Describing minimumbias and the underlying event at the LHC

1
Describing minimum-bias and the underlying event
at the LHC

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

Physics at TeV Colliders
Les Houches, 26th May 6th June 2003
2
Outline

• Describing soft interactions multiple parton
  scattering MC model Pythia6.2 .
• Minimum bias events.
• Properties of the underlying event (CDF style
  analysis).
• LHC predictions.
• Measurements at ATLAS.
• Comments and conclusion.

check QCD/SM LH 2001 Report
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
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
d
?d ?probability of hard-scattering (and
vice-versa)
5
Why multiple parton scattering?

• Violation of KNO scaling

6

• Double parton scattering measurement (CDF)

-

Phys. Rev. D 47 (11) 4857 (1993) Phys. Rev.
Lett. 79 (4) 584 (1997) Phys. Rev. D 56 (7) 3811
(1997).

• Photoproduction events at HERA

Z. Phys. C72 (1996) 637.
7
Minimum bias events
Central diffraction (1)

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

stot 102 - 118 mb
sNSD 65 - 73mb
(PYTHIA)
(PHOJET)
(PYTHIA)
(PHOJET)

• Minimum bias events are dominated by soft
  interactions, although there is also some
  contribution from hard scattering.

• constitutes unavoidable background
• prediction of radiation levels
• detector damage
• occupancy, etc.

ATL-COM-PHYS-2002-017
8
Modelling multiple parton scattering with PYTHIA

• There are two options for multiple parton
  scattering
• simple scenario MSTP(82)1 (abrupt cut at
  ptmin)
• complex scenario MSTP(82)2, 3 or 4 (smooth
  transition between high and low-pt regions)

Different matter distributions!
9
Pt cut
PARP(90)
PARP(82)

n sint
?pt0 ?n (and vice-versa)
10
Core size
PARP(84) a1/ a2

Effect on high multiplicity events.
11
Underlying event (UE) in charged jet evolution

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

• It is not only minimum bias event!
• The underlying event is everything except the
  two outgoing hard scattered jets.

Df f - fljet

• 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).

Phys. Rev. D, 65 092002 (2002)
12
lt Nchg gt in 1GeV/c bin
PYTHIA6.2 - no multiple scattering!
Considering single parton-parton scattering, the
event multiplicity is underestimated!
Pt (leading jet in GeV)
? PYTHIA6.2 - with multiple scattering!
lt Nchg gt in 1GeV/c bin
The data is best described when multiple
scattering is included
Pt (leading jet in GeV)
13
PYTHIA6.2 complex scenario
Regulating the event activity in the underlying
event
Transverse lt Nchg gt

• Pt cut ? rate of parton-parton interactions

n sint
Pt (leading jet in GeV)
PYTHIA6.2 complex scenario

• Core size ? hard component to the underlying
  event.

Transverse lt Nchg gt
Small core ? very high-pt scattering when there
is core-core overlap. Bigger core ? greater
probability of core-core overlap, but greater
chace of softer scatterings.
Pt (leading jet in GeV)
14
Minimum bias distributions
Tuning PYTHIA6.214 to minimum-bias and the
underlying event
15
(No Transcript)
16
(No Transcript)
17
Underlying event (UE)
PYTHIA6.2 14 tuned
PYTHIA6.2 14 default ?
lt Nchg gt in 1GeV/c bin
Pt (leading jet in GeV)
PYTHIA6.2 14 tuned
PYTHIA6.2 14 default ?
Transverse lt Nchg gt
Pt (leading jet in GeV)
18
PYTHIA6.2 14 tuned
lt Pt sumgt (GeV)
PYTHIA6.2 14 default ?
Df f - fljet
PYTHIA6.2 14 tuned
PYTHIA6.2 14 default ?
Transverse lt Pt sumgt (GeV)
Pt (leading jet in GeV)
19
LHC predictions
n. diff. d.diff.
CTEQ 5L
Double Gaussian
PYTHIA6.2 14 tuned
LHC prediction
PYTHIA6.2 14 default ?
Transverse lt Nchg gt
Primary vertex
Exclude 8 of chd. tracks
D PYTHIAs default
Pt (leading jet in GeV)
20

• Measurements at ATLAS
• Reasonable statistical sample 107 events ( 3
  days at 100Hz)
• (low luminosity)

Random trigger
PARP(90) 0.08
PARP(90) 0.16 (D)
PARP(90) 0.32
? UA5 53, 200, 546 and 900 GeV CDF 630
and 1800 GeV
Energy dependence
Pt dependence
Modeling beam remnants energy dependence
Protons matter distribution
First direct measurement
21
while we are waiting for the LHC

• D0 and CDF data
• max/min distributions for the transverse region
• underlying event analysis for 630 GeV and 1.8
  TeV (CDF)
• multi-jets cross section vs. Et

• Herwig JIMMY (multiple interactions)

• Underlying events and physics analysis

22
Conclusions

• Multiple parton interactions give a natural way
  of explaining the event activity for both minimum
  bias and the underlying event.

• We have tuned PYTHIA6.2 to both minimum bias and
  the UE data.

• Extrapolation to LHC energies uncertainties due
  to energy extrapolation
• (e.g. comparing PYTHIA and PHOJET ?stot15,
  ?sn.diff20, ?nchg15, ??20.... see
• ATL-COM-PHYS-2002-017)

• ATLAS can provide important measurements which
  will improve our understanding of soft
  interactions.

• JetWeb minimum bias (UA5) and UE event (CDF)
  data available.
• http//jetweb.hep.ucl.ac.uk/

• Updated results www.cern.ch/amoraes