D G/G from high-p T events in SMC
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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
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
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
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
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
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
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
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
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
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()(
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)
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
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
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
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
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
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
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
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
Statistical precision of ∆G/G
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
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
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
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
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
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