R

1
Inclusive Jet Production using the kT Algorithm
at CDF

• Régis Lefèvre
• Analysis done with Mario Martínez and Olga
  Norniella
• IFAE Barcelona

IMFP2006 XXXIV International Meeting on
Fundamental Physics April 2nd-7th 2006, Madrid,
Spain
2
The Tevatron in Run II

• Proton-antiproton collisions
• ?s 1.96 TeV
• 36 bunches crossing time 396 ns
• Peak luminosity 1.2 ?1032 cm-2 s-1

• Collecting 20 pb-1 / week
• About 1.6 fb-1 delivered

3
CDF

• Highly upgraded for Run II
• New silicon tracking
• New drift chamber
• Upgraded muon chambers
• New plug calorimeters
• New TOF

• Data taking efficiency 85
• About 1.3 fb-1 on tape
• New results based on 1 fb-1

4
Motivations

• Legacy from Run I
• Great interest on apparent excess at high ET
• SM explanation
• Gluon PDF increased at high x
• New PDFs from global fit include CDF and D0 jet
  data from Run I (CTEQ6, MRST2001)

• Stringent test of pQCD
• Over 8 order of magnitudes
• Tail sensitive to New Physics
• Probing distances 10-19 m
• PDFs at high Q2 high x
• Production enhanced at high pTthanks to new ?s

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Cone Jet Algorithms and pQCD

• Infrared and Collinear Safety
• Fixed order pQCD contains not fully cancelled
  infrared divergences
• Inclusive jet cross section affected at NNLO
• Run I Cone Algorithm JetClu
• Neither infrared nor collinear safe
• Run II Cone Algorithm Midpoint
• Uses midpoints between pairs of proto-jets as
  additional seeds
• ? Infrared and collinear safety restored
• Merging/Splitting
• NLO pQCD uses larger cone radius R R ?
  RSEPto emulate experimental merging/splitting
• Arbitrary parameter RSEP prescription RSEP
  1.3 (based on parton level approximate arguments)

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The kT Algorithm

• Inclusive kT algorithm
• Merging pairs of nearby particles in order of
  increasing relative pT
• D parameter controls merging termination and
  characterizes size of resulting jets
• pT classification inspired by pQCD gluon
  emissions
• Infrared and Collinear safeto all orders in pQCD
• No merging/splitting
• No RSEP issue comparing to pQCD

• Successfully used at LEP and HERA
• Relatively new in hadron-hadron collider
• More difficult environment
• Underlying Event
• ? Multiple Interactions per crossing (MI)

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Results from ZEUS / D0 Run I
D0 Run I

• Disagreement at low pT
• Suggests Underlying Event not properly
  accounted for

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Framework / Related Topics

• Look first at central jets 0.1 lt y lt 0.7
• Where calorimeter simulation is best
• Use D 0.5, 0.7 and 1.0
• To make sure that Underlying Event and MI
  contributions are well under control
• Data fully corrected to particle level
• Requires a good simulation of the detector
• Monte-Carlo generator should be able to reproduce
  the Jet Shapes
• Jet fragmentation and parton cascades
• NLO pQCD corrected to hadron level
• Parton level pQCD calculation correctedfor the
  Underlying Event and Hadronization
• Requires a Monte-Carlo generator able to
  reproduce the Underlying Event

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Underlying Event

• Everything but the hard scattering process
• Initial state soft radiations
• Beam-beam remnants
• Multiple Parton Interactions (MPI)
• Studied in the transverse region
• Leading jet sample
• Back-to-back sample

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Energy Flow Inside Jets

• Jet shapes governed by multi-gluon emission
  from primary parton
• Test of parton shower models
• Sensitive to underlying event structure
• Sensitive to quark and gluon mixture in the
  final state

Phys. Rev. D 71 112002 (2005)
(1-?)
37 lt pT lt 380 GeV/c
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Calorimeter Response to Jets

• (First set electromagnetic scale using Z ? ee-)
• Absolute jet energy scale
• E/p of isolated tracks used to tune the showering
  simulation (G-Flash)
• Residual discrepancies taken as systematic errors
• Induced uncertainty on jet energy scale between 1
  and 3
• Reasonable simulation of the pT spectrum of the
  particles within a jet by PYTHIA and HERWIG
  fragmentation models (fundamental as
  non-compensated calorimeters)
• Induced difference on jet energy scale lt 1
• Photon-jet balance
• Data and Simulation agree at 1 to 2 level
• Non uniformity versus ?
• Dijet balance
• Relative response known to 0.5 level
• Resolution
• Bisector method
• Jet energy resolutions known within relative
  uncertainties of few

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Theoretical Predictions
PDF uncertainty

• NLO pQCD JETRAD
• Scale ?R ?F max (PTJET) / 2
• PDFs CTEQ6.1M package
• Main uncertainty comes from PDFs
• Gluon PDF at high x
• CHAD parton-to-hadron correction factor
• Accounts for non perturbative contributions
• Underlying Event (U.E.)
• Hadronization
• PYTHIA-Tune A used as nominal
• HERWIG used for uncertainty

D0.7 and 0.1 lt y lt 0.7
CHAD
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Published Results
D 0.7 0.1 lt y lt 0.7
Phys. Rev. Lett. 96 122001 (2006)
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kT Jets vs. D (0.1 lt y lt 0.7)
D 0.5
D 1.0
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Forward Jets

• Essentials to pin down PDFs vs. eventual New
  Physicsat higher Q2 in central region
• DGLAP gives Q2 evolution
• Expend x range toward low x

High-x
Low-x
Rutherford type parton backscattering
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New Results

• D 0.7
• 5 rapidity regions up to y 2.1
• y lt 0.1
• 0.1 lt y lt 0.7
• 0.7 lt y lt 1.1
• 1.1 lt y lt 1.6
• 1.6 lt y lt 2.1
• ? L 1 fb-1

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Data / Theory
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Conclusion

• Good agreement with NLO
• Stringent test of pQCD over 8 orders of
  magnitude
• pT reach extended by 150 GeV/c with respect to
  Run I
• Careful treatment of non perturbative effects
• Underlying Event well under control
• To be used in future PDF global fits in orderto
  better constrain the gluon PDF at high x
• Forward jets essentials
• Prospect
• cos ? vs. dijet invariant mass
• Limit on contact interactions

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Backup Slides
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kT Algorithm Step-by-Step
Longitudinally invariant kT algorithm
(Ellis-Soper inclusive mode)
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Bisector Method
0.1 lt y lt 0.7

• ?// ? ISR
• ?PERP ? ISR ? Detector Resolution
• Assuming ISR democratic in ?
• ?D ? (?2PERP - ?2//) / ? 2 ? Detector
  Resolution