Jet Physics at CDF

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Jet Physics at CDF

• Sally Seidel
• University of New Mexico
• APS99
• 24 March 1999

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1. Jets at CDF 2. The Inclusive Jet Cross
Section 3. The Dijet Mass Cross Section 4.
The Differential Dijet Cross Section
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CDF A multi-purpose detector for studying
hadronic collisions at the Fermilab Tevatron
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• The motivation
• Jet distributions at colliders can
• signal new particles
• test QCD predictions
• check parton distribution functions

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The data CDF reconstructs jets using an
iterative cone algorithm with cone radius

• Jet energies are corrected for
• calorimeter non-linearity
• uninstrumented regions
• contributions from spectator partons

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• The iterative cone algorithm
• Examine all calorimeter towers with
  ET gt 1 GeV.
• Form preclusters from continuous groups of towers
  with monotonically decreasing ET.
• If a tower is outside a window of 7 x 7 towers
  from the seed of its cluster, start a new
  precluster with it.
• For each precluster, find the ET-weighted
  centroid with R 0.7.
• Define the centroid to be the new cluster axis.
• Save all towers with ET gt 100 MeV within R
  0.7 about the new axis.
• Iterate until the tower list is stable.

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The Inclusive Jet Cross Section

• For jet transverse energies in the range 40 lt ET
  lt 440 GeV this probes distances down to 10-17
  cm.
• The analysis
• For luminosity (88.8 4.1) pb-1
• Trigger on jet-like events accept 4 triggers
  with uncorrected ET thresholds at 20, 50, 70, and
  100 GeV correct for pre-scaling

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C

• Apply data quality requirements
• ?zvertex?lt 60 cm to maintain projective geometry
  of calorimeter towers
• 0.1 lt ?detector lt 0.7 for full containment of
  energy in central barrel
• Etotal lt 1800 GeV to reject accelerator loss
  events
• Define ET E?sin? and missing ET. Require
• to reject cosmic rays
  
• Correct (unsmear) observed ET for energy
  degradation and calorimeter resolution
  

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• Calculate the cross section

• where
• N number of events
• L luminosity
• ?? range is 1.2
• and ?ET bins have width 5 - 80 GeV
• Compare to EKS (Ellis, Kunszt, Soper) NLO
  calculation with CTEQ4M pdf and
  renormalization/factorization scale ? ETjet/2

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Systematic uncertainties (all uncorrelated) on
the inclusive jet cross section i. Calorimeter
response to high-pT charged hadrons ii.
Calorimeter response to low-pT charged
hadrons iii. Energy scale stability (1) iv. Jet
fragmentation model used in the simulation v.
Energy of the underlying event in the jet cone
(30) vi. Calorimeter response to electrons
photons vii. Modelling of the jet energy
resolution function viii. Luminosity (4.1)
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The Dijet Mass Cross Section

• Many classes of new particles have a larger
  branching fraction to just 2 partons than to
  modes containing a lepton or a W/Zso this can be
  a powerful way to search for new particles.
• The analysis
• For luminosity (85.9 4.1) pb-1
• Trigger on jet-like events
• Select events with ? 2 jets, both with ?event lt
  2.

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• Define ? ? (?1-?2)/2, then require ? ?
  e2? lt 5. This is the same as cos? tanh
  ? lt 2/3 where ? is the Rutherford scattering
  angle
• Apply the data quality cuts.
• Correct for trigger efficiency, zvertex cut
  efficiency, resolution, and calorimeter effects.

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• Define the dijet mass
• Calculate the cross section
• where
• N number of events, corrected for prescaling
• L luminosity
• ?Mjj 10 mass bins (consistent with detector
  resolution)
• Compare to JETRAD (Giele, Glover, Kosower) NLO
  calculation with CTEQ4M ? ETmax/2. Two
  partons are merged if they are within Rsep 1.3
  ? R.

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The dijet mass cross section compared to JETRAD
with CTEQ4M
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Compare results to data JETRAD with other pdfs
Changing ? from 0.5 ETmax to 0.25 ETmax changes
the normalization by 25.
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Compare CDF and D0 results for CTEQ4M (D0
examines ? lt 1 with no requirement on cos?)
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• Systematic uncertainties on the dijet mass cross
  section (17-34, asymmetric
  ET-dependent)
• Absolute energy scale (14-31)
• Calorimeter calibration 1.3-1.8 over the ET
  range
• Jet fragmentation model 1.2-1.7 over the ET
  range
• Calorimeter stability 1 of E
• Energy of the underlying event 1 GeV
• Unsmearing
• Parameterization of the resolution function 1-9
  depending on Mjj
• Variation between analytic and MC procedure 4
• Detector simulator energy scale 2-8

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• Relative jet energy scale (5-9 depending on Mjj
  and considering all instrumented regions)
• Other uncertainties
• luminosity 4.1
• prescale factors 1.7-3.5 depending on trigger
  used.
• zvertex cut efficiency 1
• trigger efficiency lt 1 depending on the
  statistics of the turn-on region of the trigger.

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The Dijet Differential Cross Section

• The rapidity dependence of the cross section
  probes the parton momentum fractions.
• The analysis
• For luminosity (86.0 4.1) pb-1
• Trigger on jet-like events select events with ?
  2 jets
• Apply data quality cuts

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• Order the jets by ET. Define
• The leading jet with highest ET. Require that
  it has 0.1 lt ?1 lt 0.7 and ET1 gt 40 GeV.
• The probe jet with second highest ET. Require
  that it has ET2 gt 10 GeV.
• Correct jet energies for calorimeter effects
  require ET1 gt 35 GeV.
• Classify events according to probe jet ?, ?2
• 0.1 lt ?2 lt 0.7
• 0.7 lt ?2 lt 1.4
• 1.4 lt ?2 lt 2.1
• 2.1 lt ?2 lt 3.0

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• Correct (unsmear) measured ?
• Correct for trigger efficiency, prescale, and
  vertex-finding efficiency
• For events in each of the 4 ?2 classes, calculate
  the cross section
• N number of events, corrected for prescale
• L luminosity
• ET1 bins are consistent with detector resolution
• Compare to JETRAD for 3 pdfs ?
  ETmax/2

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• Sources of systematic errors on the dijet
  differential cross section
• Same as for inclusive cross section ?
  resolution

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Probing the high-x, high-Q2 regime Notice that
for a two-body process, and so these data
examine a range in (x,Q2) including that where an
excess was observed at HERA
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