1. The gluon polarisation ΔG/G at COMPASS
Colin Bernet (DAPNIA/SPHN),
on behalf of the collaboration

2. The nucleon longitudinal spin structure
Quarksspin
Gluonsspin
Orbital momentum

3. Quarks : the spin crisis
Prediction (Ellis, Jaffe)
1988 – EMC: μ
GeV2 P
Axial anomaly

4. Polarized p-p (PHENIX, STAR)
Gluons
QCD evolution of g1
Polarized SIDIS
Photon-gluon fusion
(COMPASS,HERMES) μ
p,n
Polarized p-p (PHENIX, STAR)

5. Negatively polarized µ+ beam, 160 GeV
LHC
COMPASS
SPS

6. μ Experimental setup DeuteronTarget μ ID Dipoles HCALS
200 tracking planes
250 physicists μ
RICH

7. Open-charm tagging μ K- D0 D* π+ π+
COMPASS 2002

8. High pT tagging μ h1 h2

9. Selection of high pT eventsμ’ μ h2

10. Selection of high pT events
GeV/c
(GeV/c)2
No Q2 cut

11. Preliminary results Helicity asymmetry for high pT events, 2002 data
Careful study of false asymmetries

12. Gluon polarisation ΔG/Gμ
Monte-Carlo simulation
(PYTHIA)

13. Background – QCD Compton
Contribution to the asymmetry :

14. Background – Resolved photons
Contribution to the asymmetry :

15. Conclusion + 2003, 2004, f.o.m. improvements → stat. < 0.018
Monte-Carlo
Background
LEPTO