Transcript ΔG/G From High Pt pair production at Q2>1GeV2 at COMPASS

ΔG/G Extraction From HighPt Hadron Pairs at COMPASS
Ahmed El Alaoui
Nuclear Physics School, Erice, 16-24 September 2007
On Behalf Of COMPASS Collaboration
Outline
Introduction
COMPASS Experimental Setup
Data Analysis
Results
Summary and Conclusion
Nucleon Spin
Naive Quark Model
Pure valence description of constituent quarks:
∆u = + 4/3 ∆d = - 1/3
∆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
a0 = ΔΣ – (3αs/2π)ΔG
SPIN CRISIS
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
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
PGF Process
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 =
PGF
aLL x
(∆G/G) factorization theorem
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
COMPASS Collaboration
COMPASS
COmmon
Muon and
Proton
Apparatus for
Structure and
Spectroscopy
250 Physicists
18 Institutes
12 Countries
Experiment Layout
LHC
SPS
luminosity:
~5 1032 cm-2 s-1
beam intensity: 2.108 µ+/spill (4.8s/16.2s)
beam momentum:
160 GeV/c
COMPASS Spectrometer
Muon Wall 1 SM2
E/HCAL2
Muon Wall 2
E/HCAL1
RICH
SM1
DC
MicroMegas
Polarized
Target
160 GeV μ beam
Polarization ~ 80%
Tracking: SciFi, Silicon, MicroMegas, GEMs, MWPC, Straws
PID:
RICH, Calorimeters, μ Filters
COMPASS Target
Two 60 cm long oppositely polarized cells
6LiD
Vertex
is used as a material
distribution
dilution factor ~ 0.4
Target Polarization ~ 50%
-100
70 mrad acceptance (180 mrad for 2006 target)
-50
0
50
[cm]
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
0.0 < z1+z2 < 0.95
minv(h1,h2) > 1.5 GeV
ECalo/P > 0.3
2
High Pt Spin Asymmetry
B
μ
Aexp = (Nu - Nd)/(Nu + Nd)
The acceptance is not identical in both cells
Asymmetry bias
High Pt Spin Asymmetry
B
B
μ
μ
Aexp = (Nu - Nd)/(Nu + Nd)
Polarisation reversal
each 8 hours
‘ - Aexp)/2fPTPBD
A||/D=(Aexp
‘ = (Nu‘ - Nd‘)/(Nu‘ + Nd‘)
Aexp
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)
∆G/G Extraction at Q2<1GeV2
∆G/G at Q2<1GeV2
PGF
A||/D = RPGF∆G/G aLL
QCDC
+ RQCDC ∆q/q aLL
Resolved
photon
processes
qq
aLL
γ
qg
γ
+ Rqq∆q/q
+ Rqg∆G/G aLL (∆q/q)
(∆q/q)
gg
γ
γ
gq
+ Rgq∆q/q aLL (∆G/G) + Rgg∆G/G aLL
(∆G/G)
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
Monte Carlo vs. Data (Q2<1GeV2)
xBj
Process fractions
2
2
(Q <1GeV )
Resolved
photons
processes
32%
12%
50%
∆G/G Result at Q2<1GeV2
2002-2004 data
A||/D = 0.004 ± 0.013(stat.) ± 0.003(syst.)
∆G/G(xg,μ2) = 0.016 ± 0.058(stat.) ± 0.055(syst.)
+0.070
xg = 0.085-0.035
μ2 = 3GeV2
Contribution to Syst. error comes from
- False asymmetry
- Monte Carlo tuning
- Resolved photon process
∆G/G Extraction at Q2>1GeV2
∆G/G at
PGF
2
2
Q >1GeV
QCDC
LO
QCDC
A||/D = RPGF∆G/G aLL + RQCDC ∆Q/Q aLL
PGF
LO
+ RLO ∆Q/Q aLL
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
Monte Carlo vs. Data at Q2>1GeV2
40
30
20
34%
10
0
PGF
QCDC
LO
∆G/G Result at Q2>1GeV2
• 2002 - 2003 Data
A||/D = -0.015 ± 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…
Results ΔG/G
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