recent measurement of Δg/g at compass sébastien procureur – cea saclay on behalf of the compass...
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Recent measurement of ΔG/G at COMPASS
Recent measurement of ΔG/G at COMPASS
Sébastien Procureur – CEA Saclay
on behalf of the COMPASS collaboration
EPS2005 23/07/2005
- Quark model: ΔΣ ≈ 0.75
- QCD + s = 0 (Ellis-Jaffe): ΔΣ ≈ 0.6
- Experimentaly: a0 (=ΔΣ?) ≈ 0.2
The Nucleon spin
½= ½ΔΣ + ΔG + Lq + Lg
quarks gluons orb. mom.
ΔΣ = a0 + 3(s/2)
ΔG
Sébastien Procureur EPS2005
The Nucleon spin
½= ½ΔΣ + ΔG + Lq + Lg
quarks gluons orb. mom.
Necessary to measure ΔG
- Quark model: ΔΣ ≈ 0.75
- QCD + s = 0 (Ellis-Jaffe): ΔΣ ≈ 0.6
Sébastien Procureur EPS2005
- Experimentaly: a0 (=ΔΣ?) ≈ 0.2
ΔΣ = a0 + 3(s/2)
ΔG
ΔG/G measurementmain DIS diagrams (up to 1st order):
LO PGF QCDC
only PGF is sensitive to ΔG/G
Sébastien Procureur EPS2005
ΔG/G measurement
Apgf = ΔG/G * aLLpgf
if factorization holds (needa hard scale):
we need to select PGF events(S suppressed)… with a hard scale
hard part
soft part
Sébastien Procureur EPS2005
PGF selections• Open charm channel J.Pretz’s talk
• 2 high pT hadrons channel:
(q,q) (h1,h2) with high pT
large stat, suppress LO,but still physical backgroundscale: pT², Q²
ALL/D = Rpgf G/G aLLpgf + (ALL/D)back
?
h1
h2
Sébastien Procureur EPS2005
+ Rqq q/q aLLqq q/q
Physical background
ALL/D = Rpgf G/G aLLpgf
+ Rqcdc q/q aLLqcdc
Q² < 1
all Q²
Sébastien Procureur EPS2005
+ Rqg G/G aLLqg q/q
Physical background
ALL/D = Rpgf G/G aLLpgf
+ Rqcdc q/q aLLqcdc
+ Rqq q/q aLLqq q/q
all Q²
Q² < 1
Sébastien Procureur EPS2005
+ Rgq q/q aLLgq (G/G)
Physical background
ALL/D = Rpgf G/G aLLpgf
+ Rqcdc q/q aLLqcdc
+ Rqq q/q aLLqq q/q
+ Rqg G/G aLLqg q/q
all Q²
Q² < 1
Sébastien Procureur EPS2005
+ Rgg G/G aLLgg (G/G)
Physical background
ALL/D = Rpgf G/G aLLpgf
+ Rqcdc q/q aLLqcdc
+ Rqq q/q aLLqq q/q
+ Rqg G/G aLLqg q/q
+ Rgq q/q aLLgq (G/G)
all Q²
Q² < 1
Sébastien Procureur EPS2005
Inputs:
-generation of Monte Carlo evts-parametrization (world data)-pQCD calculations
… what about the rest (Q² < 1GeV²)??
Physical background
ALL/D = Rpgf G/G aLLpgf
+ Rqcdc q/q aLLqcdc
+ Rqq q/q aLLqq q/q
+ Rqg G/G aLLqg q/q
+ Rgq q/q aLLgq (G/G)
+ Rgg G/G aLLgg (G/G)
Sébastien Procureur EPS2005
Resolved photon processes
polarised parton distribution functions of (virtual…) photon areunknown!
BUT:
-q(x,²) < q(x,²) < q(x,²)
leads to 2 extreme (min&max) scenarios
additional (theoretical) uncertainty, but we can continue!
measured!
Sébastien Procureur EPS2005
MC/data comparison
Q²
Generator PYTHIA is used for Q²<1GeV² (+ GEANT for spectrometer simulation)
.data
- MC
data/MC
Sébastien Procureur EPS2005
Results from MC
The Monte Carlo describes our data quite well!
Sébastien Procureur EPS2005
Rpgf ≈ 31%
Systematics due to MC Relevant parameters of the MC were varied to study the dependence
of our results with MC model
Sébastien Procureur EPS2005
a typical one the critical one
Results for high pT low Q²• Asymmetry for 2002+2003 (pT > 0.7 GeV, pT² > 2.5 GeV²):
• G/G:
ALL/D = 0.002 ± 0.019(stat) ± 0.003 (syst)
ΔG/G(xg=0.1,²=3GeV²) = +0.024 ± 0.089(stat.) ± 0.014(exp.syst) ± 0.052(MC.syst) ± 0.018(photon)
0.057
Sébastien Procureur EPS2005
Summary• Most precise measurement of ΔG/G up to now:ΔG/G(xg=0.1,²=3GeV²) = +0.024 ± 0.089(stat.) ± 0.052(syst.)
ΔG is small or ΔG(xg) crosses 0 around xg = 0.1• For the 1st time, Res. Phot. asym. taken into account
• Next steps:– 2004 data analysis (ΔG/G)0.05 (also 2006 run)– NLO analysis
• Other channel: single high pT hadron
Sébastien Procureur EPS2005
Experimental systematics• Asymmetries calculated with low pT events
• Aexp = AAfalse according to microwave setting
A+Af
A-Af
1% of false asymmetries in 2003
data
Sébastien Procureur EPS2005
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