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

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) ~ Q² ~ 1GeV² ~ Q² ~ 3GeV² Determination of  G at COMPASS & RHICPAVI06, Milos S.Procureur

The nucleon spin structure ΔG contributes to scaling violation of g 1 ΔG(Q²=1) = / (AAC 2000) ΔG(Q²=1) = / (AAC 2003) ΔG(Q²=1) = / (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/

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 

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

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

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  RHIC

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

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

Results Determination of  G at COMPASS & RHICPAVI06, Milos S.Procureur Theory modelC.L.(%) GRSV-std GRSV-max (  g=g) GRSV  g= GRSV  g=−g  0 reconstruction (with background asym. subtraction)

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

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

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 

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

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

 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

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

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+ 

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

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

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

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

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)

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 ½ possible scenarios: ½  G not large  ΔΣ small (  predictions) Determination of  G at COMPASS & RHICPAVI06, Milos S.Procureur … + … + …

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

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