1. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 1 LPC Mini-Workshop on Early CMS Physics UE&MB@CMS Rick Field University of Florida (for the UE&MB@CMS Group) University of Florida D. Acosta, P. Bartalini, R. Field, K. Kotov University of Perugia F. Ambroglini, G. Bilei, L. Fano' CERN A. De Roeck Hamburg F.Bechtel, P. Schleper, G. Steinbrueck University of Trieste D. Treleani et al. DESY H. Jung, K. Borras Summer Student E. Izaguirre “Min-Bias” and the “Underlying Event” at the LHC Study charged particles and muons during the early running of the LHC!

2. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 2 1 charged particle dN chg /d  d  = 1/4  = 0.08  Study the charged particles (p T > 0.5 GeV/c, |  | < 1) and form the charged particle density, dN chg /d  d , and the charged scalar p T sum density, dPT sum /d  d . Charged Particles p T > 0.5 GeV/c |  | < 1  = 4  = 12.6 1 GeV/c PTsum dPT sum /d  d  = 1/4  GeV/c = 0.08 GeV/cdN chg /d  d  = 3/4  = 0.24 3 charged particles dPT sum /d  d  = 3/4  GeV/c = 0.24 GeV/c 3 GeV/c PTsum CDF Run 2 “Min-Bias” Observable Average Average Density per unit  -  N chg Number of Charged Particles (p T > 0.5 GeV/c, |  | < 1) 3.17 +/- 0.310.252 +/- 0.025 PT sum (GeV/c) Scalar p T sum of Charged Particles (p T > 0.5 GeV/c, |  | < 1) 2.97 +/- 0.230.236 +/- 0.018 Divide by 4  CDF Run 2 “Min-Bias” Particle Densities

3. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 3  PYTHIA was tuned to fit the “underlying event” in hard-scattering processes at 1.8 TeV and 630 GeV.  Shows the center-of-mass energy dependence of the charged particle density, dN chg /d  d  for “Min-Bias” collisions compared with PYTHIA Tune A with P T (hard) > 0.  PYTHIA Tune A predicts a 42% rise in dN chg /d  d  at  = 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). Similar to HERWIG “soft” min-bias, 4 charged particles per unit  becomes 6. LHC? PYTHIA Tune A LHC Min-Bias Predictions

4. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 4  Shows the center-of-mass energy dependence of the charged particle density, dN chg /d  d  dp T, for “Min-Bias” collisions compared with PYTHIA Tune A with P T (hard) > 0.  PYTHIA Tune A predicts that 1% of all “Min-Bias” events at 1.8 TeV are a result of a hard 2-to-2 parton-parton scattering with P T (hard) > 10 GeV/c which increases to 12% at 14 TeV! 1% of “Min-Bias” events have P T (hard) > 10 GeV/c! 12% of “Min-Bias” events have P T (hard) > 10 GeV/c! LHC? PYTHIA Tune A LHC Min-Bias Predictions

5. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 5  PYTHIA Tune A and Tune DW predict about 6 charged particles per unit  at  = 0, while the ATLAS tune predicts around 9.  Shows the predictions of PYTHIA Tune A, Tune DW, Tune DWT, and the ATLAS tune for the charged particle density dN/d  and dN/dY at 14 TeV (all p T ).  PYTHIA Tune DWT is identical to Tune DW at 1.96 TeV, but extrapolates to the LHC using the ATLAS energy dependence. PYTHIA 6.2 Tunes LHC Min-Bias Predictions

6. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 6  The ATLAS tune has many more “soft” particles than does any of the CDF Tunes. The ATLAS tune has = 548 MeV/c while Tune A has = 641 MeV/c (100 MeV/c more per particle)!  Shows the predictions of PYTHIA Tune A, Tune DW, Tune DWT, and the ATLAS tune for the charged particle p T distribution at 14 TeV (|  | p T min (|  | < 1). PYTHIA 6.2 Tunes LHC Min-Bias Predictions

7. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 7 TOTEM+CMS: p T -  coverage RP 0-12-10-8-6-4-2+2+4+6+8+10+12  0.1 1 10 100 1000 CMS Central Detectors T1 T2 p T max ~  s exp( -  ) CASTOR ZDC HF p T (GeV) CMS Forward detectors: Hadron Forward Calorimeter HF: 3 ≤|  | ≤ 5 Castor Calorimeter: 5.2 ≤|  | ≤ 6.5 Beam Scintillation counters BSC Zero-Degree Calorimeter ZDC Extended Tracking TOTEM detectors: T1 (CSC) in CMS endcaps, T2 (GEM) behind HF T1 + T2: 3 ≤ |  | ≤ 6.8 Roman Pots with Si det. up to 220 m

8. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 8 A CMS “Min-Bias” Trigger 3<|η|<5 18 wedges/side 0.175×0.175 towers Cut on the number of calorimeter cells: >10 cells hit 99% efficient >10 forward cells and >10 backward cells >15 → 86% >20 → 66% Use towers or fired cells Prescaling by a factor 4×10 7 an HF trigger with high efficiency (e.g.: at least 10 cells fired in + and - detector) → 1 Hz Tracking Efficiency p T > 0.9 GeV/c versus pseudorapidity Tracking Efficiency p T > 0.9 GeV/c versus transverse momentum

9. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 9 Detector Alignment Early track studies will help align the detector!

10. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 10 Min-Bias “Associated” Charged Particle Density  Use the maximum p T charged particle in the event, PTmax, to define a direction and look at the the “associated” density, dN chg /d  d , in “min-bias” collisions (p T > 0.5 GeV/c, |  | < 1). “Associated” densities do not include PTmax! Highest p T charged particle!  Shows the data on the  dependence of the “associated” charged particle density, dN chg /d  d , for charged particles (p T > 0.5 GeV/c, |  | < 1, not including PTmax) relative to PTmax (rotated to 180 o ) for “min-bias” events. Also shown is the average charged particle density, dN chg /d  d , for “min-bias” events. It is more probable to find a particle accompanying PTmax than it is to find a particle in the central region!

11. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 11 Min-Bias “Associated” Charged Particle Density  Shows the data on the  dependence of the “associated” charged particle density, dN chg /d  d , for charged particles (p T > 0.5 GeV/c, |  | 0.5, 1.0, and 2.0 GeV/c. Transverse Region  Shows “jet structure” in “min-bias” collisions (i.e. the “birth” of the leading two jets!). Ave Min-Bias 0.25 per unit  -  PTmax > 0.5 GeV/c PTmax > 2.0 GeV/c Rapid rise in the particle density in the “transverse” region as PTmax increases!

12. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 12 Min-Bias “Associated” Charged Particle Density Min-Bias “Associated” Charged Particle Density  Shows the data on the  dependence of the “associated” charged particle density, dN chg /d  d , for charged particles (p T > 0.5 GeV/c, |  | 0.5 GeV/c and PTmax > 2.0 GeV/c compared with PYTHIA Tune A (after CDFSIM).  PYTHIA Tune A predicts a larger correlation than is seen in the “min-bias” data (i.e. Tune A “min-bias” is a bit too “jetty”). PTmax > 2.0 GeV/c PTmax > 0.5 GeV/c Transverse Region PY Tune A

13. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 13 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). “Hard Scattering” Component “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. The “underlying event” is an unavoidable background to most collider observables and having good understand of it leads to more precise collider measurements!

14. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 14  Look at charged particle correlations in the azimuthal angle  relative to the leading charged particle jet.  Define |  | 120 o as “Away”.  All three regions have the same size in  -  space,  x  = 2x120 o = 4  /3. Charged Particle  Correlations P T > 0.5 GeV/c |  | < 1 “Transverse” region very sensitive to the “underlying event”! CDF Run 1 Analysis The Evolution of Charged Jets and the “Underlying Event” Look at the charged particle density in the “transverse” region! -2 +2 UE&MB@CMS

15. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 15 The Evolution of Charged Jets and the “Underlying Event” at the LHC  Shows average “transverse” charge particle density (|  | 0.9 GeV) versus P T (charged jet#1) prediced by PYTHIA Tune DW, HERWIG, and the ATLAS PYTHIA Tune at the LHC. MC MB JET60 JET120 PT>0.9, |  |<1 MC MB JET60 JET120 PT>0.9, |  |<1 MC MB JET60 JET120 Ratio P T >0.9GeV/P T >0.5GeV MC MB JET60 JET120 Ratio P T >0.9GeV/P T >0.5GeV  Shows average “transverse” charge particle PTsum density (|  | 0.9 GeV) versus P T (charged jet#1) prediced by PYTHIA Tune DW, HERWIG, and the ATLAS PYTHIA Tune at the LHC.  Shows average “transverse” charge particle PTsum density (|  | 0.9 GeV) versus P T (charged jet#1) at the generator level and at the detector level.  Shows average “transverse” charge particle density (|  | 0.9 GeV) versus P T (charged jet#1) at the generator level and at the detector level.  Shows the ratio, (pT>0.9 GeV/c)/(pT>0.5 GeV/c), for the “transverse” charge particle and PTsum density (|  | < 1) versus P T (charged jet#1) at the generator level and at the detector level.

16. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 16 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). “Hard Scattering” Component “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.

17. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 17 The “Central” Region in Drell-Yan Production  Look at the “central” region after removing the lepton-pair.  Study the charged particles (p T > 0.5 GeV/c, |  | < 1) and form the charged particle density, dNchg/d  d , and the charged scalar p T sum density, dPTsum/d  d , by dividing by the area in  -  space. Charged Particles (p T > 0.5 GeV/c, |  | < 1) After removing the lepton- pair everything else is the “underlying event”! Look at the charged particle density and the PT sum density in the “central” region!

18. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 18 Drell-Yan Production (Run 2 vs LHC)  Average Lepton-Pair transverse momentum at the Tevatron and the LHC for PYTHIA Tune DW and HERWIG ( without MPI ).  Shape of the Lepton-Pair p T distribution at the Z-boson mass at the Tevatron and the LHC for PYTHIA Tune DW and HERWIG ( without MPI ). Lepton-Pair Transverse Momentum Shapes of the p T (  +  - ) distribution at the Z-boson mass. is much larger at the LHC! Z

19. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 19 The “Underlying Event” in Drell-Yan Production  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 ). The “Underlying Event” Charged particle density versus M(pair) “Underlying event” much more active at the LHC! HERWIG (without MPI) is much less active than PY Tune AW (with MPI)! Z Z

20. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 20 Extrapolations to the LHC: Drell-Yan Production  Average charged particle density versus the lepton-pair invariant mass at 1.96 TeV for PYTHIA Tune A, Tune AW, Tune BW, Tune DW and HERWIG ( without MPI ).  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” Charged particle density versus M(pair) Tune DW and DWT are identical at 1.96 TeV, but have different MPI energy dependence! Z Z  Average charged particle density versus the lepton-pair invariant mass at 1.96 TeV for PYTHIA Tune A, Tune DW, ATLAS and HERWIG ( without MPI ).

21. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 21 Extrapolations to the LHC: Drell-Yan Production  Average charged PTsum density versus the lepton-pair invariant mass at 1.96 TeV for PYTHIA Tune A, Tune AW, Tune BW, Tune DW 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 ). The “Underlying Event” Charged particle charged PTsum density versus M(pair) 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 1.96 TeV for PYTHIA Tune A, DW, ATLAS, and HERWIG ( without MPI ).

22. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 22 Extrapolations to the LHC: Drell-Yan Production  Average charged particle density (p T > 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 (p T > 0.9 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” Charged particle density versus M(pair) Charged Particles (|  | 0.5 GeV/c) Z Z Charged Particles (|  | 0.9 GeV/c) The ATLAS tune has a much “softer” distribution of charged particles than the CDF Run 2 Tunes!

23. LPC CMS Workshop June 8, 2007 Rick Field – Florida/CMSPage 23 UE&MB@CMS  “Underlying Event” Studies: The “transverse region” in “leading chgjet” and “back-to-back” chgjet production and in 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,,, d  /dp T (pair) (only requires muons). Event structure for large lepton-pair p T (i.e.  +chgjets, requires muons and charged tracks).  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) Paolo Filippo Gian Mario Me (Jimmy) Livio Perugia, March 2006