1. ΔG/G Extraction From High-Pt Hadron Pairs at COMPASS
Ahmed El Alaoui
Nuclear Physics School, Erice, September 2007
On Behalf Of COMPASS Collaboration

2. Outline Introduction COMPASS Experimental Setup Data Analysis Results
Summary and Conclusion

3. Nucleon Spin SPIN CRISIS Naive Quark Model
Pure valence description of constituent quarks:
∆u = + 4/3 ∆d = - 1/ ∆Σ = 1
Relativistic Quark Model ∆Σ ≈ 0.75
QCD framework Hyperons β decay constants + SU(3) flavor symmetry
∆Σ = a0 ≈ 0.60 compatible with the Relativistic QM prediction
However, the EMC measured ∆Σ = 0.12 ± 0.09 ± 0.14
SPIN CRISIS
a0 = ΔΣ – (3αs/2π)ΔG
A measurement of ∆G is needed
- To access the gluon contribution to the nucleon spin
- To understand the role of the Axial Anomaly in the
explanation of the spin crisis

4. How To Acess ΔG/G Indirect Measurement:
QCD analysis: fit to the nucleon spin structure function g1(x)
Unfortunately, the limited range in Q2 does not allow for a precise determination of ∆G
Direct Measurement:
∆G/G can be accessed via Photon Gluon Fusion (PGF) process

5. PGF Process PGF APGF = aLL x (∆G/G) factorization theorem q = c:
Two approaches are used to tag PGF process
q = c:
- Open Charm D0, D* decay
Clean signal
Combinatorial background
- Low statistics
q = u, d, s:
- High-pt hadron Pairs
Physical background
High statistics
APGF = aLL x (∆G/G) factorization theorem
PGF

6. How To Acess ΔG/G Indirect Measurement:
QCD analysis: fit to the nucleon spin structure function g1(x)
Unfortunately, the limited range in Q2 does not allow a for precise determination of ∆G
Direct Measurement:
∆G/G can be accessed via Photon Gluon Fusion (PGF) process
Three independent measurements were done at COMPASS
- Open Charm
- High pt hadron pairs production at Q2>1GeV2
- High pt hadron pairs production at Q2<1GeV2

7. COMPASS Collaboration
COmmon
Muon and
Proton
Apparatus for
Structure and
Spectroscopy
250 Physicists
18 Institutes
12 Countries

8. Experiment Layout LHC SPS luminosity: ~5 1032 cm-2 s-1
beam intensity: µ+/spill (4.8s/16.2s)
beam momentum: GeV/c

9. COMPASS Spectrometer SAS 50 m long LAS 160 GeV μ beam
RICH
E/HCAL1
Muon Wall 1
Muon Wall 2
E/HCAL2
Polarized
Target
160 GeV μ beam
Polarization ~ 80%
50 m long
LAS
SAS
MicroMegas DC
Tracking: SciFi, Silicon, MicroMegas, GEMs, MWPC, Straws
PID: RICH, Calorimeters, μ Filters

10. COMPASS Target Two 60 cm long oppositely polarized cells
6LiD is used as a material
dilution factor ~ 0.4
Target Polarization ~ 50%
70 mrad acceptance (180 mrad for 2006 target)
Vertex distribution

11. High Pt Events Selection
Primary vertex with at least μ, μ’ and 2 hadrons
0.1 < y < 0.9 (Q2>1GeV2)
0.35 < y < 0.9 (Q2<1GeV2)
Pt > 0.7 GeV
0.0 < z, xF < 1.0
ΣPt > 2.5 GeV2 2
0.0 < z1+z2 < 0.95
minv(h1,h2) > 1.5 GeV
ECalo/P > 0.3

12. High Pt Spin Asymmetry Aexp = (Nu - Nd)/(Nu + Nd)μ B
Aexp = (Nu - Nd)/(Nu + Nd)
The acceptance is not identical in both cells Asymmetry bias

13. Polarisation reversal each 8 hours
High Pt Spin Asymmetry μ B μ B
Aexp = (Nu - Nd)/(Nu + Nd) ‘
Aexp = (Nu - Nd)/(Nu + Nd)
Polarisation reversal each 8 hours
A||/D=(Aexp- Aexp)/2fPTPBD ‘
f Dilution factor
PT(B) Target(Beam) polarization
D Depolarization factor
To improve the statistical error, a weighted method is used in the asymmetry calculation:
w = fDPB (event-wise weight)

14. ∆G/G Extraction at Q2<1GeV2

15. ∆G/G at Q2<1GeV2 γ γ γ γ γ γ A||/D = RPGF∆G/G aLL PGF
+ RQCDC ∆q/q aLL
QCDC
+ Rqq∆q/q aLL (∆q/q) qq γ
+ Rqg∆G/G aLL (∆q/q) qg
Resolved photon processes
+ Rgq∆q/q aLL (∆G/G) gq γ
+ Rgg∆G/G aLL (∆G/G) gg
qq’ qq’ γ γ
Ri (fraction of the process i), aLL, ∆q, q, q and G are obtained from
- Monte Carlo Simulation based on PYTHIA generator and Geant.
- pQCD Calculation
- pdf in the nucleon from GRSV2000 and GRV98LO parametrization
- pdf in the photon from GRS parametrization γ γ
The polarized pdfs in the photon ∆q and ∆G are not available. Therefore the positivity limit is used to constrain them which leads to 2 extreme scenarios.
Included as systematic error in the estimation of ∆G/G

16. Monte Carlo vs. Data (Q2<1GeV2)
xBj

17. Process fractions (Q2<1GeV2)
Resolved photons processes
32%
12%
50%

18. ∆G/G Result at Q2<1GeV2
data
A||/D = ± 0.013(stat.) ± 0.003(syst.)
∆G/G(xg,μ2) = ± 0.058(stat.) ± 0.055(syst.)
xg =
+0.070
μ2 = 3GeV2
Contribution to Syst. error comes from
- False asymmetry
- Monte Carlo tuning
- Resolved photon process

19. ∆G/G Extraction at Q2>1GeV2

20. ∆G/G at Q2>1GeV2 A||/D = RPGF∆G/G aLL + RQCDC ∆Q/Q aLLLO
A||/D = RPGF∆G/G aLL + RQCDC ∆Q/Q aLL
PGF
QCDC
+ RLO ∆Q/Q aLL LO
Contribution from resolved photon precesses is negligible in this case
At Q2>1GeV2 analysis, Lepto generator seems to describe the real data much better than PYTHIA. It was then used to estimate the fraction of each process

21. Monte Carlo vs. Data at Q2>1GeV2
34%

22. ∆G/G Result at Q2>1GeV2
Data
A||/D = ± 0.080(stat.) ± 0.013(syst.)
∆G/G(xg, μ2) = 0.06 ± 0.31(stat.) ± 0.06(syst.)
<xg> = 0.13
μ2 = 3GeV2
Contribution to Syst. error comes from
- False asymmetry
- Monte Carlo tuning
2004 Data
Analysis is in progress…

23. Results ΔG/G

24. Summary and Conclusion
- High-Pt asymmetries at Q2>1GeV2 and Q2<1GeV2 were presented
- The measured ∆G/G is compatible with zero at xg = 0.1
The most precise measurement up to now
Analysis of 2004 data (at Q2>1GeV2) is almost finished. It will be
released soon
- The new solenoid, installed in 2006, has an acceptance (180 mrad) three times larger than the previous one.
Double the statistics obtained in 2004
Access higher value of xg
Thank you