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Determination of the gluon polarisation at COMPASS & RHIC Sébastien Procureur (CEA - Saclay) Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur STAR

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The nucleon spin structure ½= ½ΔΣ + ΔG + L q + L g quark spingluon spinorbital momenta Prediction 2: (QCD & s=0) ΔΣ ~ 0.6 ΔΣ ~ 0.75Prediction 1:(Naive quark model) Experiment:a 0 = ΔΣ – (3 S /2 ) ΔG ~ 0.2!(EMC, SMC, SLAC) expect G(Q²) = dx g G(x g,Q²) ~ 1.5 @ Q² ~ 1GeV² ~ 2.5 @ Q² ~ 3GeV² Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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The nucleon spin structure ΔG contributes to scaling violation of g 1 ΔG(Q²=1) = 0.53 +/- 1.93 (AAC 2000) ΔG(Q²=1) = 0.50 +/- 1.27 (AAC 2003) ΔG(Q²=1) = 0.47 +/- 1.08 (AAC 2006) Small available Q² range for g 1 large uncertainties on G Direct measurement of G needed Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur hep-ph/0312112

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Direct measurement of ΔG Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur or lepton beam or nucleon target heavy flavor, high p T or proton beam 0, (PHENIX), jets (STAR) STAR polarized p-p collider (RHIC)polarized semi-inc. DIS Hard probe

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RHIC @ BNL Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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AGS LINAC BOOSTER Polarized Source Spin Rotators Partial Snake Siberian Snakes 200 MeV Polarimeter AGS Polarimeter Rf Dipole RHIC pC Polarimeters Absolute Polarimeter (H jet) P HENIX P HOBOS B RAHMS & PP2PP S TAR Siberian Snakes Helical Partial Snake Strong Snake Spin Flipper RHIC Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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P1P1 P2P2 (R = L ++ /L -- : relative luminosity) Run number L integrated P Figure of Merit (P 4 L ) Run 3 (2003)350 nb -1 27%1.86nb -1 Run 4 (2004)120 nb -1 40%3.1nb -1 Run 5 (2005)3.8 pb -1 47%185nb -1 G/G @ RHIC

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Muon ID Panels Central Arms North Muon Arm South Muon Arm Ring Imaging Cerenkov EM Calorimeter Muon Tracking Chambers Beam-Beam Counter Multiplicity/Vertex Detector Time Expansion Chamber Drift Chambers Pad Chambers Time of Flight Panels Four spectrometer arms with excellent trigger and DAQ capabilities. PHENIX detector Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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0 cross section Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur Very good agreement with NLO Xsection calculation!

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Results Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur Theory modelC.L.(%) GRSV-std17.1-21.7 GRSV-max ( g=g) 0.0-0.0 GRSV g=0 16.7-18.4 GRSV g=−g 0.0-0.7 0 reconstruction (with background asym. subtraction)

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Large acceptance TPC and EMC -1<η<2 STAR detector Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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Results Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur STAR Inclusive Jet Cross section STAR Preliminary Large G already disfavored 2003-2004 data: + results from 2005 soon…

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RHIC projections Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur STAR Results of unprecedent precision in the next years: pQCD analysis + PHENIX data on

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COMPASS SPS LHC Beam: 2. 10 8 µ + / spill (4.8s / 16.2s) Luminosity ~5. 10 32 cm -2 s -1 Beam polarization: -80% Beam momentum: 160 GeV/c COMPASS @ CERN Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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COMPASS Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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G/G @ COMPASS LOCompton Photon gluon fusion (PGF) Spin asymmetries A LL of the main diagrams in DIS: Only PGF is sensitive to ( G/G)(x g ) (but S suppressed) A LL = * A LL Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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Open charm channel q = c, then c D 0 or D* K, c c : no physical background : low statistics Selections of PGF scale: m c Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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ΔG/G with open charm A LL = D * D 0 + S K+ + S Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur D 0 K+

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Open charm channel q = c, then c D or D* K, c c : no physical background : low statistics High p T hadrons channel (q,q) (h 1,h 2 ) with high transverse momentum wrt * direction : physical background : large statistics Selections of PGF scale: m c scale: p T, Q² Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc + R qq q/q a LL gq ( G/G) + R qg G/G a LL qg ( q/q) + … Need a Monte Carlo simulation Theoretical inputs for polarisations of partons in virtual photon… + resolved photons! (low Q²) G/G with high p T hadrons Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur high Q²

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pTpT Monte Carlo simulation PYTHIA generator for low Q² + GEANT for spectro simul. Good description of the data by the simulation! R PGF 30% R Res.Phot 50% Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur Data/MC Data & MC

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Results Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur 2002-2004 data (2002-2003 data: PLB 633 (2006) 25-32) Favours small values of G

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Next steps Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur Results from 2004 data for Q² > 1 GeV² Improved high p T selection with Neural Network New magnet larger acceptance larger x g Improved RICH (particle identification) Extraction of G/G at different x g very soon! New run in 2006 NLO analysis (theoretical inputs)

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Conclusion Efforts on G measurement start to pay! Agreement between RHIC and COMPASS data Very exciting results are expected in the next few years! ½= ½ΔΣ + ΔG + L q + L g ½0.2 + 0.4 + 0.0 + 0.0 possible scenarios: ½0.2 + 0.0 + 0.4 G not large ΔΣ small ( predictions) Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur … + … + …

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Spares Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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Resolved photon asymmetry 1 st estimation of the resolved photon asymmetry! Problem: polarized PDFs of virtual photons not measured! -q ,npert (x p, 0 ²) < q ,npert (x p, 0 ²) < q ,npert (x p, 0 ²) Solution: measured! Allows us to obtain a range for ( q/q) and ( G/G) ! Adds a limited uncertainty to the estimation of ( G/G)(x g ) q ,pert (x p, ²) calculable Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

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