1. Determination of the gluon polarisation
in CERN
Adam Mielech, INFN Trieste
on behalf of the COMPASS collaboration

2. Nucleon spin structure
quarks
gluons
orbital momentum

3. Quarks polarisation
Evaluated from the spin structure function g1(x) measured in Deep Inelastic Scattering of polarised lepton on the polarised nucleon :
g1(x,Q2) world data (HERMES 2002)
e,m (k’)
e,m (k,s)
QED
g1(x,Q2),
g2(x ,Q2)
g*(Q2, n)
P (P,S)
Q2=-q2=-(k-k’)2
n=E-E’
x=Q2/2Mn
lab
Proton momentum fraction carried by quark
G1 plots….
Assuming Quark Parton Model:

4. “The spin crisis” experiment theory
Measured at CERN, SLAC, DESY
Result from SMC fit to proton world data
At Q2 =5GeV 2 Phys Lett B (1994)
experiment
theory
Ellis –Jaffe sum rule prediction
According to triangle anomaly, there is no unambiguous way to separate the quark contribution and the anomalous gluonic contribution

5. Accesing gluon polarisation
From QCD evolution of g1(x) stucture function:
or from direct measurements
Photon Gluon Fusion(SMC, HERMES,COMPASS)
jets from polarised pp2jets scattering (RHIC)
Fit to the g1(x,Q2) world data, by Blümlein & Böttcher,
Nucl. Phys. B636 (02) 225 p

6. DG/G from open charm
50%
40%
-76% C

7. DG/G from high pT hadrons
QCD-Compton
Leading Order q g* q g*
Photon Gluon Fusion

8. Photon Gluon Fusion ps+ h1 D+* C K- D0 p+ h2
q=c, Open Charm Production
Looking for charged K and p
coming from D0 and D±* decays C D0 K- p+
D+*
ps+ h1 h2
62% 4%
q =u,d,s,c at high pT
Looking for high pT hadron pairs

9. Localisation LHC SPS N ~200 physicists from 12 countries
Luminosity: ~ cm-2 s-1
intensity: µ+/spill (4.8s / 16.2s)
polarization: %
momentum: GeV/c
~200 physicists from 12 countries
polarised muon beam
polarised target
particle tracking and momentum measurement
particle identification
calorimetry measurement
Localisation
LHC
SPS N

10. Experimental setup Target μ ID HCALS m+ beam + 200 tracking planes
Spectrometers magnets
μ ID
HCALS
m+ beam
+ 200 tracking planes
RICH

11. Two 60 cm long target cells with opposite polarisation
Polarised 6LiD target
Two 60 cm long target cells
with opposite polarisation
Superconducting
Solenoid (2.5 T)
3He – 4He dilution
refrigerator (T~50mK)
Dipol (0.5 T)
Polarisation: %
Dilution: 40%
Reconstructed
interaction vertices μ

12. Particle identification
5 m
6 m
3 m
photon
detectors:
CsI MWPC
mirror
wall
vessel
radiator:
C4F10
q(mrad)
p(GeV/c) p K
p, m
RICH
hadrons m
m – hadron separation is also possible using hadron calorimeters

13. Tracking detectors

14. Open charm – selection in COMPASSD0 C K- p+
D+*
ps+
~300
COMPASS 2002 data

15. High pT hadrons- selection
Current fragmentation
xF > 0.1
z > 0.1
2 high pT hadrons
pT > 0.7 GeV/c
pT12+ pT22 > 2.5 (GeV/c)2
m(h1h2) > 1.5 GeV/c2

16. DG/G from high pT hadrons
QCD-Compton
Leading Order
Photon Gluon Fusion
fractions of cross section determined by Monte Carlo

17. Additional background for Q2<1(GeV2)
Resolved Photon
VMD - Pomeron

18. DG/G at COMPASS –present status
Open charm
Theoretically clean DG/G extraction, experiment challenging because of difficulty of the charm reconstruction in the large target.
We are able to reconstruct charmed mesons.
We are still collecting the data.
Projected error on DG/G from data: 0.24
High pT hadron pairs
Experimental signature easy, background subprocesses extraction based on Monte Carlo
Measured asymmetry from 2002 data:
+ 2003, 2004 → stat. < 0.018
Up to now systematic error contains only studies on
false asymmetries due to target or spectrometer effects

19. SUMMARY Quarks spin distribution is known.
Gluon spin contribution is going to be measured soon in different processes.
Orbital momentum components of the spin → next generation experiments.
Nucleon spin puzzle is still very exciting subject.