QCD: Approaching True Precision or, Latest Jet Results from the TeVatron

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QCD Approaching True Precisionor, Latest Jet
Results from the TeVatron
Presented by John Krane Iowa State University

• Experimental Details
• SubJets and Event Quantities
• Cross Sections

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Changing Paradigms in QCD

• Inclusion of error estimates in the PDFs
• Progress toward NNLO predictions
• More rigorous treatment of experimental errors
• More consistent ET calculations between
  experiments

calculation of virtual corrections
covariance matrices
jet algorithms workshop
hope for better underlying event
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Jet Definition
Iteration vs ratcheting

• Cone DefinitionR0.7 in h-fmerge/split
• KT Definition
• cells/clusters are combined if their relative kT2
  is small(D1.0 or 0.5 -- scaling parameter)
• For subjets, also define large
• (ycut, 10-3)

Centroid found with4-vector addition vs Snowmass
addition
0.5 vs 0.75
DÆ uses Ellis and Sopers definition
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Jet Cuts

• Typical cuts on EM fraction, hotcells, missing
  ET, vertex position, etc.
• gt 97 efficient
• gt 99 pure

Muon BC
Magnet
Muon A-Layer
Hadronic Calorimeter
EM Calorimeter
Central Tracking
jet
e
g
m
noise
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Jet Energy Corrections
no distinction between jetsof different kinds

• Response functions
• Noise and underlying event
• Showering
• ResolutionsEstimated with dijet balancing or
  simulation

d2s dET dh
ET
the symmetric part
d2s dET dh
ET
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Jet and Event Quantities

• Subjet Multiplicity
• Underlying event structure
• Low ET Multijet studies

New !
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Subjet Multiplicity
Using the kT algorithm

• Linear Combination
• ltMgt fg Mg (1-fg) MQ
• Assume Mg, MQ independent of vs
• Measure M at two vs energies andextract the g
  and Q components
• Largest uncertainty comes fromthe gluon
  fractions in the PDFs

D0 Preliminary
Mean Jet Multiplicity
0.5 0.4 0.3 0.2 0.1
Quark Jet Fraction
Gluon Jet Fraction
1 2 3 4
Subjet Multiplicity M
Coming soon as a PRD
HEWIG prediction 1.860.08(stat)
no attempt to developa likelihood function
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Underlying event structure

• Counting charged particles in azimuthal sectors
• Improve modeling of underlying event

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Low ET Multijet events

• At high-ET, QCD does quite well.
• But try counting jets at low-ETcompare to Pythia

Each jets ETgt20 GeV. For 2 jets or more,
normalization is off, so correct to gt40 GeV
spectrum.
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Low ET Multijet events

• Strong pT ordering in DGLAP suppresses spectator
  jets
• BFKL has diffusion in log(pT)

Looking also at Jetrad and Herwig
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Cross Sections

• R32
• 630/1800 ratio of jet cross sections
• Forward cross sections
• Two PRDs
• KT central inclusive

Published! Published! Published! Submitted
New !
c2 analysis
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Inclusive R32

• A study of soft jet emission rate
• Ratios exploit error correlations
• We observe lack of PDF sensitivity
• Investigate mR scale sensitivity with Jetrad

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Inclusive R32

• several formulae for mR
• mR not always same for jets in the same event
  (yet agreement not improved)
• single-scales seem better than mixed-scales,
• 0.3HT is most robust
• (As minimum ET threshold climbs, the hardness
  of the event is less well-represented by ETmax?)

PRL 86, 1955 (2001)
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Inclusive Ratio

• Published yesterday in PRL
• c2 probabilities fall between 30 and 60
• DÆ has tried a normalization-only test, which
  yields generally poor agreement (lt1), except for
  2ET (7) and 0.25ET (23).

PDF Variations
mfmR Variations
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A few words on c2
100 trials
The calculations work great,root-diagonal
elements alonenot sufficient for plots

• 10 MC simulations of systematic errors

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Rapidity-dependent Inclusive

• DÆs most complete cross section measurement
• Uncertainty in theory is larger than uncertainty
  is data!

?d2?? dET d?? (fb/GeV)
ET (GeV)
PRL 86, 1707 (2001)
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Jets PRD 1 CDFs Run 1b inclusive jet

• Has discussion of Dc2 technique.

• Includes comparisons to Run 1a dataDÆs run 1b
  data

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Jets PRD 2 DÆs Run 1b jet results

• Tour de Force
• 1800 central inclusive,dijet mass,dijet
  angular distribution,630/1800 inclusive ratio

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KT Inclusive Jet Cross Section

• -0.5 lt h lt 0.5
• D 1.0
• Predictions IR and UV safe
• Merging behavior well-defined for both exp. and
  theory

DÆ Preliminary
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KT Comparison
DÆ Preliminary

• Normalization differs by 20 or more
• pT dependence
• DÆ has an error matrix expect c2 numbers soon
• No significant deviations of predictions from
  data

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Summary

• TeVatron stopped running in 1996 but several
  results are still in the queue
• Jet substructure
• Event structure
• Cross sections
• Growing sophistication in results
• Greater consistency between experiments
• Error matrices
• Better corrections

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Backup slides
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Large HT

• Sum of jet ETs gt 500 GeV
• Shape of the cross section limits compositness
  scale

• Limits on Quark Compositeness from High Energy
  Jets in pbarp Collisions at1800 GeV
• Phys. Rev. D Rapid Comm. 62 031101 2000

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Energy Scale

• Offset noise, pileup, multiple
  interactions, underlying
  event
• Response observed energy vs. True energy
• Showering removes edge-effects at the
  jet cone boundary

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Jet Resolution and Smearing Effects
Observed Cross Section

• Finite jet energy resolution has the effect of
  inflating the cross section, especially where the
  
• cross section is steepest.
• The correction (unsmearing) is determined with
  dijet asymmetry measurements.

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Cross Section Uncertainties The Covariance
Matrix

• separate errors into categories based on
  degree of ET correlation
• determine change in cross section at each
  data point from each error
• repeat for each uncertainty

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Jet Cross Section at 1800 GeV

• Comparing DÆ and CDF

/
/
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• Simulation of Jet Cross Sections
• using CTEQ4M

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• Backup Slides

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NLO QCD Glossary

• Parton Distribution Function (PDF)
• Factorization Scale (mf)
• Renormalization Scale (m)
• Rsep

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• Uncertainties in the Theory

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The DÆ Calorimeter

Liquid ArgonUranium absorberFull coverage to
h 4.2 0.1 x 0.1 segmentation6 nuclear
interaction lengths
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Luminosity
as determined by World Average

• 1800 GeV
• 630 GeV

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Luminosity

• Determined with a fit between 1800 and 546 GeV

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Total Efficiency Correction

• also...Vertex efficiency 90

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The MPF method
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Other Systematics

• Jet selection (cuts)
• Event selection (missing ET cut, vertex
  cut)630 lt 1 1800 1-2
• Unsmearing full covariance (qg fractions
  are correlated)
• scaling to dimensionless variables 0.8

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Energy Scale Uncertainty(A toy Monte Carlo Study)

• Generate Monte Carlo Jets with appropriate
  ET spectrum, weight
• Generate lum, number of interactions, vertex
• Call energy scale correction to get errors
  component-by-component

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Energy Scale Uncertainties
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Classifying Uncertainties

• The error correlations
• Completely Correlated
• no symbol Uncorrelated (independent)

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Total Ratio Correlation

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Augmented QCD