Recent measurement of ?G/G at COMPASS

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Recent measurement of ?G/G at COMPASS
Sébastien Procureur CEA Saclay on behalf of
the COMPASS collaboration
EPS2005 23/07/2005
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The Nucleon spin
½ ½?S ?G Lq Lg
Sébastien Procureur EPS2005
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The Nucleon spin
½ ½?S ?G Lq Lg

• Quark model ?S 0.75

- QCD ?s 0 (Ellis-Jaffe) ?S 0.6
- Experimentaly a0 (?S?) 0.2
?S a0 3(?s/2?) ?G
Sébastien Procureur EPS2005
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The Nucleon spin
½ ½?S ?G Lq Lg
quarks gluons orb. mom.

• Quark model ?S 0.75

- QCD ?s 0 (Ellis-Jaffe) ?S 0.6
- Experimentaly a0 (?S?) 0.2
?S a0 3(?s/2?) ?G
Necessary to measure ?G
Sébastien Procureur EPS2005
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?G/G measurement
main DIS diagrams (up to 1st order)
PGF
QCDC
LO
? only PGF is sensitive to ?G/G
Sébastien Procureur EPS2005
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?G/G measurement
hard part
if factorization holds (need a hard scale)
Apgf ?G/G aLLpgf
soft part
? we need to select PGF events(?S suppressed)
with a hard scale
Sébastien Procureur EPS2005
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PGF selections

• Open charm channel ? J.Pretzs talk
• 2 high pT hadrons channel

(q,q) (h1,h2) with high pT
h1

• large stat, suppress LO,
• but still physical background

scale pT², Q²
?
h2
ALL/D Rpgf ?G/G aLLpgf (ALL/D)back
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Physical background
ALL/D Rpgf ?G/G aLLpgf
all Q²
Rqcdc ?q/q aLLqcdc
Sébastien Procureur EPS2005
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Physical background
ALL/D Rpgf ?G/G aLLpgf
all Q²
Rqcdc ?q/q aLLqcdc
Rqq ?q/q aLLqq ?q/q ?
Q² lt 1
Sébastien Procureur EPS2005
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Physical background
ALL/D Rpgf ?G/G aLLpgf
all Q²
Rqcdc ?q/q aLLqcdc
Rqq ?q/q aLLqq ?q/q ?
Q² lt 1
Rqg ?G/G aLLqg ?q/q ?
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Physical background
ALL/D Rpgf ?G/G aLLpgf
all Q²
Rqcdc ?q/q aLLqcdc
Rqq ?q/q aLLqq ?q/q ?
Rqg ?G/G aLLqg ?q/q ?
Q² lt 1
Rgq ?q/q aLLgq (?G/G) ?
Sébastien Procureur EPS2005
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Physical background
ALL/D Rpgf ?G/G aLLpgf
all Q²
Rqcdc ?q/q aLLqcdc
Rqq ?q/q aLLqq ?q/q ?
Rqg ?G/G aLLqg ?q/q ?
Q² lt 1
Rgq ?q/q aLLgq (?G/G) ?
Rgg ?G/G aLLgg (?G/G) ?
Sébastien Procureur EPS2005
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Physical background
ALL/D Rpgf ?G/G aLLpgf
Rqcdc ?q/q aLLqcdc

• Inputs
• generation of Monte Carlo evts
• parametrization (world data)
• pQCD calculations

Rqq ?q/q aLLqq ?q/q ?
Rqg ?G/G aLLqg ?q/q ?
Rgq ?q/q aLLgq (?G/G) ?
Rgg ?G/G aLLgg (?G/G) ?
what about the rest (Q² lt 1GeV²)??
Sébastien Procureur EPS2005
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Resolved photon processes

• polarised parton distribution functions of
  (virtual) photon are
• unknown!

BUT
-q?(x,?²) lt ?q?(x,?²) lt q?(x,?²)
measured!

• leads to 2 extreme (minmax) scenarios

? additional (theoretical) uncertainty, but we
can continue!
Sébastien Procureur EPS2005
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MC/data comparison
Generator PYTHIA is used for Q²lt1GeV² ( GEANT
for spectrometer simulation)
Q²
.data - MC
data/MC
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MC/data comparison
.data - MC
.data - MC
pT
data/MC
data/MC
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Results from MC
The Monte Carlo describes our data quite well!
Rpgf 31
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Systematics due to MC

• Relevant parameters of the MC were varied to
  study the dependence
• of our results with MC model

a typical one
the critical one
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Results for high pT low Q²

• Asymmetry for 20022003 (pT gt 0.7 GeV, ?pT² gt
  2.5 GeV²)
• ?G/G

ALL/D 0.002 0.019(stat) 0.003 (syst)
?G/G(xg0.1,?²3GeV²) 0.024 0.089(stat.)
0.014(exp.syst)
0.052(MC.syst)
0.018(photon)
0.057
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Comparison with other results
?G 2.48
?G 0.62
?G 0.16
(GRSV param.)
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Summary

• Most precise measurement of ?G/G up to now
• ?G/G(xg0.1,?²3GeV²) 0.024 0.089(stat.)
  0.052(syst.)
• ? ?G is small or ?G(xg) crosses 0 around xg 0.1
• For the 1st time, Res. Phot. asym. taken into
  account

• Next steps
• 2004 data analysis ? ?(?G/G)?0.05 (also 2006 run)
• NLO analysis

• Other channel single high pT hadron

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COMPASS acceptance
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Results from RHIC-PHENIX
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Asymmetry versus pT
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Experimental systematics

• Asymmetries calculated with low pT events
• Aexp A??Afalse according to microwave setting

A?-Af
? ?1 of false asymmetries in 2003 data
A?Af
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Sébastien Procureur EPS2005