The

1
The Underlying Event at CDF and CMS
CMS
CDF Run 2
Outline of Talk

• Review the CDF Run2 analysis of the underlying
  event in Leading Jet and Back-to-Back
  events.

UEMB_at_CMS

• Discuss using Drell-Yan muon-pair production to
  study the underlying event at CDF and CMS.

• Show some extrapolations from CDF ?CMS.

• Discuss the people and the plan.

CMS NOTE-2006/067
2
QCD Monte-Carlo ModelsHigh Transverse Momentum
Jets
Underlying Event

• Start with the perturbative 2-to-2 (or sometimes
  2-to-3) parton-parton scattering and add initial
  and final-state gluon radiation (in the leading
  log approximation or modified leading log
  approximation).

• The underlying event consists of the beam-beam
  remnants and from particles arising from soft or
  semi-soft multiple parton interactions (MPI).

The underlying event is an unavoidable
background to most collider observables and
having good understand of it leads to more
precise collider measurements!

• Of course the outgoing colored partons fragment
  into hadron jet and inevitably underlying
  event observables receive contributions from
  initial and final-state radiation.

3
The Transverse Regionsas defined by the
Leading Jet
Charged Particle Df Correlations pT gt 0.5 GeV/c
h lt 1
Look at the charged particle density in the
transverse region!
Transverse region is very sensitive to the
underlying event!

• Look at charged particle correlations in the
  azimuthal angle Df relative to the leading
  calorimeter jet (JetClu R 0.7, h lt 2).
• Define Df lt 60o as Toward, 60o lt -Df lt 120o
  and 60o lt Df lt 120o as Transverse 1 and
  Transverse 2, and Df gt 120o as Away. Each
  of the two transverse regions have area DhDf
  2x60o 4p/6. The overall transverse region is
  the sum of the two transverse regions (DhDf
  2x120o 4p/3).

4
Charged Particle Density Df Dependence
Refer to this as a Leading Jet event
Subset
Refer to this as a Back-to-Back event

• Look at the transverse region as defined by the
  leading jet (JetClu R 0.7, h lt 2) or by the
  leading two jets (JetClu R 0.7, h lt 2).
  Back-to-Back events are selected to have at
  least two jets with Jet1 and Jet2 nearly
  back-to-back (Df12 gt 150o) with almost equal
  transverse energies (ET(jet2)/ET(jet1) gt 0.8)
  and with ET(jet3) lt 15 GeV.

• Shows the Df dependence of the charged particle
  density, dNchg/dhdf, for charged particles in the
  range pT gt 0.5 GeV/c and h lt 1 relative to
  jet1 (rotated to 270o) for 30 lt ET(jet1) lt 70
  GeV for Leading Jet and Back-to-Back events.

5
Transverse PTsum Density versus ET(jet1)
Leading Jet
Back-to-Back
Min-Bias 0.24 GeV/c per unit h-f

• Shows the average charged PTsum density,
  dPTsum/dhdf, in the transverse region (pT gt 0.5
  GeV/c, h lt 1) versus ET(jet1) for Leading
  Jet and Back-to-Back events.

• Compares the (uncorrected) data with PYTHIA Tune
  A and HERWIG (without MPI) after CDFSIM.

6
The Underlying Event inHigh PT Jet Production
(Run 2 vs LHC)
Charged particle density versus PT(jet1)
The Underlying Event
Underlying event much more active at the LHC!

• Average charged particle density in the
  Transverse region versus PT(jet1) at 1.96 TeV
  for PY Tune AW and HERWIG (without MPI).

• Average charged particle density in the
  Transverse region versus PT(jet1) at 14 TeV
  for PY Tune AW and HERWIG (without MPI).

7
Extrapolations to the LHCHigh PT Jet Production
Transverse Charged particle density and charged
PTsum density versus PT(jet1)
The Underlying Event
Tune DW and DWT are identical at 1.96 TeV, but
have different MPI energy dependence!
The ATLAS tune has a much softer distribution
of charged particles than the CDF Run 2 Tunes!

• Average charged particle density in the
  Transverse region versus PT(jet1) at 14 TeV
  for PY Tune AW, Tune DWT, ATLAS, and HERWIG
  (without MPI).

• Average charged particle density in the
  Transverse region versus PT(jet1) at 14 TeV
  for PY Tune AW, Tune DWT, ATLAS, and HERWIG
  (without MPI).

8
The Underlying Event inHigh PT Charged Jet
Production (LHC)
Charged densities versus PT(jet1) and PT(charged
jet1)
The Underlying Event
The calorimeter jet belonging to the charged
particle jet has more transverse momentum!
You can study the underlying event with either
charged particle jets or calorimeter jets!

• Average charged particle density in the
  Transverse region versus PT(jet1) versus
  PT(charged jet1) at 14 TeV for PY Tune DW.

• Average charged PTsum density in the Transverse
  region versus PT(jet1) versus PT(charged jet1)
  at 14 TeV for PY Tune DW.

9
QCD Monte-Carlo ModelsLepton-Pair Production
Underlying Event

• Start with the perturbative Drell-Yan muon pair
  production and add initial-state gluon radiation
  (in the leading log approximation or modified
  leading log approximation).

• The underlying event consists of the beam-beam
  remnants and from particles arising from soft or
  semi-soft multiple parton interactions (MPI).

• Of course the outgoing colored partons fragment
  into hadron jet and inevitably underlying
  event observables receive contributions from
  initial and final-state radiation.

10
The Central Regionin Drell-Yan Production
Look at the charged particle density and the
PTsum density in the central region!
Charged Particles (pT gt 0.5 GeV/c, h lt 1)
After removing the lepton-pair everything else is
the underlying event!

• Look at the central region after removing the
  lepton-pair.
• Study the charged particles (pT gt 0.5 GeV/c, h
  lt 1) and form the charged particle density,
  dNchg/dhdf, and the charged scalar pT sum
  density, dPTsum/dhdf, by dividing by the area in
  h-f space.

11
Drell-Yan Production (Run2 vs LHC)
Lepton-Pair Transverse Momentum
ltpT(mm-)gt is much larger at the LHC!
Shapes of the pT(mm-) distribution at the
Z-boson mass.
Z

• Average Lepton-Pair transverse momentum at the
  Tevatron and the LHC for PYTHIA Tune DW and
  HERWIG (without MPI).

• Shape of the Lepton-Pair pT distribution at the
  Z-boson mass at the Tevatron and the LHC for
  PYTHIA Tune DW and HERWIG (without MPI).

12
The Underlying Event inDrell-Yan Production
(Run2 vs LHC)
Charged particle density versus M(pair)
The Underlying Event
HERWIG (without MPI) is much less active than PY
Tune AW (with MPI)!
Underlying event much more active at the LHC!
Z
Z

• Charged particle density versus the lepton-pair
  invariant mass at 1.96 TeV for PYTHIA Tune AW and
  HERWIG (without MPI).

• Charged particle density versus the lepton-pair
  invariant mass at 14 TeV for PYTHIA Tune AW and
  HERWIG (without MPI).

13
Extrapolations to the LHCDrell-Yan Production
Charged particle density and charged PTsum
density versus M(pair)
The Underlying Event
Tune DW and DWT are identical at 1.96 TeV, but
have different MPI energy dependence!
The ATLAS tune has a much softer distribution
of charged particles than the CDF Run 2 Tunes!
Z
Z

• Average charged particle density versus the
  lepton-pair invariant mass at 14 TeV for PYTHIA
  Tune DW, Tune DWT, ATLAS and HERWIG (without
  MPI).

• Average charged PTsum density versus the
  lepton-pair invariant mass at 14 TeV for PYTHIA
  Tune DW, Tune DWT, ATLAS and HERWIG (without
  MPI).

14
Extrapolations to the LHCDrell-Yan Production
Charged particle density versus M(pair)
The Underlying Event
The ATLAS tune has a much softer distribution
of charged particles than the CDF Run 2 Tunes!
Charged Particles (hlt1.0, pT gt 0.5 GeV/c)
Charged Particles (hlt1.0, pT gt 0.9 GeV/c)
Z
Z

• Average charged particle density (pT gt 0.5 GeV/c)
  versus the lepton-pair invariant mass at 14 TeV
  for PYTHIA Tune DW, Tune DWT, ATLAS and HERWIG
  (without MPI).

• Average charged particle density (pT gt 0.9 GeV/c)
  versus the lepton-pair invariant mass at 14 TeV
  for PYTHIA Tune DW, Tune DWT, ATLAS and HERWIG
  (without MPI).

15
UEMB_at_CMS
Livio
UEMB_at_CMS Rick Field (Florida) Darin Acosta
(Florida) Paolo Bartalini (Florida) Albert De
Roeck (CERN) Livio Fano' (INFN/Perugia at
CERN) Filippo Ambroglini (INFN/Perugia at
CERN) Khristian Kotov (UF Student, Acosta)
Filippo
Paolo

• Measure Min-Bias and the Underlying Event at CMS

• The plan involves two phases.
• Phase 1 would be to measure min-bias and the
  underlying event as soon as possible (when the
  luminosity is low), perhaps during commissioning.
  We would then tune the QCD Monte-Carlo models
  for all the other CMS analyses. Phase 1 would be
  a service to the rest of the collaboration. As
  the measurements become more reliable we would
  re-tune the QCD Monte-Carlo models if necessary
  and begin Phase 2.
• Phase 2 is physics and would include comparing
  the min-bias and underlying event measurements
  at the LHC with the measurements we have done
  (and are doing now) at CDF and then writing a
  physics publication.

Perugia, Italy, March 2006
UEMB_at_CMS
Florida-Perugia
University of Perugia
16
UEMB_at_CMS
UEMB_at_CMS

• Min-Bias Studies Charged particle distributions
  and correlations. Construct charged particle
  jets and look at mini-jet structure and the
  onset of the underlying event. (requires only
  charged tracks)

Study the underlying event by using charged
particles and muons! (start during the pilot run)

• Underlying Event Studies The transverse
  region in leading Jet and back-to-back
  charged particle jet production and the central
  region in Drell-Yan production. (requires
  charged tracks and muons for Drell-Yan)

• Drell-Yan Studies Transverse momentum
  distribution of the lepton-pair versus the mass
  of the lepton-pair, ltpT(pair)gt, ltpT2(pair)gt,
  ds/dpT(pair) (only requires muons). Event
  structure for large lepton-pair pT (i.e. mm
  jets, requires muons).