Models for minimum bias and the underlying event

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Models for minimum bias and the underlying event

• A. Moraes, C. Buttar and I. Dawson
• University of Sheffield

HERA and the LHC
CERN, 26th March 2004
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Underlying event in charged jet evolution (CDF
style analysis)
Minimum bias events

• Experimental definition depends on the
  experiment
• trigger!
• Minimum bias is usually associated to
• non-single-diffractive events (NSD),
• e.g. ISR, UA5, E735, CDF,

• It is not only minimum bias event!

• The underlying event is everything except the
  two outgoing hard scattered jets.

• In a hard scattering process, the underlying
  event has a hard component (initial final-state
  radiation and particles from the outgoing hard
  scattered partons) and a soft component
  (beam-beam remnants).

• CDF analysis
• charged particles
• ptgt0.5 GeV and ?lt1
• cone jet finder

Df f - fljet
stot 102 - 118 mb
sNSD 65 - 73mb
(PYTHIA)
(PHOJET)
(PYTHIA)
(PHOJET)
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Model for hadron-hadron collisions (Pythia)

• Defines two interaction regions in pt scale hard
  and soft processes.
• Uses perturbative QCD to describe high-pt
  parton-parton scattering.

• Attempts to extend perturbative (high-pt) picture
  down to low-pt region.
• Considers the possibility that multiple parton
  scattering takes place in hadron-hadron
  collisions.

n sint
?pt0 ?n (and vice-versa)
pt0

• Different options for hadronic matter
  distributions.

?d ?probability of hard-scattering (and
vice-versa)
d
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(No Transcript)
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Comparing different PYTHIA tunings to the data
dNchg/d? at ?0
F(z) ltnchg gt P(nchg)
LHC
z nchg /ltnchg gt
vs (GeV)
High-multiplicity events are described
differently by each tuning
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Comments
Central Region (data dNchg/d? 4)
Transverse lt Nchg gt

• ATLAS TDR it hasnt been tuned to UE data
  doesnt include double diffraction for
• minimum bias events uses a model with a small
  hadronic core size doesnt correct for particle
  decays which affect minimum bias distributions
  and doesnt include
• a pTmin energy dependence.

dNchg/d? 15

• CDF tuning it has been tuned particularly to UE
  data doesnt correct for particle decays which
  affect minimum bias distributions.

dNchg/d? 10
Ratio (MC/Data)
Pt (leading jet in GeV)
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PHOJET

• Developed mainly for soft and semi-hard particle
  production.

• Implements ideas of Dual Parton Model for low-pT
  processes.

• Multiple Pomeron exchanges (sea-quark
  multi-chains) enhances the event activity.

• Limited to production mechanisms of strong
  interactions.

• However, useful tool for MB and UE studies where
  jets are involved.

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PYTHIA vs PHOJET
F(z) ltnchg gt P(nchg)
z nchg /ltnchg gt
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PYTHIA vs PHOJET
dNchg/d? at ?0
Transverse lt Nchg gt
LHC
Pt (leading jet in GeV)
vs (GeV)

• PYTHIA models favour ln2(s)
• PHOJET suggests a ln(s) dependence.

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Combined ?2 (not the minimum ?2 !)
Multiplicity information nch, dN/d?, KNO
Transverse region (UE) nch and pTsum

• PHOJET1.12 and PYTHIA6.214 tuned give the best
  global agreement to the data!

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LHC predictions PYTHIA6.214 tuned vs. CDF
tuning
Central Region (min-bias dNchg/d? 7)
Transverse lt Nchg gt
LHC
dNchg/d? 30
x 3
x 2
dNchg/d? 20
Tevatron
Pt (leading jet in GeV)
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LHC predictions PYTHIA6.214 tuned vs.
PHOJET1.12
Central Region (min-bias dNchg/d? 7)
LHC
Transverse lt Nchg gt
dNchg/d? 30
x 3
dNchg/d? 15
x1.5
Tevatron
Pt (leading jet in GeV)
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LHC predictions pp collisions at vs 14 TeV
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Conclusions

• Current minimum bias and underlying event data
  for pp and pp can be described with appropriate
  tunings for PYTHIA and PHOJET.

• PYTHIA6.214 tuned and PHOJET1.12 with its
  default settings give the best global agreement
  to the data (?2/407 d.o.f 9.27 and 7.25,
  respectively).

• CDF tuning (tune A) is the best model
  describing UE at the Tevatron, however, fails to
  reproduce several minimum bias distributions at
  lower energies.

• There are sizeable uncertainties in LHC
  predictions generated by different models

• PYTHIA6.214 tuned and PHOJET1.12 generate LHC
  predictions with 30 difference for minimum
  bias event, and a factor of 2 for underlying
  event distributions.

• Updated results www.cern.ch/amoraes