d g/g from high-p t events in smc
DESCRIPTION
D G/G from high-p T events in SMC. E.Rondio for Spin Muon Collaboration (SMC) Sołtan Institute for Nuclear Studies Warsaw, Poland. Determination of ∆G/G from Photon Gluon Fusion Analysis in Leading Order where it can be separated based on simulations with LEPTO - PowerPoint PPT PresentationTRANSCRIPT
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G/G from high-pT events in SMC
•Determination of ∆G/G from Photon Gluon Fusion•Analysis in Leading Order where it can be separated•based on simulations with LEPTO•Search for sample with high PGF contribution•application for DIS region, SMC data with Q2 >1GeV2
E.Rondio
for Spin Muon Collaboration (SMC)
Sołtan Institute for Nuclear Studies
Warsaw, Poland
Workshop on Hadron Structure and Spectroscopy, Paris, March 1st to 3rd 2004
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History
• Idea proposed by R.D.Carlitz, J.C.Collins and A.H.Mueller, Phys.Lett.B 214, 229 (1988)
• Revisited by A.Bravar,D.von Harrach and A.Kotzinian, Phys.Lett.B 421, 349 (1998)
• Method used in HERMES for photoproductionHERMES, A.Airapetian et al., Phys.Rev.Lett.84, 2584 (2000)
• Here application for DIS region, SMC data with Q2 >1GeV2
SMC, B.Adeva et al.., submitted to Phys.Rev.D, hep-ex/0402010
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QCDCQCDCLL
LPLPLL
PGFPGFLL
lhhXlN
RaRaq
q
RaG
ΔGA
G/G evaluation from measured asymmetry
where: AlNlhhX measured asymmetry,
q/q approximated by A1/D asymmetry N,
aLL partonic asymmetry,
R fraction of contributing processes
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Applicability and restrictionsSplitting between processes only in LO >>> when higher order effects expected to be important
it can not be used >>> here scale dependence was checked and found
small, so no clear signal of such strong dependenceUsing information which is not an observable (which type of interaction given event is) >>> so it has to be taken from simulation >>> the above makes analysis model dependent (using Lepto or eg. Pythia can give different results) but … a tool to check reliability is comparison of data with
MC Spin effects do not have to be simulated >>>measurement is independent of assumptions about
polarized parton distributions and spin effects in fragmentation
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Why events with high-pT hadrons ?
PGF
signal
LP QCDC
• Two high-pT hadrons more likely in QCDC and PGF because in LP source of pT only fragmentation in PGF and QCDC in addition pT from hard scattering
background
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Target: butanol, ammonia –
proton d-butanol - deuteron
Beam:
µ+ 190 GeV
Pµ= -0.78±0.03
Measured asymmetry:
lhhXlNTμ fAPP
NN
NN
where: beam, target
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Selected events cover following x, y, Q2
region
xBj xBj
yQ2
[GeV]
Conditions on hadrons in the final state
2 hadrons: pT> 0.7GeV, z>0.1, xF>0.1
(no electron contamination observed after these cuts)
Event selection for asymmetry
vertex in target half, beam through full target length, stable conditions
Kinematic cuts and regions: Q2>1GeV2, 0.4<y<0.9, acceptance for and h
Statistics after selections
proton deuteron
81 178 75 266
below 0.5% of the inclusive sample
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Monte Carlo studies
→ studies for DIS µp interactions at 190 GeV→ LEPTO simulations, Q2 1 GeV2
→ detector and reconstruction effects• geometrical acceptance for hadrons• simulations of trigger conditions• looses in reconstruction (chamber efficiencies)• smearing for scattered µ and hadrons (1/p, angles)• secondary interaction in target for hadrons
→conditions in MC generation scale for hard processes (syst.errors only)
cut-off’s in matrix element calculation parameters of symmetric fragmentation function
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Data and Monte Carlo agree at the level of 10-25%
To be used for selections of PGF and ∆G evaluation
Data and Monte Carlo comparison
Event kinematicsSensitive to trigger mixture, smearing
Hadron variablesSensitive to smearing and MC generation (ff)
Data
MC
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Simulation of exp. conditions
Sensitive to details of target:
position, angle
Good description after inclusion
of hadron secondary interactions
Modification of fragm. function
a=0.5, b=0.1 (stand.)
zbma Tezzzf /1 2
)1()(
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Contribution of PGF processFor SMC experimental
conditions Lepto at generation level RPGF = 8% events with two hadrons
(phad>5GeV) RPGF = 12% additionally pT
had > 0.7 GeV RPGF = 24%
How to get more? Two methods tried:• kinematical selections
(cuts) and • Neural Network
classification (NN)
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The criteria to judge the selection:
PGF(in)
PGF(out)Efficiency
PGF(out)QCDC(out)LP(out)
PGF(out)Purity
Several variables tried
Opposite charges of hadrons –
small effect, 1/3 events lost
Azimuthal angle between hadrons
– no improvement
Best - ∑p2T
Cuts on hadron variables
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Neural network
• input layer: event kinematics (x, y, Q2) and hadron variables (E1,2, pT1,2, charge, azimuthal angle between pT of two selected hadrons), • best way to use correlations• output layer: single unit number within range (0,1)
NN response Architecture: multi-layer feed-forward configuration
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Neural Network responsenumber within range <0,1.> events at high values of NN response are more likely to
be PGF
PGF enriched sample
selected by setting the threshold
on the NN response
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NN treshold
Processes
contributions
for two selection
method
PDG
QCDC
LO
PGF
LO
QCDC
Best result of cut
selection based
on pT2
compared to NN
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Asymmetry AlNlhhX
Systematic uncertainties:
•False asymmetries from acceptance variation
•Calculation of radiative effects (unpolarized and polarized part)
Effect due to restricted phace space
•Polarization of beam and target
•Target material
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Selection Proton AlNlhhX Q2
Deuteron AlNlhhX Q2
pT2 0.0180.0710.010 7.07 0.054
0.0930.008 7.91
NN 0.0300.0570.010 3.30 0.070 0.0770.010
4.00Interpretation of A lN→ lhhX in terms of ∆G/G requires
additional information from MC simulation.
AlNlhhX
pT0.7GeV pT22.5GeV2
NN0.26
Results on Asymmetry
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Input for calculation of ∆G/G
∆q/q approximated by A1·D
neglecting PGF contribution in inclusive
A1 measurements,
ok. only if RPDG(incl)<< RPDG(selected)
From other measurements:
A1 asymmetry taken from fit
to all experimental data
f(x)=xa·(1-ebx)+c ,
Q2 dependence neglected
proton
deuteron
Hermes
Hermes
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Input for calculation of ∆G/G From MC simulations:
• aLL calculated in POLDIS
aLLLP 0.8
aLLQCDC 0.6
aLLPGF -0.44
• fractions of processes Selection RLP RQCDC RPGF
pT22.5GeV2 26% 42% 32%
NN 0.26 38% 30% 32%
Important consistency between data and MC
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Statistical precision of ∆G/G
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Gluon polarization
Separately for proton and deuteron
∆G/G determined for a given fraction of nucleon momentum carried by gluons η
Selection G/G (G/G)stat genPGF
pT22.5GeV2 -0.07 0.40 0.09
NN 0.26 -0.20 0.29 0.07
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Average value final SMC result on
∆G/G =-0.200.290.11
SMCHermes
NNpT1
2+pT22
comparison
• Difference < 2 σ
• Different process DIS vs. Photoproduction
• Factor 2 difference
in ηgluon
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Systematic uncertainty on ∆G/G Contribution to the systematic
due to uncertainty on parameters used in MC :
• sensitivity to fragmentation, • cutoffs in matrix elements calculations• scale dependence (2Q2,Q2/2),
Changes in RPGF < 5%
Similar effect for
pT of faster hadron
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Error source uncertainty on ∆G/G
Precision of A1 fit 0.026Scale change Q2/2 ; 2Q2 0.010Fragmentation function 0.034Cutoffs in matrix elements
0.008
err. from MC and A1 0.053Syst.error from Alhh 0.062
Total 0.115
+20%R / -20%R 0.067 / 0.100
+4% aLL / -4% aLL 0.015 / 0.017
Systematic uncertainty on ∆G/G
Changing only R or aLL
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Summary• The method of ∆G/G evaluation from asymmetry for
events with high-pT hadrons was applied to SMC data in DIS region
• Results obtained for cut selection and neural network ∆G/Gstat. sys. -0.07 ± 0.40 ± 0.11 cut ∑pT
2
∆G/Gstat. sys. -0.20 0.29 0.11 NNpoints to rather small value of gluon polarization
• precision of ∆G/G limited by the statistical error, • systematic error controlable (and can be reduced for
high statistics by precise data/MC comparison)
• Improvement on accuracy of ∆G/G in future: COMPASS at CERN, RHIC at BNL, E161 at SLAC