1. New measurement of G/G at COMPASS
Sébastien Procureur (CEA - Saclay)
on behalf of the COMPASS collaboration
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

2.  A large G = G(xg) is expected!
The nucleon spin
½= ½ΔΣ + ΔG + Lq + Lg
quark spin
gluon spin
orbital momenta
Prediction 1:
ΔΣ ~ 0.75
(Naive quark model)
Prediction 2:
ΔΣ ~ 0.6
(QCD & s=0)
Experiment:
a0 = ΔΣ – (3S/2)ΔG ~ 0.2!
(EMC,…)
 A large G = G(xg) is expected!
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

3. COMPASS experiment LHC COMPASS SPS
Beam: µ+/ spill (4.8s / 16.2s) Luminosity: ~ cm-2 s-1
Beam polarization: -76%
Beam momentum: GeV/c
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

4. How to access to (G/G)(xg)?
Spin asymmetries ALL of the main diagrams in DIS:
Photon gluon
fusion (PGF) LO
Compton
ALL 
ALL = *
 Only PGF is sensitive to (G/G)(xg) (but S suppressed)
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

5. Selections of PGF Open charm channel c c q = c, then c D0 or D* K,
scale: mc c
q = c, then c D0 or D* K,
 : no physical background
 : low statistics
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

6. (ΔG/G)(xg) with open charm
ALL/D =
D*  D0+   K+  +  :
(G/G)(xg)=  0.73(stat) :
[(G/G)(xg)] = 0.43
(analysis in progress)
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

7. Selections of PGF Open charm channel High pT hadrons channel c c
scale: mc
scale: pT , Q² c
q = c, then c D or D* K,
(q,q) (h1,h2) with high transverse
momentum wrt * direction
 : no physical background
 : low statistics
 : physical background
 : large statistics
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

8. (G/G)(xg) with high pT hadrons
ALL/D = Rpgf G/G aLLpgf
+ Rqcdc q/q aLLqcdc
+ Rqq q/q aLLgq (G/G)
+ Rqg G/G aLLqg (q/q) 
+ …
Resolved
photons!
(low Q²)
 Need a Monte Carlo simulation
 What about polarisations of partons in virtual photon?
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

9. Monte Carlo simulation
Event generator PYTHIA is used for low Q² analysis
(+ GEANT for spectrometer simulation) Q² pT
Data & MC Q² pT
Data/MC
 Good description of the data by the simulation!
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

10. Results of simulation RPGF  30% RRes.Phot  50%
Relevant parameters were then varied to
estimate the uncertainties due to MC
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

11. Resolved photon asymmetry
Problem: polarized PDFs of virtual photons not measured!
q,pert(xp,²) calculable
Solution:
-q,npert(xp,0²) < q,npert(xp,0²) < q,npert(xp,0²)
measured!
Allows us to obtain a range
for (q/q) and (G/G)  !
 Adds a limited uncertainty to the estimation of (G/G)(xg)
1st estimation of the resolved photon asymmetry!
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

12. Results (1) 02-03 0.024 0.089 Values used for extraction of (G/G)(xg)
data
(G/G)(xg)
stat
exp.syst
MC.syst 
02-03
0.024
0.089
0.014
0.052
0.018
[PLB 633
(2006) 25-32]
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

13. Results (1)
Equiv. stat. * ~2.4 with 2004 data!
Asymmetries compatible with
Values used for extraction of (G/G)(xg)
data
(G/G)(xg)
stat
exp.syst
MC.syst 
02-03
0.024
0.089
0.014
0.052
0.018
02-04
0.016
0.058
0.013
[PLB 633
(2006) 25-32]
NEW
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

14. Results (2)  Favours small values of G
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

15. Outlook More results soon available with 2004 data: For the future:
Q² > 1 (GeV/c)² high pT events:
Open charm selection
 No resolved photons!
 No physical background!
For the future:
Optimization of event selection with a neural network
Bins in xg (requires improvement of xg reconstruction)
2006 data with new COMPASS magnet (larger xg)
already
in progress!
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

16. Conclusion 2004 data confirms results from 02-03 (FOM *2.4)
G is small (or G(xg) has a node around 0.1)
still the most precise determination of (G/G)(xg)
 G not large  ΔΣ small ( predictions)
½= ½ΔΣ + ΔG + Lq + Lg ½
possible scenarios: ½
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

17. Spares
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

18. A1 at COMPASS (2002-2003) Agreement with previous experiments
Improve stat at small x
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

19. g1 at COMPASS
PLB 612(2005),154
 = – (without COMPASS)
 = – (with COMPASS)
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

20. Transversity at COMPASS
Collins
Sivers
PRL 94(2005),202002
Asymetries compatible with 0 ( HERMES)
 p/n compensation?
New measurement of G/G at COMPASS 41st Rencontres de Moriond S.Procureur

21. Results of PHENIX Theory model C.L. (%) GRSV-std 17.1-21.7 *GRSV-max
(PANIC05)
Theory model
C.L. (%)
GRSV-std
*GRSV-max
(Dg=g)
*GRSV Dg=0
*GRSV Dg=−g
GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, Phys. Rev. D 53 (1996) 4775.
* At input scale: Q2 = .4 GeV