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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. - PowerPoint PPT Presentation

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Compass Future Plans

Prospects for GPD studies at COMPASSE. Burtin CEA-Saclay Irfu/SPhNOn behalf of the COMPASS CollaborationCERN , March 4th, 2010 Physics Motivations Sensitivity of observables Experimental issues Beam tests resultsGeneralized Parton Distributionscontains pdfH(x,0,0) = q(x) measured in DIS

contains form factors measured in elastic scatteringFactorisation:Q2 large, -t>

Known expression

*p6Angular dependence analysisCase of COMPASS : +(P=-0.8) and -(P=+0.8) unpolarized H2 target *pDU,CS : d+- d-= 2 P dDVCSpol + e aBH Re ADVCS

=> Re (F1H)

SU,CS : d++ d-= 2(dBH+dDVCSunpol ) + 2 e P aBH Im ADVCS=> Im (F1H)=> d/dt7From SU,CS : t-slope measurement Using SU,CS : dDVCS / dt ~ exp(-Bt) B ~

Ansatz at small x :B(x) = b0 + 2 ln(x0/x) =0.125 GeV-2 (FFS)

160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1global = 10 %

1 < Q2 < 8 GeV2 3 slope measurement for : > 0.30 (ECAL 1+2 ) > 0.16 (ECAL 0+1+2)

Assuming 3% systematic error on BH8Angular dependence analysisCase of COMPASS : +(P=-0.8) and -(P=+0.8) unpolarized H2 target *pDU,CS : d+- d-= 2 P dDVCSpol + e aBH Re ADVCS

=> Re (F1H)

SU,CS : d++ d-= 2(dBH+dDVCSunpol ) + 2 e P aBH Im ADVCS=> Im (F1H)=> d/dt9DU,CS : Beam Charge & Spin Difference*p160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1global = 10 %

Bj=> Re (F1H)

DU,CS : d+- d-= 2 P dDVCSpol + e aBH Re ADVCS

10BCSA() over the kinematical domain (deg)1 < Q2 < 22 < Q2 < 44 < Q2 < 80.03< x < 0.070.02 < x < 0.030.01< x < 0.020.005< x < 0.01Points: VGG prediction Phys. Rev. D60:094017,1999 Statistical errors onlyCurves: FFS prediction Phys. Rev. D59:119901,1999160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1global = 10 %

BCSA = DU,CS /SU,CS11statistical precision of the cos modulationBBBBBBBCSA = DU,CS /SU,CS = A0 + ACS,U cos + A2 cos2 12Meson production : filter of GPDs

H0 = 1/2 (2/3 Hu + 1/3 Hd + 3/8 Hg)

H = 1/2 (2/3 Hu 1/3 Hd + 1/8 Hg)

H = -1/3 Hs - 1/8 HgTransversely polarized target asymmetry on vector meson : E/H ( studied at COMPASS without RPD ) : : 9 : 1 : 2 at large Q2Cross section measurement :Vector meson : ,, H & EPseudo-scalar : , H & E ~ ~Would allow for flavor separation :13Continuation of the GPD program :constrain the GPD E

with +, - beam and transversely polarized NH3 (proton) target DT,CS dT ( +) - dT ( -) Im(F2 H F1 E) sin(- S) cos

160 GeV muon beam1.2 m polarized NH3 target (f=0.26)2 years - global = 10 %14GPD program : new equipments

p DVCS p p ECal1 + ECal2 10 2.5 m liquid H2 target

4m long Recoil Proton Detector Hermetic calorimetrynew ECAL0Later stage Transversely polarized targetAssociated RPDRecoil Proton Detector

Gandalf Project:1 GHz digitalisation of the PMT signal tocope for high rate 4 m long scintillator slabs ~ 300ps timing resolution Full scale prototype tested successfully

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 readoutTested

DVCS+BHECAL1&22008 beam test

Ring ARing BCompass hadron runTarget regionSelection of events : - one vertex with and - no other charged tracks - only 1 high energy photon (t1GeV2Deep VCSBethe-HeitlerDetection efficiency : +p->+p+ = 0.32 +/- 0.13 Global efficiency :- +p->+p+ efficiency SPS & COMPASS availability Dead time trigger efficiency

global = 0.13 +/- 0.05Projections of errors are realistic=> Bethe-Heitler observed~ 10 times more data taken in 2009Conclusions & perspectivesCOMPASS has a great potential in GPDs physics

Study of the GPD H with a LH2 target : measurement ot t-slopes transverse partonic structure of the nucleon measurement of Beam Charge and Spin differences & asymmetries Equipements needed : 4m long RPD, 2.5m LH2 target, Extended & improved calorimetry at a later stage : study of the GPD E with a transversely polarized target

DU,CS /SU,CS : Beam Charge & Spin Asymetry*pBCSA = DU,CS /SU,CS 160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1global = 10 %

Bj23

proton2008 DVCS beam testExperimental realisationCompass hadron set-up New superRPD and H2 target Hermetic calorimetry : Move ECALs upstream and/or complete ECAL2 New ECAL0 upstream of SM1

proton2008 DVCS beam testECAL0 and ECAL1VerticalHorizontalECAL0superRPDSM1ECAL1ECAL2Hadron program RPD

Proton identification in RPDElastic scattering (hadron beam)

Kinematical consistency : *Events rejected

proton*With , and :With , and proton :

+p->++p

Exclusive photon production signal*pBefore cutsAll Q2After cutsAll Q2

The peak at =0 remains => caracteristic of BH Liquid H2 Target & RPD

Ring ARing B

RPD(2008)MuRex (2006)BL=1m; th=1cmAtten length = 0.7mB = 300 psL=4m; th=5cmAtten length = 4mB = 200 ps AL=50cm; th=5mmA = 180 psL=2.83m; th=4mmA = 270 psToFToF = 350 ps RB-RA= 85-12 = 63 cm ToF = 310 psRB-RA= 110-25 = 85cm

Measurements and Estimations for resolutiontmin=-0.06 GeV2

Good resolution in t

Importance for thethe transverse imaging

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