ΔG/G Extraction From High-Pt Hadron Pairs at COMPASS Ahmed El Alaoui Nuclear Physics School, Erice, 16-24 September 2007 On Behalf Of COMPASS Collaboration

Outline Introduction COMPASS Experimental Setup Data Analysis Results Summary and Conclusion

Nucleon Spin SPIN CRISIS Naive Quark Model Pure valence description of constituent quarks: ∆u = + 4/3 ∆d = - 1/3 ∆Σ = 1 Relativistic Quark Model ∆Σ ≈ 0.75 QCD framework Hyperons β decay constants + SU(3) flavor symmetry ∆Σ = a0 ≈ 0.60 compatible with the Relativistic QM prediction However, the EMC measured ∆Σ = 0.12 ± 0.09 ± 0.14 SPIN CRISIS a0 = ΔΣ – (3αs/2π)ΔG A measurement of ∆G is needed - To access the gluon contribution to the nucleon spin - To understand the role of the Axial Anomaly in the explanation of the spin crisis

How To Acess ΔG/G Indirect Measurement: QCD analysis: fit to the nucleon spin structure function g1(x) Unfortunately, the limited range in Q2 does not allow for a precise determination of ∆G Direct Measurement: ∆G/G can be accessed via Photon Gluon Fusion (PGF) process

PGF Process PGF APGF = aLL x (∆G/G) factorization theorem q = c: Two approaches are used to tag PGF process q = c: - Open Charm D0, D* decay Clean signal Combinatorial background - Low statistics q = u, d, s: - High-pt hadron Pairs Physical background High statistics APGF = aLL x (∆G/G) factorization theorem PGF

How To Acess ΔG/G Indirect Measurement: QCD analysis: fit to the nucleon spin structure function g1(x) Unfortunately, the limited range in Q2 does not allow a for precise determination of ∆G Direct Measurement: ∆G/G can be accessed via Photon Gluon Fusion (PGF) process Three independent measurements were done at COMPASS - Open Charm - High pt hadron pairs production at Q2>1GeV2 - High pt hadron pairs production at Q2<1GeV2

COMPASS Collaboration COmmon Muon and Proton Apparatus for Structure and Spectroscopy 250 Physicists 18 Institutes 12 Countries

Experiment Layout LHC SPS luminosity: ~5 1032 cm-2 s-1 beam intensity: 2.108 µ+/spill (4.8s/16.2s) beam momentum: 160 GeV/c

COMPASS Spectrometer SAS 50 m long LAS 160 GeV μ beam RICH E/HCAL1 Muon Wall 1 Muon Wall 2 E/HCAL2 Polarized Target 160 GeV μ beam Polarization ~ 80% 50 m long LAS SAS MicroMegas DC Tracking: SciFi, Silicon, MicroMegas, GEMs, MWPC, Straws PID: RICH, Calorimeters, μ Filters

COMPASS Target Two 60 cm long oppositely polarized cells 6LiD is used as a material dilution factor ~ 0.4 Target Polarization ~ 50% 70 mrad acceptance (180 mrad for 2006 target) Vertex distribution

High Pt Events Selection Primary vertex with at least μ, μ’ and 2 hadrons 0.1 < y < 0.9 (Q2>1GeV2) 0.35 < y < 0.9 (Q2<1GeV2) Pt > 0.7 GeV 0.0 < z, xF < 1.0 ΣPt > 2.5 GeV2 2 0.0 < z1+z2 < 0.95 minv(h1,h2) > 1.5 GeV ECalo/P > 0.3

High Pt Spin Asymmetry Aexp = (Nu - Nd)/(Nu + Nd) μ B Aexp = (Nu - Nd)/(Nu + Nd) The acceptance is not identical in both cells Asymmetry bias

Polarisation reversal each 8 hours High Pt Spin Asymmetry μ B μ B Aexp = (Nu - Nd)/(Nu + Nd) ‘ Aexp = (Nu - Nd)/(Nu + Nd) Polarisation reversal each 8 hours A||/D=(Aexp- Aexp)/2fPTPBD ‘ f Dilution factor PT(B) Target(Beam) polarization D Depolarization factor To improve the statistical error, a weighted method is used in the asymmetry calculation: w = fDPB (event-wise weight)

∆G/G Extraction at Q2<1GeV2

∆G/G at Q2<1GeV2 γ γ γ γ γ γ A||/D = RPGF∆G/G aLL PGF + RQCDC ∆q/q aLL QCDC + Rqq∆q/q aLL (∆q/q) qq γ + Rqg∆G/G aLL (∆q/q) qg Resolved photon processes + Rgq∆q/q aLL (∆G/G) gq γ + Rgg∆G/G aLL (∆G/G) gg qq’ qq’ γ γ Ri (fraction of the process i), aLL, ∆q, q, q and G are obtained from - Monte Carlo Simulation based on PYTHIA generator and Geant. - pQCD Calculation - pdf in the nucleon from GRSV2000 and GRV98LO parametrization - pdf in the photon from GRS parametrization γ γ The polarized pdfs in the photon ∆q and ∆G are not available. Therefore the positivity limit is used to constrain them which leads to 2 extreme scenarios. Included as systematic error in the estimation of ∆G/G

Monte Carlo vs. Data (Q2<1GeV2) xBj

Process fractions (Q2<1GeV2) Resolved photons processes 32% 12% 50%

∆G/G Result at Q2<1GeV2 2002-2004 data A||/D = 0.004 ± 0.013(stat.) ± 0.003(syst.) ∆G/G(xg,μ2) = 0.016 ± 0.058(stat.) ± 0.055(syst.) xg = 0.085-0.035 +0.070 μ2 = 3GeV2 Contribution to Syst. error comes from - False asymmetry - Monte Carlo tuning - Resolved photon process

∆G/G Extraction at Q2>1GeV2

∆G/G at Q2>1GeV2 A||/D = RPGF∆G/G aLL + RQCDC ∆Q/Q aLL LO A||/D = RPGF∆G/G aLL + RQCDC ∆Q/Q aLL PGF QCDC + RLO ∆Q/Q aLL LO Contribution from resolved photon precesses is negligible in this case At Q2>1GeV2 analysis, Lepto generator seems to describe the real data much better than PYTHIA. It was then used to estimate the fraction of each process

Monte Carlo vs. Data at Q2>1GeV2 34%

∆G/G Result at Q2>1GeV2 2002 - 2003 Data A||/D = -0.015 ± 0.080(stat.) ± 0.013(syst.) ∆G/G(xg, μ2) = 0.06 ± 0.31(stat.) ± 0.06(syst.) <xg> = 0.13 μ2 = 3GeV2 Contribution to Syst. error comes from - False asymmetry - Monte Carlo tuning 2004 Data Analysis is in progress…

Results ΔG/G

Summary and Conclusion - High-Pt asymmetries at Q2>1GeV2 and Q2<1GeV2 were presented - The measured ∆G/G is compatible with zero at xg = 0.1 The most precise measurement up to now Analysis of 2004 data (at Q2>1GeV2) is almost finished. It will be released soon - The new solenoid, installed in 2006, has an acceptance (180 mrad) three times larger than the previous one. Double the statistics obtained in 2004 Access higher value of xg Thank you