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Describing minimumbias and the underlying event at the LHC

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Defines two interaction regions in pt scale: hard and soft processes. ... In a hard scattering process, the underlying event has a hard component (initial ... – PowerPoint PPT presentation

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Title: 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
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16
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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
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