The gluon polarisation ?G/G

1

• The gluon polarisation ?G/G
• at COMPASS

Colin Bernet (DAPNIA/SPHN), on behalf of the
collaboration
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The nucleon longitudinal spin structure
Orbital momentum
Quarksspin
Gluonsspin
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Quarks the spin crisis

• Prediction (Ellis, Jaffe)
• 1988 EMC

GeV2
µ
P

• Axial anomaly

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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
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Experimental setup
DeuteronTarget



µ ID
Dipoles
HCALS

• 200 tracking planes
• 250 physicists

µ
RICH
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Open-charm tagging
K-
µ
D0
D
p
p
COMPASS 2002
8
High pT tagging
µ
9
Selection of high pT events
h1
µ
µ
h2
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Selection of high pT events

• GeV/c
• (GeV/c)2
• No Q2 cut

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Preliminary results

• Helicity asymmetry for high pT events, 2002 data
• Careful study of false asymmetries

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Gluon polarisation ?G/G
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Background QCD Compton

• Contribution to the asymmetry

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Background Resolved photons

• Contribution to the asymmetry

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Conclusion

• 2003, 2004, f.o.m. improvements
• ? stat. lt
  0.018
• Monte-Carlo
• Background
• LEPTO