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The “Underlying Event” at CDF and CMS

The “Underlying Event” at CDF and CMS. Review the CDF Run2 analysis of the “underlying event” in “Leading Jet” and “Back-to-Back” events. CDF Run 2. CMS. Outline of Talk. UE&MB@CMS. Discuss using Drell-Yan muon-pair production to study the “underlying event” at CDF and CMS.

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The “Underlying Event” at CDF and CMS

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  1. The “Underlying Event” at CDF and CMS • Review the CDF Run2 analysis of the “underlying event” in “Leading Jet” and “Back-to-Back” events. CDF Run 2 CMS Outline of Talk UE&MB@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 Rick Field – Florida/CDF/CMS

  2. “Hard Scattering” Component QCD Monte-Carlo Models:High Transverse Momentum Jets • 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). “Underlying Event” • 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. Rick Field – Florida/CDF/CMS

  3. The “Transverse” Regionsas defined by the Leading Jet • 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). 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”! Rick Field – Florida/CDF/CMS

  4. Charged Particle Density Df Dependence • 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. Refer to this as a “Leading Jet” event Subset Refer to this as a “Back-to-Back” event • 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. Rick Field – Florida/CDF/CMS

  5. Hard Radiation! “Transverse” PTsum Density versus ET(jet#1) “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 > 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. Rick Field – Florida/CDF/CMS

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

  8. The “Underlying Event” inHigh PT Charged Jet Production (LHC) • Average charged particle density in the “Transverse” region versus PT(jet#1) versus PT(charged jet#1) at 14 TeV for PY Tune DW. Charged densities versus PT(jet#1) and PT(charged jet#1) 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 PTsum density in the “Transverse” region versus PT(jet#1) versus PT(charged jet#1) at 14 TeV for PY Tune DW. Rick Field – Florida/CDF/CMS

  9. “Hard Scattering” Component QCD Monte-Carlo Models:Lepton-Pair Production • 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). “Underlying Event” • 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. Rick Field – Florida/CDF/CMS

  10. The “Central” Regionin Drell-Yan Production Look at the charged particle density and the PTsum density in the “central” region! • 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. Charged Particles(pT > 0.5 GeV/c, |h| < 1) After removing the lepton-pair everything else is the “underlying event”! Rick Field – Florida/CDF/CMS

  11. Drell-Yan Production (Run2 vs LHC) • Average Lepton-Pair transverse momentum at the Tevatron and the LHC for PYTHIA Tune DW and HERWIG (without MPI). 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 • 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). Rick Field – Florida/CDF/CMS

  12. The “Underlying Event” inDrell-Yan Production (Run2 vs LHC) • 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 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 14 TeV for PYTHIA Tune AW and HERWIG (without MPI). Rick Field – Florida/CDF/CMS

  13. Extrapolations to the LHC:Drell-Yan Production Charged particle density and charged PTsum density versus M(pair) • Average charged particle density versus the lepton-pair invariant mass at 14 TeV for PYTHIA Tune DW, Tune DWT, ATLAS and HERWIG (without MPI). 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 PTsum density versus the lepton-pair invariant mass at 14 TeV for PYTHIA Tune DW, Tune DWT, ATLAS and HERWIG (without MPI). Rick Field – Florida/CDF/CMS

  14. Extrapolations to the LHC:Drell-Yan Production Charged particle density versus M(pair) • 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). 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 > 0.9 GeV/c) versus the lepton-pair invariant mass at 14 TeV for PYTHIA Tune DW, Tune DWT, ATLAS and HERWIG (without MPI). Rick Field – Florida/CDF/CMS

  15. UE&MB@CMS Livio UE&MB@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 UE&MB@CMS Florida-Perugia University of Perugia Rick Field – Florida/CDF/CMS

  16. UE&MB@CMS UE&MB@CMS • “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 andmuons for Drell-Yan) • 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) • 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). Rick Field – Florida/CDF/CMS

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