1. The “Underlying Event” at CDF and CMS
CDF Run 2
CMS
Outline of Talk
Review the CDF Run2 analysis of the “underlying event” in “Leading Jet” and “Back-to-Back” events.
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
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

2. QCD Monte-Carlo Models: High Transverse Momentum Jets
“Hard Scattering” Component
“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).
Of course the outgoing colored partons fragment into hadron “jet” and inevitably “underlying event” observables receive contributions from initial and final-state radiation.
The “underlying event” is an unavoidable background to most collider observables and having good understand of it leads to more precise collider measurements!
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

3. The “Transverse” Regions as defined by the Leading Jet
Charged Particle Df Correlations pT > 0.5 GeV/c |h| < 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| < 2).
Define |Df| < 60o as “Toward”, 60o < -Df < 120o and 60o < Df < 120o as “Transverse 1” and “Transverse 2”, and |Df| > 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).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

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| < 2) or by the leading two jets (JetClu R = 0.7, |h| < 2). “Back-to-Back” events are selected to have at least two jets with Jet#1 and Jet#2 nearly “back-to-back” (Df12 > 150o) with almost equal transverse energies (ET(jet#2)/ET(jet#1) > 0.8) and with ET(jet#3) < 15 GeV.
Shows the Df dependence of the charged particle density, dNchg/dhdf, for charged particles in the range pT > 0.5 GeV/c and |h| < 1 relative to jet#1 (rotated to 270o) for 30 < ET(jet#1) < 70 GeV for “Leading Jet” and “Back-to-Back” events.
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

5. “Transverse” PTsum Density versus ET(jet#1)
“Leading Jet”
Hard Radiation!
“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 > 0.5 GeV/c, |h| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events.
Compares the (uncorrected) data with PYTHIA Tune A and HERWIG (without MPI) after CDFSIM.
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

6. The “Underlying Event” in High PT Jet Production (Run 2 vs LHC)
Charged particle density versus PT(jet#1)
The “Underlying Event”
“Underlying event” much more active at the LHC!
Tevatron Run 2
Average charged particle density in the “Transverse” region versus PT(jet#1) at 1.96 TeV for PY Tune AW and HERWIG (without MPI).
Average charged particle density in the “Transverse” region versus PT(jet#1) at 14 TeV for PY Tune AW and HERWIG (without MPI).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

7. Extrapolations to the LHC: High PT Jet Production
“Transverse” Charged particle density and charged PTsum density versus PT(jet#1)
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(jet#1) at 14 TeV for PY Tune AW, Tune DWT, ATLAS, and HERWIG (without MPI).
Average charged particle density in the “Transverse” region versus PT(jet#1) at 14 TeV for PY Tune AW, Tune DWT, ATLAS, and HERWIG (without MPI).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

8. The “Underlying Event” in High PT Charged Jet Production (LHC)
Charged densities versus PT(jet#1) and PT(charged jet#1)
The “Underlying Event”
You can study the “underlying event” with either charged particle jets or calorimeter jets!
The calorimeter jet belonging to the charged particle jet has more transverse momentum!
Average charged particle density in the “Transverse” region versus PT(jet#1) versus PT(charged jet#1) at 14 TeV for PY Tune DW.
Average charged PTsum density in the “Transverse” region versus PT(jet#1) versus PT(charged jet#1) at 14 TeV for PY Tune DW.
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

9. QCD Monte-Carlo Models: Lepton-Pair Production
“Hard Scattering” Component
“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.
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

10. The “Central” Region in Drell-Yan Production
Look at the charged particle density and the PTsum density in the “central” region!
Charged Particles (pT > 0.5 GeV/c, |h| < 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 > 0.5 GeV/c, |h| < 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.
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

11. Drell-Yan Production (Run2 vs LHC)
Lepton-Pair Transverse Momentum
<pT(m+m-)> is much larger at the LHC!
Shapes of the pT(m+m-) 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).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

12. The “Underlying Event” in Drell-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).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

13. Extrapolations to the LHC: Drell-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).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

14. Extrapolations to the LHC: Drell-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 (|h|<1.0, pT > 0.5 GeV/c)
Charged Particles (|h|<1.0, pT > 0.9 GeV/c) Z Z
Average charged particle density (pT > 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 > 0.9 GeV/c) versus the lepton-pair invariant mass at 14 TeV for PYTHIA Tune DW, Tune DWT, ATLAS and HERWIG (without MPI).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS

15. UE&MB@CMS UE&MB@CMS UE&MB@CMS
Livio
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
Florida-Perugia
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS
University of Perugia

16.
Study the “underlying event” by
using charged particles and muons!
(start during the pilot run)
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)
“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, <pT(pair)>, <pT2(pair)>, ds/dpT(pair) (only requires muons). Event structure for large lepton-pair pT (i.e. mm +jets, requires muons).
SE CMS Physics Workshop May 31, 2006
Rick Field – Florida/CDF/CMS