A' Moraes, C' Buttar and I' Dawson

1
Describing minimum bias and the underlying event
at the LHC with ATLAS

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

ATLAS Physics Plenary
CERN, 26th February 2004
2
Outline

• Minimum bias.
• Underlying event (CDF style analysis).
• PYTHIA tunings for minimum bias and underlying
  event.
• PYTHIA vs PHOJET.
• LHC predictions.
• Comments and conclusions.

3
Introduction
p

• What do we expect from a proton-proton collision
  at a centre-of-mass energy of 14 TeV?

?
p

• There will be a combination of soft (most of
  the time) and hard (occasionally) interactions.

• Perturbative QCD is highly successful when
  applied to hard processes (large-pt) but cannot
  be applied to soft interactions (low-pt).

• Alternative approaches to describe soft processes
  are therefore required.

4
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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Minimum bias events Trigger simulation vs MC
direct selection
-

• all processes are switched on elastic
  inelastic
• use forward counter requirements to select events.

-
2.0 lt ? lt 5.6 (UA5)
-
3.2 lt ? lt 5.9 (CDF)
-
Switch NSD inelastic processes in the MC PYTHIA
ISUB 94 and 95 PHOJET IPRON 1, 4 and 7.

• Selecting NSD inelastic processes in the MC event
  generators, minimum bias events are reproduced
  reasonably well.

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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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Transverse lt Nchg gt
Pt (leading jet in GeV)
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F(z) ltnchg gt P(nchg)
F(z) ltnchg gt P(nchg)
z nchg /ltnchg gt
z nchg /ltnchg gt
High-multiplicity events are described
differently by each tuning
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Comments
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.

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

Ratio (MC/Data)
Pt (leading jet in GeV)
12
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
UE LHC predictions
PYTHIA6.214 - tuned
PHOJET1.12
Transverse lt Nchg gt
x 3
PYTHIA predicts significant increase in event
activity in the UE at the LHC. While
tuned-PYTHIA and PHOJET both agree well with CDF
data, their predictions at LHC energies differ by
more than a factor of 2.
x1.5
Pt (leading jet in GeV)
PYTHIA6.214 - tuned
LHC predictions differ by factor of two in
transverse region. Need to investigate
further energy dependence, Trans MAX/MIN,
PHOJET1.12
LHC
lt Nchg gt in 3.6 bins
Pt ljet gt 30 GeV/c
f - fljet
Tevatron
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LHC predictions pp collisions at vs 14 TeV
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VBF Signal (H?WW?l?l?)
The Underlying Event at the LHC
Prospects for the search for a standard model
Higgs boson in ATLAS using VBF, S.Asai et al,
SN-ATLAS-2003-024 -gt EPJ

• forward tagging jets
• correlated leptons
• low hadronic activity in central region
• central Higgs production

D. Gaussian PYTHIA6.214 tuned Default PYTHIA6.2
Pythia6.214
e-µ channel only! (MH 160 GeV)
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ATLAS Data Challenges tunings for pile-up events
Tuned to UE (CDF) data only.
Multiple interactions ON CTEQ 5L PARP(85)0.33
PARP(86)0.66 PARP(89)1 TeV PARP(90)0.16
Tuned to min-bias (ISR UA5 CDF) and UE (CDF)
data.

• PYTHIA6.214 tuned is also being used as a
  reference by CMS and ALICE .

20
Conclusions

• Current minimum bias and underlying event data
  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.

• 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.

• There is more activity (particle density) in the
  UE than predicted for an average minimum bias
  event! PYTHIA6.214 tuned predicts an increase
  (factor of 2) in this activity when extrapolating
  from Tevatron to LHC, whereas PHOJET1.12 suggests
  the ratio UE/Min-bias will remain the same.

• Updated results www.cern.ch/amoraes