1. Recent measurement of ΔG/G at COMPASS Sébastien Procureur – CEA Saclay on behalf of the COMPASS collaboration EPS200523/07/2005

2. The Nucleon spin ½= ½ΔΣ + ΔG + L q + L g Sébastien ProcureurEPS2005

3. - Quark model: ΔΣ 0.75 - QCD + s = 0 (Ellis-Jaffe): ΔΣ 0.6 - Experimentaly: a 0 (=ΔΣ?) 0.2 The Nucleon spin ½= ½ΔΣ + ΔG + L q + L g quarks gluons orb. mom. ΔΣ = a 0 + 3( s /2 ) ΔG Sébastien ProcureurEPS2005

4. The Nucleon spin ½= ½ΔΣ + ΔG + L q + L g quarks gluons orb. mom. Necessary to measure ΔG - Quark model: ΔΣ 0.75 - QCD + s = 0 (Ellis-Jaffe): ΔΣ 0.6 Sébastien ProcureurEPS2005 - Experimentaly: a 0 (=ΔΣ?) 0.2 ΔΣ = a 0 + 3( s /2 ) ΔG

5. ΔG/G measurement main DIS diagrams (up to 1 st order): LO PGFQCDC only PGF is sensitive to ΔG/G Sébastien ProcureurEPS2005

6. ΔG/G measurement A pgf = ΔG/G * a LL pgf if factorization holds (need a hard scale): we need to select PGF events( S suppressed)… with a hard scale hard part soft part Sébastien ProcureurEPS2005

7. PGF selections Open charm channel J.Pretzs talk 2 high p T hadrons channel: (q,q) (h 1,h 2 ) with high p T large stat, suppress LO, but still physical background scale: p T ², Q² A LL /D = R pgf G/G a LL pgf + (A LL /D) back ? h1h1 h2h2 Sébastien ProcureurEPS2005

8. Physical background A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc all Q² Sébastien ProcureurEPS2005

9. + R qq q/q a LL qq q/q Physical background A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc Q² < 1 all Q² Sébastien ProcureurEPS2005

10. + R qg G/G a LL qg q/q Physical background A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc + R qq q/q a LL qq q/q all Q² Q² < 1 Sébastien ProcureurEPS2005

11. + R gq q/q a LL gq ( G/G) Physical background A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc + R qq q/q a LL qq q/q + R qg G/G a LL qg q/q all Q² Q² < 1 Sébastien ProcureurEPS2005

12. + R gg G/G a LL gg ( G/G) Physical background A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc + R qq q/q a LL qq q/q + R qg G/G a LL qg q/q + R gq q/q a LL gq ( G/G) all Q² Q² < 1 Sébastien ProcureurEPS2005

13. Inputs: -generation of Monte Carlo evts -parametrization (world data) -pQCD calculations … what about the rest (Q² < 1GeV²)?? Physical background A LL /D = R pgf G/G a LL pgf + R qcdc q/q a LL qcdc + R qq q/q a LL qq q/q + R qg G/G a LL qg q/q + R gq q/q a LL gq ( G/G) + R gg G/G a LL gg ( G/G) Sébastien ProcureurEPS2005

14. Resolved photon processes polarised parton distribution functions of (virtual…) photon are unknown! BUT: -q (x, ²) < q (x, ²) < q (x, ²) leads to 2 extreme (min&max) scenarios additional (theoretical) uncertainty, but we can continue! measured! Sébastien ProcureurEPS2005

15. MC/data comparison Q² Generator PYTHIA is used for Q²<1GeV² (+ GEANT for spectrometer simulation). data - MC data/MC Sébastien ProcureurEPS2005

16. MC/data comparison pTpT. data - MC. data - MC data/MC Sébastien ProcureurEPS2005

17. Results from MC The Monte Carlo describes our data quite well! Sébastien ProcureurEPS2005 R pgf 31%

18. Systematics due to MC Relevant parameters of the MC were varied to study the dependence of our results with MC model Sébastien ProcureurEPS2005 a typical onethe critical one

19. Results for high p T low Q² Asymmetry for 2002+2003 (p T > 0.7 GeV, p T ² > 2.5 GeV²): G/G: A LL /D = 0.002 ± 0.019(stat) ± 0.003 (syst) ΔG/G(x g =0.1, ²=3GeV²) = +0.024 ± 0.089(stat.) ± 0.014(exp.syst) ± 0.052(MC.syst) ± 0.018(photon) 0.057 Sébastien ProcureurEPS2005

20. Comparison with other results G = 2.48 G = 0.62 G = 0.16 (GRSV param.) Sébastien ProcureurEPS2005

21. Summary Most precise measurement of ΔG/G up to now: ΔG/G(x g =0.1, ²=3GeV²) = +0.024 ± 0.089(stat.) ± 0.052(syst.) ΔG is small or ΔG(x g ) crosses 0 around x g = 0.1 For the 1 st time, Res. Phot. asym. taken into account Next steps: –2004 data analysis (ΔG/G) 0.05 (also 2006 run) –NLO analysis Other channel: single high p T hadron Sébastien ProcureurEPS2005

22. Spares Sébastien ProcureurEPS2005

23. COMPASS acceptance Sébastien ProcureurEPS2005

24. Results from RHIC-PHENIX Sébastien ProcureurEPS2005

25. Asymmetry versus p T Sébastien ProcureurEPS2005

26. Experimental systematics Asymmetries calculated with low p T events A exp = A A false according to microwave setting A +A f A - A f 1% of false asymmetries in 2003 data Sébastien ProcureurEPS2005

27. Polarised 6 LiD Target P D : 50% Dilution factor: 40% Two 60 cm long target cells with opposite polarisation Superconducting Solenoid (2.5 T ) 3 He – 4 He dilution refrigerator (T~50mK) Dipole (0.5 T) Reconstructed interaction vertices Sébastien ProcureurEPS2005