Arthur M' Moraes

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

• Arthur M. Moraes
• University of Sheffield
• HEP - ATLAS

University of Glasgow
Glasgow, 20th February 2004
2
Outline

• Describing minimum bias and the underlying
  event.
• Comparison of different PYTHIA tunings.
• PYTHIA vs PHOJET.
• LHC predictions.
• Activation levels in the SCT-barrel system
• 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)
5
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
6
(No Transcript)
7
F(z) ltnchg gt P(nchg)
z nchg /ltnchg gt
High-multiplicity events are described
differently by each tuning
8
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)
9
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.

10
PYTHIA vs PHOJET
F(z) ltnchg gt P(nchg)
z nchg /ltnchg gt
11
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.

12
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!

13
LHC predictions pp collisions at vs 14 TeV
14
Comments
ATLAS DC

• PYTHIA6.214 tuned is going to be used for
  ATLAS DC2 production. It is also being used as
  a reference by CMS and ALICE Collaborations.

• Minimum bias DC samples will be used to validate
  ATLAS Software.

• Reconstructed minimum bias DC data-samples will
  also be used to develop strategies aimed at
  measuring minimum bias events during the ATLAS
  Commissioning Stage.

Model Improvements GRID resources

• Model improvements are being investigated in
  collaboration with T. Sjöstrand (PYTHIA6.3 new
  MPI model) and R. Field (CDF run II data).

• GridPP resources are already being used to
  generate minimum bias and underlying event
  samples.

15
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)
16
Activation of SCT barrel-modules at ATLAS

• The harsh radiation environment at ATLAS will
  cause the radioactivation of detector components.

• There can be severe implications for any access
  or maintenance operations

• Our predictions are based on both calculations
  and measurements.

• Consider the effects from both neutron capture
  and spallation induced activation.

17
Estimating activity levels
irradiation (180d) and cooling times.
Produced radionuclides per pp event per module
Average number of pp events/s (4 x 108 s-1)

• 64Cu, 198Au and 110mAg are the dominant
  radionuclides from (n,?).
• Notice that 1 week after shut down, 110mAg will
  dominate!

• Concerning spallation induced activation 3H, 7B,
  22Na and 24Na are the dominant radionuclides.
  22Na will be the most important for dose analysis!

18
? and ß dose estimates
? (ß) dose rates in nSv/h for one SCT barrel
module (10 years irradiating)

• Our estimates indicate that individual
  SCT-barrel modules will fall into the
  slightly-radioactive category.

Maximum ? (ß) dose µSv/h (10 years irradiating)

• Dose rates from the SCT-barrel will be small
  when compared to other elements of the ATLAS
  experiment. It should not create any serious
  problems.

19
Conclusions and Outlook

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

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

• We have successfully tuned PYTHIA to both
  minimum bias and the underlying event
  distributions. The tuned model is now being used
  to generate LHC predictions.

• Our investigations on soft interactions can be
  extended to applications beyond the LHC
  experiment (Tevatron, HERA, ).

• The levels of radioactivation of SCT-barrel
  modules should not pose any serious problems
  after operation in high radiation environments.