MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 1 Physics and Techniques of Event Generators Rick Field University of Florida (for the CDF & CMS Collaborations) CDF Run 2 Min-Bias and the Underlying Event at the TEVATRON and the LHC CMS at the LHC 1 st Lecture IPPP Durham, April 18-20, 2007  The early days of event generators.  “Min-Bias” at the Tevatron.  Studying the “underlying event” in Run 1 at CDF. and extrapolations to the LHC.

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 2 Toward and Understanding of Hadron-Hadron Collisions  From 7 GeV/c  0 ’s to 600 GeV/c Jets. The early days of trying to understand and simulate hadron-hadron collisions. Feynman-Field Phenomenology FeynmanandField 1 st hat!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 3 “Feynman-Field Jet Model” The Feynman-Field Days  FF1: “Quark Elastic Scattering as a Source of High Transverse Momentum Mesons”, R. D. Field and R. P. Feynman, Phys. Rev. D15, (1977).  FFF1: “Correlations Among Particles and Jets Produced with Large Transverse Momenta”, R. P. Feynman, R. D. Field and G. C. Fox, Nucl. Phys. B128, 1-65 (1977).  FF2: “A Parameterization of the properties of Quark Jets”, R. D. Field and R. P. Feynman, Nucl. Phys. B136, 1-76 (1978).  F1: “Can Existing High Transverse Momentum Hadron Experiments be Interpreted by Contemporary Quantum Chromodynamics Ideas?”, R. D. Field, Phys. Rev. Letters 40, (1978).  FFF2: “A Quantum Chromodynamic Approach for the Large Transverse Momentum Production of Particles and Jets”, R. P. Feynman, R. D. Field and G. C. Fox, Phys. Rev. D18, (1978)  FW1: “A QCD Model for e + e - Annihilation”, R. D. Field and S. Wolfram, Nucl. Phys. B213, (1983). My 1 st graduate student!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 4 Hadron-Hadron Collisions  What happens when two hadrons collide at high energy?  Most of the time the hadrons ooze through each other and fall apart (i.e. no hard scattering). The outgoing particles continue in roughly the same direction as initial proton and antiproton.  Occasionally there will be a large transverse momentum meson. Question: Where did it come from?  We assumed it came from quark-quark elastic scattering, but we did not know how to calculate it! FF (preQCD) Feynman quote from FF1 “The model we shall choose is not a popular one, so that we will not duplicate too much of the work of others who are similarly analyzing various models (e.g. constituent interchange model, multiperipheral models, etc.). We shall assume that the high P T particles arise from direct hard collisions between constituent quarks in the incoming particles, which fragment or cascade down into several hadrons.” “Black-Box Model”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 5 Quark-Quark Black-Box Model FF (preQCD) Quark Distribution Functions determined from deep-inelastic lepton-hadron collisions Quark Fragmentation Functions determined from e + e - annihilations Quark-Quark Cross-Section Unknown! Deteremined from hadron-hadron collisions. No gluons! Feynman quote from FF1 “Because of the incomplete knowledge of our functions some things can be predicted with more certainty than others. Those experimental results that are not well predicted can be “used up” to determine these functions in greater detail to permit better predictions of further experiments. Our papers will be a bit long because we wish to discuss this interplay in detail.”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 6 Quark-Quark Black-Box Model FF (preQCD) Predict particle ratios Predict increase with increasing CM energy W Predict overall event topology (FFF1 paper 1977) “Beam-Beam Remnants” 7 GeV/c  0 ’s!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 7 QCD Approach: Quarks & Gluons FFF Parton Distribution Functions Q 2 dependence predicted from QCD Quark & Gluon Fragmentation Functions Q 2 dependence predicted from QCD Quark & Gluon Cross-Sections Calculated from QCD Feynman quote from FFF2 “We investigate whether the present experimental behavior of mesons with large transverse momentum in hadron-hadron collisions is consistent with the theory of quantum-chromodynamics (QCD) with asymptotic freedom, at least as the theory is now partially understood.”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 8 High P T Jets 30 GeV/c! Predict large “jet” cross-section Feynman, Field, & Fox (1978) CDF (2006) 600 GeV/c Jets! Feynman quote from FFF “At the time of this writing, there is still no sharp quantitative test of QCD. An important test will come in connection with the phenomena of high P T discussed here.”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 9 Proton-AntiProton Collisions at the Tevatron  tot =  EL  SD   DD   HC 1.8 TeV: 78mb = 18mb + 9mb + (4-7)mb + (47-44)mb The CDF “Min-Bias” trigger picks up most of the “hard core” cross-section plus a small amount of single & double diffraction. The “hard core” component contains both “hard” and “soft” collisions. Beam-Beam Counters 3.2 < |  | < 5.9 CDF “Min-Bias” trigger 1 charged particle in forward BBC AND 1 charged particle in backward BBC  tot =  EL  IN

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 10 No-Bias vs Min-Bias What you see for “Min-Bias” depends on your triggger! By comparing different “Min-Bias” triggers one can learn about the components!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 11 No-Bias vs Min-Bias  Charged particle (all p T ) pseudo-rapidity distribution, dN chg /d  d , at 1.96 TeV with no trigger (i.e. no-bias) from PYTHIA Tune DW. About 2.5 charged particles per unit  at  = 0. About 0.9 charged particles (p T > 0.5 GeV/c) per unit  at  = 0.  Charged particle (p T > 0.5 GeV/c) pseudo- rapidity distribution, dN chg /d  d , at 1.96 TeV with the CDF Min-Bias trigger from PYTHIA Tune DW. About 1.5 charged particles (p T > 0.5 GeV/c) per unit  at  = 0 with CDF min-bias trigger.

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage cm Min-Bias Particle Types  Using c  = 10 mm reduces the charged particle density by almost 10%! Mostly from K s →  +  - (68.6%) and  →p  - (64.2%). With c  = 10mm With Stable Particles charged particle density: 10mm vs Stable This is a bigger effect than I expected! No-Bias at 14 TeV Proton AntiProton CDF Run 2

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 13 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  = GeV/c PTsum dPT sum /d  d  = 1/4  GeV/c = 0.08 GeV/cdN chg /d  d  = 3/4  = 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) / / PT sum (GeV/c) Scalar p T sum of Charged Particles (p T > 0.5 GeV/c, |  | < 1) / / Divide by 4  CDF Run 2 “Min-Bias” Particle Densities

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 14  Shows CDF “Min-Bias” data on the number of charged particles per unit pseudo-rapidity at 630 and 1,800 GeV. There are about 4.2 charged particles per unit  in “Min-Bias” collisions at 1.8 TeV (|  | < 1, all p T ). = 4.2 = 0.67  Convert to charged particle density, dN chg /d  d  by dividing by 2 . There are about 0.67 charged particles per unit  -  in “Min-Bias” collisions at 1.8 TeV (|  | < 1, all p T )  There are about 0.25 charged particles per unit  -  in “Min-Bias” collisions at 1.96 TeV (|  | 0.5 GeV/c) CDF Run 1 “Min-Bias” Data Charged Particle Density

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 15  Shows the center-of-mass energy dependence of the charged particle density, dN chg /d  d  for “Min-Bias” collisions at  = 0. Also show a log fit (Fit 1) and a (log) 2 fit (Fit 2) to the CDF plus UA5 data. = 0.51  = GeV  What should we expect for the LHC? = 0.63  = TeV LHC? 24% increase CDF Run 1 “Min-Bias” Data Energy Dependence

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 16  Shows the center-of-mass energy dependence of the charged particle density, dN chg /d  d  for “Min-Bias” collisions compared with the HERWIG “Soft” Min-Bias Monte-Carlo model. Note: there is no “hard” scattering in HERWIG “Soft” Min-Bias.  HERWIG “Soft” Min-Bias contains no hard parton-parton interactions and describes fairly well the charged particle density, dN chg /d  d , in “Min-Bias” collisions.  HERWIG “Soft” Min-Bias predicts a 45% rise in dN chg /d  d  at  = 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged particles per unit  becomes 6. Can we believe HERWIG “soft” Min-Bias? Can we believe HERWIG “soft” Min-Bias? No! LHC? Herwig “Soft” Min-Bias

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 17  Shows the p T dependence of the charged particle density, dN chg /d  d  dP T, for “Min-Bias” collisions at 1.8 TeV collisions compared with HERWIG “Soft” Min-Bias.  HERWIG “Soft” Min-Bias does not describe the “Min-Bias” data! The “Min-Bias” data contains a lot of “hard” parton-parton collisions which results in many more particles at large P T than are produces by any “soft” model.  Shows the energy dependence of the charged particle density, dN chg /d  d  for “Min-Bias” collisions compared with HERWIG “Soft” Min-Bias. Lots of “hard” scattering in “Min-Bias”! HERWIG “Soft” Min-Bias CDF Run 1 “Min-Bias” Data p T Distribution

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 18  HERWIG “hard” QCD with P T (hard) > 3 GeV/c describes well the high p T tail but produces too many charged particles overall. Not all of the “Min-Bias” collisions have a hard scattering with P T (hard) > 3 GeV/c!  One cannot run the HERWIG “hard” QCD Monte-Carlo with P T (hard) < 3 GeV/c because the perturbative 2-to-2 cross-sections diverge like 1/P T (hard) 4 ? HERWIG “soft” Min-Bias does not fit the “Min-Bias” data!  HC PYTHIA cuts off the divergence. Can run P T (hard)>0! CDF Run 1 “Min-Bias” Data Combining “Soft” + “Hard” HERWIG diverges! No easy way to “mix” HERWIG “hard” with HERWIG “soft”.

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 19 PYTHIA Tune A Min-Bias “Soft” + ”Hard”  PYTHIA regulates the perturbative 2-to-2 parton-parton cross sections with cut-off parameters which allows one to run with P T (hard) > 0. One can simulate both “hard” and “soft” collisions in one program.  The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned. Tuned to fit the CDF Run 1 “underlying event”! 12% of “Min-Bias” events have P T (hard) > 5 GeV/c! 1% of “Min-Bias” events have P T (hard) > 10 GeV/c!  This PYTHIA fit predicts that 12% of all “Min-Bias” events are a result of a hard 2-to-2 parton-parton scattering with P T (hard) > 5 GeV/c (1% with P T (hard) > 10 GeV/c)! Lots of “hard” scattering in “Min-Bias” at the Tevatron! PYTHIA Tune A CDF Run 2 Default

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 20  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! CDF Run 2 Min-Bias “Associated” Charged Particle Density

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 21  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 CDF Run 2 Min-Bias “Associated” Charged Particle Density Rapid rise in the particle density in the “transverse” region as PTmax increases!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 22  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 CDF Run 2 Min-Bias “Associated” Charged Particle Density

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 23  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

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 24  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

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 25  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

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 26  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

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 27  Shows the average transverse momentum of charged particles (|  | 0.5 GeV) versus the number of charged particles, N chg, for the CDF Run 2 Min-Bias events. The charged rises with Nchg! Charged versus N chg

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 28 Using Pile-Up to Study Min-Bias  The primary vertex is the highest PTsum of charged particles pointing towards it. Proton AntiProton 60 cm  Normally one only includes those charged particles which point back to the primary vertex.  However, the primary vertex is presumably the collision that satisfied the trigger and is hence biased.  Perhaps the pile-up is not biases and can serve as a new type of “Min-Bias” trigger.  This assumes that the pile-up is not affected by the trigger (i.e. it is the same for all primary processes). High PT Jet MB CDF Run 2

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 29 Using Pile-Up to Study Min-Bias  Shows the the charged particle density, dN chg /d  for charged particles (p T > 0.5 GeV/c) pointing to the primary vertex for “Min-Bias” collisions at 1.96 TeV. About 2.6 charged particles per unit  at  = 0. About 1.6 charged particles per unit  at  = 0 per pile-up interaction.  Shows the the charged particle density, dN chg /d  (per interaction) for charged particles (p T > 0.5 GeV/c) pointing to the  pile-up vertices for “Min- Bias” collisions at 1.96 TeV.  Clearly the pile-up “min-bias” is biased because there must to be some particles in the central region to form a vertex (e.g. elestic scattering does not contribute), but it is less biased than CDF “min-bias”.

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 30 Is the Pile-Up Biased?  Shows the the charged particle multiplicity distribution  (per interaction)  for charged particles (p T > 0.5 GeV/c, |  | <1) pointing to the  pile-up vertices for “Min-Bias” collisions at 1.96 TeV.  Shows the the charged particle multiplicity distribution (per interaction) for charged particles (p T > 0.5 GeV/c, |  | 150 GeV/c) at 1.96 TeV. The pile-up is different for Min-bias collisions and high p T jet production! Amasing! Warning! This data is very preliminary and not “blessed” by CDF. So do not believe it yet!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 31 Is the Pile-Up Biased?  Shows the data on the  dependence of the charged particle density, dN chg /d  d , for charged particles (p T > 0.5 GeV/c, |  | < 1) pointing to the  primary vertex relative to the leading calorimeter jet (rotated to 270 o ) for 150 < PT(jet#1) < 250 GeV/c |  (jet#1)| < 2.  Shows the data on the  dependence of the charged particle density, dN chg /d  d , for charged particles (p T > 0.5 GeV/c, |  | < 1) (per interaction) pointing to the  pile-up vertices relative to the leading calorimeter jet (rotated to 270 o ) for 150 < PT(jet#1) < 250 GeV/c |  (jet#1)| < 2. Transverse Region The pile-up knows the direction of the leading high p T jet! Amasing! The pile-up conspires to help give you what you ask for (i.e. satisfy your “trigger” or your event selection)! Warning! This data is very preliminary and not “blessed” by CDF. So do not believe it yet!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 32 Min-Bias Summary  “Min-Bias” is not well defined. What you see depends on what you trigger on! Every trigger produces some biases. We learn about “min-bias” by comparing different “low bias” triggers.  Preliminary results seem to show that pile- up is biased! and that it conspires to help give you what you ask for (i.e. satisfy your “trigger” or your event selection)! If true this means the pile-up is not the same for all processes. It is process (i.e. trigger) dependent! This would have big implications for the LHC! I must double check my analysis and get it “blessed”! Leading Jet

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 33 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!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 34  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 Look at the charged particle density in the “transverse” region! “Transverse” region very sensitive to the “underlying event”! CDF Run 1 Analysis CDF Run 1: Evolution of Charged Jets “Underlying Event”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 35  Compares the average “transverse” charge particle density with the average “Min-Bias” charge particle density (|  | 0.5 GeV). Shows how the “transverse” charge particle density and the Min-Bias charge particle density is distributed in p T. CDF Run 1 Min-Bias data = 0.25 P T (charged jet#1) > 30 GeV/c “Transverse” = 0.56 Factor of 2! “Min-Bias” Run 1 Charged Particle Density “Transverse” p T Distribution

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 36  Plot shows average “transverse” charge particle density (|  | 0.5 GeV) versus P T (charged jet#1) compared to the QCD hard scattering predictions of ISAJET 7.32 (default parameters with P T (hard)>3 GeV/c).  The predictions of ISAJET are divided into two categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants); and charged particles that arise from the outgoing jet plus initial and final-state radiation (hard scattering component). Beam-Beam Remnants ISAJET “Hard” Component ISAJET 7.32 “Transverse” Density ISAJET uses a naïve leading-log parton shower-model which does not agree with the data!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 37  Plot shows average “transverse” charge particle density (|  | 0.5 GeV) versus P T (charged jet#1) compared to the QCD hard scattering predictions of HERWIG 5.9 (default parameters with P T (hard)>3 GeV/c).  The predictions of HERWIG are divided into two categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants); and charged particles that arise from the outgoing jet plus initial and final-state radiation (hard scattering component). Beam-Beam Remnants HERWIG “Hard” Component HERWIG uses a modified leading- log parton shower-model which does agrees better with the data! HERWIG 6.4 “Transverse” Density

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 38 Herwig P T (chgjet#1) > 5 GeV/c = 0.40 Herwig P T (chgjet#1) > 30 GeV/c “Transverse” = 0.51  Compares the average “transverse” charge particle density (|  | 0.5 GeV) versus P T (charged jet#1) and the p T distribution of the “transverse” density, dN chg /d  d  dP T with the QCD hard scattering predictions of HERWIG 6.4 (default parameters with P T (hard)>3 GeV/c. Shows how the “transverse” charge particle density is distributed in p T. HERWIG has the too steep of a p T dependence of the “beam-beam remnant” component of the “underlying event”! HERWIG 6.4 “Transverse” P T Distribution

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 39 MPI: Multiple Parton Interactions  PYTHIA models the “soft” component of the underlying event with color string fragmentation, but in addition includes a contribution arising from multiple parton interactions (MPI) in which one interaction is hard and the other is “semi-hard”.  The probability that a hard scattering events also contains a semi-hard multiple parton interaction can be varied but adjusting the cut-off for the MPI.  One can also adjust whether the probability of a MPI depends on the P T of the hard scattering, P T (hard) (constant cross section or varying with impact parameter).  One can adjust the color connections and flavor of the MPI (singlet or nearest neighbor, q-qbar or glue-glue).  Also, one can adjust how the probability of a MPI depends on P T (hard) (single or double Gaussian matter distribution).

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 40 ParameterDefaultDescription PARP(83)0.5Double-Gaussian: Fraction of total hadronic matter within PARP(84) PARP(84)0.2Double-Gaussian: Fraction of the overall hadron radius containing the fraction PARP(83) of the total hadronic matter. PARP(85)0.33Probability that the MPI produces two gluons with color connections to the “nearest neighbors. PARP(86)0.66Probability that the MPI produces two gluons either as described by PARP(85) or as a closed gluon loop. The remaining fraction consists of quark-antiquark pairs. PARP(89)1 TeVDetermines the reference energy E 0. PARP(90)0.16Determines the energy dependence of the cut-off P T0 as follows P T0 (E cm ) = P T0 (E cm /E 0 )  with  = PARP(90) PARP(67)1.0A scale factor that determines the maximum parton virtuality for space-like showers. The larger the value of PARP(67) the more initial- state radiation. Hard Core Take E 0 = 1.8 TeV Reference point at 1.8 TeV Determine by comparing with 630 GeV data! Affects the amount of initial-state radiation! Tuning PYTHIA: Multiple Parton Interaction Parameters

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 41 Default parameters give very poor description of the “underlying event”! Note Change PARP(67) = 4.0 (< 6.138) PARP(67) = 1.0 (> 6.138) Parameter MSTP(81)1111 MSTP(82)1111 PARP(81) PARP(82) PARP(89)1,000 PARP(90)0.16 PARP(67)  Plot shows the “Transverse” charged particle density versus P T (chgjet#1) compared to the QCD hard scattering predictions of PYTHIA (P T (hard) > 0) using the default parameters for multiple parton interactions and CTEQ3L, CTEQ4L, and CTEQ5L. PYTHIA default parameters PYTHIA Defaults MPI constant probability scattering

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 42 Old PYTHIA default (more initial-state radiation) New PYTHIA default (less initial-state radiation) ParameterTune BTune A MSTP(81)11 MSTP(82)44 PARP(82)1.9 GeV2.0 GeV PARP(83)0.5 PARP(84)0.4 PARP(85) PARP(86) PARP(89)1.8 TeV PARP(90)0.25 PARP(67) Old PYTHIA default (more initial-state radiation) New PYTHIA default (less initial-state radiation)  Plot shows the “transverse” charged particle density versus P T (chgjet#1) compared to the QCD hard scattering predictions of two tuned versions of PYTHIA (CTEQ5L, Set B (PARP(67)=1) and Set A (PARP(67)=4)). Run 1 Analysis Run 1 PYTHIA Tune A PYTHIA CTEQ5L CDF Default!

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 43  Shows the data on the average “transverse” charge particle density (|  | 0.5 GeV) as a function of the transverse momentum of the leading charged particle jet from Run 1.  Compares the Run 2 data (Min-Bias, JET20, JET50, JET70, JET100) with Run 1. The errors on the (uncorrected) Run 2 data include both statistical and correlated systematic uncertainties. Excellent agreement between Run 1 and 2! PYTHIA Tune A was tuned to fit the “underlying event” in Run I!  Shows the prediction of PYTHIA Tune A at 1.96 TeV after detector simulation (i.e. after CDFSIM). Run 1 vs Run 2: “Transverse” Charged Particle Density “Transverse” region as defined by the leading “charged particle jet”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 44  Shows the data on the average “transverse” charged PTsum density (|  | 0.5 GeV) as a function of the transverse momentum of the leading charged particle jet from Run 1.  Compares the Run 2 data (Min-Bias, JET20, JET50, JET70, JET100) with Run 1. The errors on the (uncorrected) Run 2 data include both statistical and correlated systematic uncertainties. Excellent agreement between Run 1 and 2!  Shows the prediction of PYTHIA Tune A at 1.96 TeV after detector simulation (i.e. after CDFSIM). PYTHIA Tune A was tuned to fit the “underlying event” in Run I! Run 1 vs Run 2: “Transverse” Charged PTsum Density “Transverse” region as defined by the leading “charged particle jet”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 45  Compares the average “transverse” charge particle density (|  | 0.5 GeV) versus P T (charged jet#1) with the p T distribution of the “transverse” density, dN chg /d  d  dP T. Shows how the “transverse” charge particle density is distributed in p T.  Compares the Run 2 data (Min-Bias, JET20, JET50, JET70, JET100) with Run 1. Excellent agreement between Run 1 and 2! Charged Particle Density “Transverse” p T Distribution

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 46  Look at charged particle correlations in the azimuthal angle  relative to the leading JetClu 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 Away-side “jet” (sometimes) Perpendicular to the plane of the 2-to-2 hard scattering “Transverse” region is very sensitive to the “underlying event”! Look at the charged particle density in the “transverse” region! “Underlying Event” as defined by “Calorimeter Jets”

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 47  Shows the data on the average “transverse” charge particle density (|  | 0.5 GeV) as a function of the transverse energy of the leading JetClu jet (R = 0.7, |  (jet)| < 2) from Run 2.  Compares the “transverse” region of the leading “charged particle jet”, chgjet#1, with the “transverse” region of the leading “calorimeter jet” (JetClu R = 0.7), jet#1., compared with PYTHIA Tune A after CDFSIM. “Transverse” region as defined by the leading “calorimeter jet” “Transverse” Charged Particle Density

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 48  Shows the data on the average “transverse” charged PTsum density (|  | 0.5 GeV) as a function of the transverse energy of the leading JetClu jet (R = 0.7, |  (jet)| < 2) from Run 2.  Compares the “transverse” region of the leading “charged particle jet”, chgjet#1, with the “transverse” region of the leading “calorimeter jet” (JetClu R = 0.7), jet#1., compared with PYTHIA Tune A after CDFSIM. “Transverse” region as defined by the leading “calorimeter jet” “Transverse” Charged PTsum Density

MCnet07 - Durham - Part 1 April 18-20, 2007 Rick Field – Florida/CDF/CMSPage 49  Shows the ratio of P T (chgjet#1) to the “matched” JetClu jet E T versus P T (chgjet#1).  Shows the “matched” JetClu jet E T versus the transverse momentum of the leading “charged particle jet” (closest jet within R = 0.7 of the leading chgjet). The leading chgjet comes from a JetClu jet that is, on the average, about 90% charged! In lecture 2 I will show you A more detailed study of the “underlying event” in Run 2 at CDF! Relationship Between “Calorimeter” and “Charged Particle” Jets