Jet Studies at CDF

1
Jet Studies at CDF

• Anwar Ahmad Bhatti
• The Rockefeller University
• CDF Collaboration
• DIS03 St. Petersburg Russia
• April 24,2003

• Inclusive Jet Cross Section
• Di-Jet Mass distribution
• Jet shape and energy Flow in the event

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Jet Cross Section Measurement

• Measure parton distribution functions at high
• Look for deviations from QCD predictions
• Backgrounds for various new physics signals
• A step towards more complicated analyses

3
Results from Run I

• In Run I, CDF found that the jet cross section
  is higher than prediction using PDF at that
  time (1996).
• A global fit by the CTEQ collaboration found
  that gluon distributions at high x are not
  constrained by other data. (Direct photon data is
  not precise enough, both due to theoretical and
  experimental uncertainties.)
• They introduced one more parameter.

CTEQ4M
Statistical Errors only
CTEQ4HJ
(Data-Theory)/Theory
MRST
Jet Transverse Energy (GeV)
10
100
The CTEQ6 set includes, D0 high ? (high x, low Q)
data. In this fit large gluon density at high x
is a natural choice.
CDF Run 1b
D0 Run I
ds/dpt (nb/GeV)
500
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Improvements

• TeV
  
• Better DAQ/ upgraded trigger, higher statistics.
• New plug calorimeter
• Better modeling of calorimeter at low Et and
  shower spreading (work
• in progress)

CTEQ 6.1 Run II/Run I 0.1lt ylt0.7
Run II/ Run I
Transverse Energy of Jet (GeV)
Theory predicts x2 higher cross section at 400
GeV x5 higher cross section at 600 GeV.
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Data Set (Feb 2002-Jan 2003)

• Luminosity
• Central Jet
• Event vertex
• cm
• Cleanup using missing
• and visual scan
• Four triggers, use data where trigger gt99
  efficient.

Events/ 10 GeV
Jet Transverse Energy (GeV)
Good match between triggers in overlap region
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Trigger Efficiency
Trigger Efficiency
Measure trigger efficiency using lower Et
threshold trigger
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A High Et Jet Event
GeV
GeV
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Jet Clustering and Jet Energy Corrections

• Iterative cone clustering with JetClu algorithm
  R0.7
• Correct calorimeter energy to particles energy
  within a cone radius R
• No out-of-cone corrections
• Calorimeter scale set to Run I scale based on
  photon jet balancing results.
• corrections to raw cal
  energy.
• Correct for
• underlying event /multiple interactions
• calorimeter non-linearity
• smearing due to resolution.

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Comparison with NLO QCDCTEQ6.1 PDFs
Cross Section Ratio Data/ CTEQ6.1
Transverse Energy of the jet (GeV)
Reasonable agreement within large uncertainties
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Comparison with Run I

• Higher due to higher
• 1.8 TeV 1.96 TeV
• Systematic errors mostly cancel but RunII jet
  energy scale uncertainty is dominant.
• Reasonable agreement
• but more work needed to
• understand the details.

Cross Section Ratio
Jet Transverse Energy (GeV)
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Systematic Uncertainties

• Response
• (Test beam and data)
• Raw Energy Scale
• Jet Fragmentation (measured from CDF data)
• Jet Energy Resolution
• Underlying Event Energy
• Luminosity

Percent uncertainty in cross section
Systematic uncertainty dominated by energy scale
of calorimeter in Run II.
Transverse Energy of Jet (GeV)
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Jet Cross Section at large pseudorapidity
Raw Cross Section

• Determine high x, low PDFs from CDF data

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DiJet Mass Spectrum

A good place to look for new physics Antoni
Munars talk April 25, 255 pm EW and
Physics Beyond SM Session
Run II extend the range by 300 GeV due to
higher cross section at vs 1.96 TeV
Mass (corrected) 1364 GeV
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Jet Shape and Energy Flow in an Event

• Internal structure of jet
• Test pQCD/ parton shower models
• Hadronization/fragmentation, essential for jet
  energy determination
• Compare with Herwig/ Pythia
• Previous (PRL70, 1993) measurement, good
  agreement with pQCD calculations(
  ).

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Energy Distribution within a Jet (differential)
Herwig after detector simulation
Pythia after detector simulation
r/R
CDF II Preliminary
Good agreements with Herwig and Pythia in
central region Slightly wider jets in forward
region at low
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Energy Distribution within Jet
?(r0.4)/?(r0.7)
Jet Transverse Energy (GeV)
Jets become narrower as their Et
increases. Smaller fraction of energy in R0.4
as ? of the jet increases.
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Energy Flow in an event

Detector Level

• Reconstruct jet using JetClu.
• Define
• Measure transverse energy along f direction
  within ?? for various separations between two
  leading jets.
• Compare with Herwig prediction
• after detector simulation.

CDFII Preliminary
Good agreement between data and Herwig (Parton
Shower Underlying Event)
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Conclusions

• The Run II inclusive jet cross section extends
  to jet GeV.
• The cross section is consistent with NLO QCD
  predictions
• The dijet mass spectrum extends to
  GeV.
• The energy distribution within a jet measured
  for GeV.
• The jet shape and energy flow in event is well
  modeled by Herwig Monte Carlo and Pythia Monte
  Carlo.
• We are working on
• Angular Distributions
• Inclusive jet cross section to higher ?
• Jet Cross section using MidPoint and kt
  clustering
• b-jet cross section
• W/Z Jet cross sections
• Photon Production
• Many and more accurate results in near future.