Wouldallow for flavorseparation : Hρ0= 1/2 (2/3 Hu + 1/3 Hd + 3/8 Hg) Hω= 1/2 (2/3 Hu – 1/3 Hd + 1/8 Hg) H = -1/3 Hs - 1/8 Hg  ρ: ω :   9 : 1 : 2 at large Q2 Transverselypolarizedtargetasymmetry on vectormeson : E/H ( studiedat COMPASS without RPD )

  14. Continuation of the GPD program :constrain the GPD E with+, - beamand transverselypolarized NH3 (proton) target DT,CS dT(+) -dT(-) Im(F2H– F1E)sin(- S) cos  160 GeV muon beam 1.2 m polarized NH3 target (f=0.26) 2 years - εglobal = 10 %

  15. GPD program : new equipments DVCSμp μ’p’ μ’ ECal1 + ECal2 10° • 2.5 m liquid H2target μ Hermeticcalorimetry new ECAL0 p’ 4m long Recoil Proton Detector Later stage… Transverselypolarizedtarget Associated RPD

  16. Recoil Proton Detector • 4 m long scintillatorslabs • ~ 300ps timing resolution • Full scale prototype • testedsuccessfully Gandalf Project: 1 GHz digitalisation of the PMT signal to cope for high rate

  17. ECAL 0 • Requirements • - Photon energy range 0.2- 30 GeV • - Size: 320cm x 320cm ; • - Granularity 4x4 – 6x6cm2 • - Energy resolution < 10.0%/√E (GeV) • - Thickness < 50 cm, • - Insensitive to the magnetic field. • Prototype under studies • Shaschlyk module with AMPD readout • Tested DVCS+BH ECAL1&2

  18. Ring B Ring A 2008 beam test Compass hadron run Target region Selection of events : - one vertex with μ and μ’ - no other charged tracks - only 1 high energy photon (Δt<5ns) - 1 proton in RPD with p < 1. GeV/c DVCS Bethe-Heitler

  19. 2008 beam test : exclusivity cuts μ’+γ Δp =|Pμ’+γ|-|PRPD| Δpperp < 0.2 GeV γ μ’ miss Δϕ proton Transverse plane Δϕ=ϕmiss- ϕRPD Δϕ < 36 deg

  20. 2008 beam test : Bethe-Heitler signal Monte-Carlo simulation of BH (dominant) and DVCS Deep VCS Bethe-Heitler => Bethe-Heitlerobserved Detection efficiency : εμ+p->μ+p+γ = 0.32 +/- 0.13 After all cuts, Q2>1GeV2 • Global efficiency : • - μ+p->μ+p+γ efficiency • SPS & COMPASS availability • Dead time • trigger efficiency • εglobal = 0.13 +/- 0.05 Projections of errors are realistic ~ 10 times more data taken in 2009

  21. Conclusions & perspectives • COMPASS has a great potential in GPDs physics • Study of the GPD H with a LH2 target : • measurement ott-slopes – transversepartonic structure of the nucleon • measurement of Beam Charge and Spin differences & asymmetries • Equipementsneeded : • 4m long RPD, 2.5m LH2 target, Extended & improved calorimetry • at a later stage : • study of the GPD E with a transverselypolarized target

  22. μ’ * θ μ p φ DU,CS /SU,CS : Beam Charge & Spin Asymetry BCSA = DU,CS /SU,CS 160 GeVmuon beam 2.5m LH2 target 2 years L = 1222 pb-1 εglobal = 10 % Bj

  23. Experimentalrealisation 2008 DVCS beam test Compass hadron set-up μ’ γ proton • New « superRPD » and H2target • Hermeticcalorimetry : Move ECALsupstream and/or completeECAL2 • New ECAL0 upstream of SM1

  24. ECAL0 and ECAL1 2008 DVCS beam test Horizontal proton γ μ’ “superRPD” SM1 Vertical ECAL0 ECAL1 ECAL2

  25. Hadron program RPD Proton identification in RPD Elasticscattering (hadron beam)

  26. 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

  27. μ’ * θ μ p φ Exclusive photon production signal After cuts All Q2 Before cuts All Q2 The peakatφ=0 remains => caracteristic of BH

  28. Ring B Ring A Liquid H2 Target & RPD

  29. Measurements and Estimations for resolution tmin=-0.06 GeV2 Good resolution in t Importance for the the transverse imaging

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