Determination of the gluon polarisation at COMPASS

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Determination of the gluon polarisation at
COMPASS RHIC

• Sébastien Procureur (CEA - Saclay)

Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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The nucleon spin structure
½ ½?S ?G Lq Lg
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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The nucleon spin structure
?G contributes to scaling violation of g1
?G(Q²1) 0.53 /- 1.93 (AAC 2000)
?G(Q²1) 0.50 /- 1.27 (AAC 2003)
?G(Q²1) 0.47 /- 1.08 (AAC 2006)
Small available Q² range for g1 ? large
uncertainties on ?G
Direct measurement of ?G needed
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Direct measurement of ?G
?
polarized p-p collider (RHIC)
polarized semi-inc. DIS
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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RHIC _at_ BNL
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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RHIC
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
Siberian Snakes
BRAHMS PP2PP
PHOBOS
Siberian Snakes
Spin Flipper
PHENIX
STAR
Spin Rotators
Partial Snake
Strong Snake
Helical Partial Snake
Polarized Source
LINAC
AGS
BOOSTER
200 MeV Polarimeter
Rf Dipole
AGS Polarimeter
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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?G/G _at_ RHIC
P1
P2
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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PHENIX detector
EM Calorimeter
Beam-Beam Counter
Time Expansion Chamber
Muon Tracking Chambers
Central Arms
Muon ID Panels
Pad Chambers
North Muon Arm
Multiplicity/Vertex Detector
Drift Chambers
South Muon Arm
Time of Flight Panels
Four spectrometer arms with excellent trigger
and DAQ capabilities.
Ring Imaging Cerenkov
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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?0 cross section
Very good agreement with NLO Xsection
calculation!
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Results
?0 reconstruction (with background asym.
subtraction)
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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STAR detector
Large acceptance TPC and EMC -1lt?lt2
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Results
2003-2004 data
Large ?G already disfavored
results from 2005 soon
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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RHIC projections
Results of unprecedent precision in the next
years
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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COMPASS _at_ CERN
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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COMPASS
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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?G/G _at_ COMPASS
Spin asymmetries ALL of the main diagrams in DIS
Photon gluon fusion (PGF)
LO
Compton
? Only PGF is sensitive to (?G/G)(xg) (but ?S
suppressed)
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Selections of PGF
Open charm channel
c
scale mc
c
q c, then c D0 or D K,?
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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?G/G with open charm
ALL
D ? D0 ?S ? K ? ?S
D0 ? K ?
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Selections of PGF
Open charm channel
High pT hadrons channel
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
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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?G/G with high pT hadrons
high Q²
resolved photons! (low Q²)
? Need a Monte Carlo simulation
? Theoretical inputs for polarisations of partons
in virtual photon
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Monte Carlo simulation
PYTHIA generator for low Q² GEANT for spectro
simul.
Data MC
Data/MC
pT
Good description of the data by the simulation!
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Results
2002-2004 data
(2002-2003 data PLB 633 (2006) 25-32)
Favours small values of ?G
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Next steps
? Results from 2004 data for Q² gt 1 GeV²
? Improved high pT selection with Neural Network
very soon!
? Extraction of ?G/G at different xg
? NLO analysis (theoretical inputs)
New run in 2006
? New magnet ? larger acceptance ? larger xg
? Improved RICH (particle identification)
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Conclusion

• Efforts on ?G measurement start to pay!
• Agreement between RHIC and COMPASS data
• Very exciting results are expected in the next
  few years!

?G not large ? ?S small (? predictions)
½ ½?S ?G Lq Lg
½0.2 0.4 0.0 0.0
possible scenarios
½0.2 0.0 0.4

Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Spares
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur
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Resolved photon asymmetry

• Problem polarized PDFs of virtual photons not
  measured!

?q?,pert(xp,?²) calculable

• Solution

-q?,npert(xp,?0²) lt ?q?,npert(xp,?0²) lt
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!
Determination of ?G at COMPASS RHIC PAVI06,
Milos S.Procureur