Low-pT Multijet Cross Sections

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Low-pT Multijet Cross Sections

• John Krane
• Iowa State University

Part I Data vs MC, interpreted as
physics Part II Data vs MC, interpretedas a
tuning problem
MC Workshop Oct. 4 2002, Fermilab
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Motivation

• High-pT inclusive jet spectra appear to be well
  described by NLO QCD
• Possible exceptions include kT algorithm
  analysis, possibly also the ratio of cross
  sections at 630/1800 GeV, large-h dijets (BFKL).
  But only at 1 or 2 s,not actual disagreement
• Originally, this was a search for BFKL effects,
  which could produce extra jets in low-Q2 events

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The Analysis

• Jets with ET gt 20 GeV, usual jet and event
  cuts,efficiencies applied but no unsmearing
• Study inclusive samples of events having at
  least 1-jet, 2-jets, 3-jets, 4-jets
• Compare to normalized Pythia GEANT and Herwig
  GEANT, tune if necessary

The usual sample
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Data and Pythia
pointsdata, histoPythia

• Inclusive xsec looks fine
• Multijet xsecs exhibit deviations from Pythia
• Lets pretend its physics

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(D-T)/T

• Solid lines energy scale Ålum uncertainty
• Dash smearing uncertainty
• Dotted total error in ratio

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Data and Herwig
pointsdata, histoHerwig

• Started generating jets at 0.5 GeV
• Multijet xsecs exhibit similar deviations

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Vector sum pT
(Data and Pythia)

• Define
• The more jets in the event, the more imbalance
  in energy
• Could this be
• ISR, with pT
• lost down the
• Beampipe?

Events gt 250 are the excess in 3 jet events
gt150 in 4
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Angles in 3-jet events
(Data and Pythia)

• Find which jet is the third one by isolating
  the two jets with minimal SpT

Many back-to-backin the data
Usually, third jet is near one of the first two,
but more so in Pythia
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Angles in 3-jet events
(Data and Herwig)

• Find which jet is the third one by isolating
  the two jets with minimal SpT

Min SpT not bad
Third jet is often at 90 degrees, often composed
of underlying event E
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Early impressions of these results

• Signs point to initial state radiation effects in
  data
• DGLAP style?
• BFKL style?
• or a need to tune the MC

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Try tuning Pythia, also compare to Herwigsee
what works

• Herwig defaults also did poorly
• Many iterations required
• Only compared to distributions shown today (and a
  few other very similar ones)

Pythia works if PARP(83) 0.32 (from
0.5) Fraction of matterin the proton core
Herwig works if PTMIN 3.7 GeV pT generation
threshold
Does this changeunderlying eventin some
way? Both higher and lowervalues do worse!
A Multiplepartonscattering parameter
Didnt try Jimmy
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Data and Tuned MC

• Points Data
• Solid Pythia
• Dash Herwig

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(D-T)/T

• No remaining deviations from data
• Is this because there were no ISR effects?
• Answer lies in the validity of our tuning

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Vector sum pT

• Tuned MC reproduced the small shoulderin
  addition to the 3 and 4

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Angles with Pythia (and Jetrad)

• Points Data
• Solid Pythia
• (error bands and)
• Dash-dot Jetrad

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Angles with Herwig
Pythias CKIN(3) showsno such sensitivity
Cross section shapes verystrange if ptminlt3.7 GeV

• Points Data
• Solid Herwig

Peak (from underlying event jets) becomes
enormous if ptmingt3.7 GeV
Dot Herwig with cut on merged jets
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Conclusions

• Results not entirely satisfying
• Would like to make definitive statements about
  ISR
• or provide solid tuning suggestions
• Instead, we found sensitivity to several params.
• Think the multiple parton scattering is
  constrainedby other data, we provide a new
  handle
• We dont understand the Herwig tuning at all
• Our decision publish the data, leave tuning to
  experts with a more global view
• Tuning isnt really our forté
• If we do it, we probably want a second paper out
  of it!

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Backup Slides
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Cone Algorithm Details (Run I)

• Draw a cone around a seed
• Calc sum ET, and ET-weighted position
• Draw new cone here and recalculate sum ET,
  position
• Reiterate until stable

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Energy Scale
calorimeter jet

• Correction back to the particle level
• Remove noise, underlying event,extra pp
  interactions
• Correct for detector response
• Undo misassignment of particle energies to jets

p
K
particle jet
Time
parton jet