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New measurement of G/G at COMPASS
Sébastien Procureur (CEA - Saclay) on behalf of the COMPASS collaboration New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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A large G = G(xg) is expected!
The nucleon spin ½= ½ΔΣ + ΔG + Lq + Lg quark spin gluon spin orbital momenta Prediction 1: ΔΣ ~ 0.75 (Naive quark model) Prediction 2: ΔΣ ~ 0.6 (QCD & s=0) Experiment: a0 = ΔΣ – (3S/2)ΔG ~ 0.2! (EMC,…) A large G = G(xg) is expected! New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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COMPASS experiment LHC COMPASS SPS
Beam: µ+/ spill (4.8s / 16.2s) Luminosity: ~ cm-2 s-1 Beam polarization: -76% Beam momentum: GeV/c New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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How to access to (G/G)(xg)?
Spin asymmetries ALL of the main diagrams in DIS: Photon gluon fusion (PGF) LO Compton ALL ALL = * Only PGF is sensitive to (G/G)(xg) (but S suppressed) New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Selections of PGF Open charm channel c c q = c, then c D0 or D* K,
scale: mc c q = c, then c D0 or D* K, : no physical background : low statistics New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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(ΔG/G)(xg) with open charm
ALL/D = D* D0+ K+ + : (G/G)(xg)= 0.73(stat) : [(G/G)(xg)] = 0.43 (analysis in progress) New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Selections of PGF Open charm channel High pT hadrons channel c c
scale: mc scale: pT , Q² c q = c, then c D or D* K, (q,q) (h1,h2) with high transverse momentum wrt * direction : no physical background : low statistics : physical background : large statistics New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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(G/G)(xg) with high pT hadrons
ALL/D = Rpgf G/G aLLpgf + Rqcdc q/q aLLqcdc + Rqq q/q aLLgq (G/G) + Rqg G/G aLLqg (q/q) + … Resolved photons! (low Q²) Need a Monte Carlo simulation What about polarisations of partons in virtual photon? New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Monte Carlo simulation
Event generator PYTHIA is used for low Q² analysis (+ GEANT for spectrometer simulation) Q² pT Data & MC Q² pT Data/MC Good description of the data by the simulation! New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Results of simulation RPGF 30% RRes.Phot 50%
Relevant parameters were then varied to estimate the uncertainties due to MC New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Resolved photon asymmetry
Problem: polarized PDFs of virtual photons not measured! q,pert(xp,²) calculable Solution: -q,npert(xp,0²) < q,npert(xp,0²) < q,npert(xp,0²) measured! Allows us to obtain a range for (q/q) and (G/G) ! Adds a limited uncertainty to the estimation of (G/G)(xg) 1st estimation of the resolved photon asymmetry! New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Results (1) 02-03 0.024 0.089 Values used for extraction of (G/G)(xg)
data (G/G)(xg) stat exp.syst MC.syst 02-03 0.024 0.089 0.014 0.052 0.018 [PLB 633 (2006) 25-32] New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Results (1) Equiv. stat. * ~2.4 with 2004 data! Asymmetries compatible with Values used for extraction of (G/G)(xg) data (G/G)(xg) stat exp.syst MC.syst 02-03 0.024 0.089 0.014 0.052 0.018 02-04 0.016 0.058 0.013 [PLB 633 (2006) 25-32] NEW New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Results (2) Favours small values of G
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Outlook More results soon available with 2004 data: For the future:
Q² > 1 (GeV/c)² high pT events: Open charm selection No resolved photons! No physical background! For the future: Optimization of event selection with a neural network Bins in xg (requires improvement of xg reconstruction) 2006 data with new COMPASS magnet (larger xg) already in progress! New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Conclusion 2004 data confirms results from 02-03 (FOM *2.4)
G is small (or G(xg) has a node around 0.1) still the most precise determination of (G/G)(xg) G not large ΔΣ small ( predictions) ½= ½ΔΣ + ΔG + Lq + Lg ½ possible scenarios: ½ New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Spares New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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A1 at COMPASS (2002-2003) Agreement with previous experiments
Improve stat at small x New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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g1 at COMPASS PLB 612(2005),154 = – (without COMPASS) = – (with COMPASS) New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Transversity at COMPASS
Collins Sivers PRL 94(2005),202002 Asymetries compatible with 0 ( HERMES) p/n compensation? New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur
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Results of PHENIX Theory model C.L. (%) GRSV-std 17.1-21.7 *GRSV-max
(PANIC05) Theory model C.L. (%) GRSV-std *GRSV-max (Dg=g) *GRSV Dg=0 *GRSV Dg=−g GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, Phys. Rev. D 53 (1996) 4775. * At input scale: Q2 = .4 GeV
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