Toward an Understanding of Hadron-Hadron Collisions

Toward an Understanding of Hadron-Hadron Collisions
From Feynman-Field to the LHC Rick Field University of Florida Outline of Talk TRIUMF September The early days of Feynman-Field Phenomenology. Studying “min-bias” collisions and the “underlying event” at CDF. Using Drell-Yan lepton-pair production to study the “underlying event”. Extrapolations to the LHC. CDF Run 2 CMS at the LHC TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Toward and Understanding of Hadron-Hadron Collisions
Feynman-Field Phenomenology 1st hat! Feynman and Field From 7 GeV/c p0’s to 600 GeV/c Jets. The early days of trying to understand and simulate hadron-hadron collisions. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

The Feynman-Field Days
“Feynman-Field Jet Model” 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 1st graduate student! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Hadron-Hadron Collisions
FF (preQCD) What happens when two hadrons collide at high energy? 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 PT particles arise from direct hard collisions between constituent quarks in the incoming particles, which fragment or cascade down into several hadrons.” 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! “Black-Box Model” TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Quark-Quark Black-Box Model
No gluons! FF (preQCD) Quark Distribution Functions determined from deep-inelastic lepton-hadron collisions 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.” Quark Fragmentation Functions determined from e+e- annihilations Quark-Quark Cross-Section Unknown! Deteremined from hadron-hadron collisions. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Quark-Quark Black-Box Model
FF (preQCD) Predict particle ratios Predict increase with increasing CM energy W “Beam-Beam Remnants” Predict overall event topology (FFF1 paper 1977) 7 GeV/c p0’s! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

SAW CRONIN AM NOW CONVINCED WERE RIGHT TRACK QUICK WRITE
Telagram from Feynman July 1976 SAW CRONIN AM NOW CONVINCED WERE RIGHT TRACK QUICK WRITE FEYNMAN TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Feynman Talk at Coral Gables (December 1976)
1st transparency Last transparency “Feynman-Field Jet Model” TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

QCD Approach: Quarks & Gluons
Quark & Gluon Fragmentation Functions Q2 dependence predicted from QCD FFF2 1978 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.” Parton Distribution Functions Q2 dependence predicted from QCD Quark & Gluon Cross-Sections Calculated from QCD TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

A Parameterization of the Properties of Jets
Field-Feynman 1978 Secondary Mesons (after decay) Assumed that jets could be analyzed on a “recursive” principle. (bk) (ka) Let f(h)dh be the probability that the rank 1 meson leaves fractional momentum h to the remaining cascade, leaving quark “b” with momentum P1 = h1P0. Rank 2 Rank 1 Assume that the mesons originating from quark “b” are distributed in presisely the same way as the mesons which came from quark a (i.e. same function f(h)), leaving quark “c” with momentum P2 = h2P1 = h2h1P0. Primary Mesons (cb) (ba) cc pair bb pair continue Add in flavor dependence by letting bu = probabliity of producing u-ubar pair, bd = probability of producing d-dbar pair, etc. Calculate F(z) from f(h) and bi! Let F(z)dz be the probability of finding a meson (independent of rank) with fractional mementum z of the original quark “a” within the jet. Original quark with flavor “a” and momentum P0 TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Feynman-Field Jet Model
R. P. Feynman ISMD, Kaysersberg, France, June 12, 1977 Feynman quote from FF2 “The predictions of the model are reasonable enough physically that we expect it may be close enough to reality to be useful in designing future experiments and to serve as a reasonable approximation to compare to data. We do not think of the model as a sound physical theory, ....” TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Monte-Carlo Simulation of Hadron-Hadron Collisions
FF1-FFF1 (1977) “Black-Box” Model FF2 (1978) Monte-Carlo simulation of “jets” F1-FFF2 (1978) QCD Approach FFFW “FieldJet” (1980) QCD “leading-log order” simulation of hadron-hadron collisions “FF” or “FW” Fragmentation the past today ISAJET (“FF” Fragmentation) HERWIG (“FW” Fragmentation) PYTHIA tomorrow SHERPA PYTHIA 6.3 TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

High PT Jets CDF (2006) Feynman, Field, & Fox (1978) 30 GeV/c! Predict
large “jet” cross-section 30 GeV/c! 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 PT discussed here.” 600 GeV/c Jets! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Proton-AntiProton Collisions at the Tevatron
The CDF “Min-Bias” trigger picks up most of the “hard core” cross-section plus a small amount of single & double diffraction. stot = sEL + sIN stot = sEL + sSD + sDD + sHC 1.8 TeV: 78mb = 18mb mb (4-7)mb + (47-44)mb CDF “Min-Bias” trigger 1 charged particle in forward BBC AND 1 charged particle in backward BBC The “hard core” component contains both “hard” and “soft” collisions. Beam-Beam Counters 3.2 < |h| < 5.9 TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

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! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Particle Densities Charged Particles pT > 0.5 GeV/c |h| < 1
DhDf = 4p = 12.6 CDF Run 2 “Min-Bias” CDF Run 2 “Min-Bias” Observable Average Average Density per unit h-f Nchg Number of Charged Particles (pT > 0.5 GeV/c, |h| < 1) 3.17 +/- 0.31 / PTsum (GeV/c) Scalar pT sum of Charged Particles 2.97 +/- 0.23 / 1 charged particle dNchg/dhdf = 1/4p = 0.08 dNchg/dhdf = 3/4p = 0.24 3 charged particles 1 GeV/c PTsum dPTsum/dhdf = 1/4p GeV/c = 0.08 GeV/c dPTsum/dhdf = 3/4p GeV/c = 0.24 GeV/c 3 GeV/c PTsum Divide by 4p 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. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

CDF Run 1 “Min-Bias” Data Charged Particle Density
<dNchg/dh> = 4.2 <dNchg/dhdf> = 0.67 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 h in “Min-Bias” collisions at 1.8 TeV (|h| < 1, all pT). Convert to charged particle density, dNchg/dhdf, by dividing by 2p. There are about 0.67 charged particles per unit h-f in “Min-Bias” collisions at 1.8 TeV (|h| < 1, all pT). 0.25 0.67 There are about 0.25 charged particles per unit h-f in “Min-Bias” collisions at 1.96 TeV (|h| < 1, pT > 0.5 GeV/c). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

CDF Run 2 Min-Bias “Associated” Charged Particle Density
Highest pT charged particle! “Associated” densities do not include PTmax! Use the maximum pT charged particle in the event, PTmax, to define a direction and look at the the “associated” density, dNchg/dhdf, in “min-bias” collisions (pT > 0.5 GeV/c, |h| < 1). It is more probable to find a particle accompanying PTmax than it is to find a particle in the central region! Shows the data on the Df dependence of the “associated” charged particle density, dNchg/dhdf, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative to PTmax (rotated to 180o) for “min-bias” events. Also shown is the average charged particle density, dNchg/dhdf, for “min-bias” events. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

CDF Run 2 Min-Bias “Associated” Charged Particle Density
Rapid rise in the particle density in the “transverse” region as PTmax increases! PTmax > 2.0 GeV/c Transverse Region Transverse Region Ave Min-Bias 0.25 per unit h-f PTmax > 0.5 GeV/c Shows the data on the Df dependence of the “associated” charged particle density, dNchg/dhdf, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative to PTmax (rotated to 180o) for “min-bias” events with PTmax > 0.5, 1.0, and 2.0 GeV/c. Shows “jet structure” in “min-bias” collisions (i.e. the “birth” of the leading two jets!). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

CDF Run 1: Evolution of Charged Jets “Underlying Event”
Charged Particle Df Correlations PT > 0.5 GeV/c |h| < 1 Look at the charged particle density in the “transverse” region! “Transverse” region very sensitive to the “underlying event”! CDF Run 1 Analysis Look at charged particle correlations in the azimuthal angle Df relative to the leading charged particle jet. Define |Df| < 60o as “Toward”, 60o < |Df| < 120o as “Transverse”, and |Df| > 120o as “Away”. All three regions have the same size in h-f space, DhxDf = 2x120o = 4p/3. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Run 1 Charged Particle Density “Transverse” pT Distribution
Factor of 2! PT(charged jet#1) > 30 GeV/c “Transverse” <dNchg/dhdf> = 0.56 “Min-Bias” CDF Run 1 Min-Bias data <dNchg/dhdf> = 0.25 Compares the average “transverse” charge particle density with the average “Min-Bias” charge particle density (|h|<1, pT>0.5 GeV). Shows how the “transverse” charge particle density and the Min-Bias charge particle density is distributed in pT. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

ISAJET 7.32 “Transverse” Density
ISAJET uses a naïve leading-log parton shower-model which does not agree with the data! ISAJET “Hard” Component Beam-Beam Remnants Plot shows average “transverse” charge particle density (|h|<1, pT>0.5 GeV) versus PT(charged jet#1) compared to the QCD hard scattering predictions of ISAJET 7.32 (default parameters with PT(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). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

HERWIG 6.4 “Transverse” Density
HERWIG uses a modified leading-log parton shower-model which does agrees better with the data! HERWIG Beam-Beam Remnants “Hard” Component Plot shows average “transverse” charge particle density (|h|<1, pT>0.5 GeV) versus PT(charged jet#1) compared to the QCD hard scattering predictions of HERWIG 5.9 (default parameters with PT(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). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

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 PT of the hard scattering, PT(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 PT(hard) (single or double Gaussian matter distribution). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Tuning PYTHIA: Multiple Parton Interaction Parameters
Default Description PARP(83) 0.5 Double-Gaussian: Fraction of total hadronic matter within PARP(84) PARP(84) 0.2 Double-Gaussian: Fraction of the overall hadron radius containing the fraction PARP(83) of the total hadronic matter. PARP(85) 0.33 Probability that the MPI produces two gluons with color connections to the “nearest neighbors. PARP(86) 0.66 Probability 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 TeV Determines the reference energy E0. PARP(90) 0.16 Determines the energy dependence of the cut-off PT0 as follows PT0(Ecm) = PT0(Ecm/E0)e with e = PARP(90) PARP(67) 1.0 A 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 Determine by comparing with 630 GeV data! Affects the amount of initial-state radiation! Take E0 = 1.8 TeV Reference point at 1.8 TeV TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

PYTHIA default parameters
PYTHIA Defaults MPI constant probability scattering PYTHIA default parameters Parameter 6.115 6.125 6.158 6.206 MSTP(81) 1 MSTP(82) PARP(81) 1.4 1.9 PARP(82) 1.55 2.1 PARP(89) 1,000 PARP(90) 0.16 PARP(67) 4.0 1.0 Plot shows the “Transverse” charged particle density versus PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA (PT(hard) > 0) using the default parameters for multiple parton interactions and CTEQ3L, CTEQ4L, and CTEQ5L. Default parameters give very poor description of the “underlying event”! Note Change PARP(67) = 4.0 (< 6.138) PARP(67) = 1.0 (> 6.138) TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Run 1 PYTHIA Tune A PYTHIA 6.206 CTEQ5L
CDF Default! PYTHIA CTEQ5L Parameter Tune B Tune A MSTP(81) 1 MSTP(82) 4 PARP(82) 1.9 GeV 2.0 GeV PARP(83) 0.5 PARP(84) 0.4 PARP(85) 1.0 0.9 PARP(86) 0.95 PARP(89) 1.8 TeV PARP(90) 0.25 PARP(67) 4.0 Run 1 Analysis Plot shows the “transverse” charged particle density versus PT(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)). Old PYTHIA default (more initial-state radiation) Old PYTHIA default (more initial-state radiation) New PYTHIA default (less initial-state radiation) New PYTHIA default (less initial-state radiation) TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Run 1 vs Run 2: “Transverse” Charged Particle Density
“Transverse” region as defined by the leading “charged particle jet” Excellent agreement between Run 1 and 2! Shows the data on the average “transverse” charge particle density (|h|<1, pT>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. 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). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

PYTHIA Tune A Min-Bias “Soft” + ”Hard”
Tuned to fit the CDF Run 1 “underlying event”! PYTHIA Tune A CDF Run 2 Default 12% of “Min-Bias” events have PT(hard) > 5 GeV/c! 1% of “Min-Bias” events have PT(hard) > 10 GeV/c! PYTHIA regulates the perturbative 2-to-2 parton-parton cross sections with cut-off parameters which allows one to run with PT(hard) > 0. One can simulate both “hard” and “soft” collisions in one program. Lots of “hard” scattering in “Min-Bias” at the Tevatron! The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned. This PYTHIA fit predicts that 12% of all “Min-Bias” events are a result of a hard 2-to-2 parton-parton scattering with PT(hard) > 5 GeV/c (1% with PT(hard) > 10 GeV/c)! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

PYTHIA Tune A LHC Min-Bias Predictions
12% of “Min-Bias” events have PT(hard) > 10 GeV/c! LHC? Shows the center-of-mass energy dependence of the charged particle density, dNchg/dhdfdPT, for “Min-Bias” collisions compared with PYTHIA Tune A with PT(hard) > 0. 1% of “Min-Bias” events have PT(hard) > 10 GeV/c! 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 PT(hard) > 10 GeV/c which increases to 12% at 14 TeV! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

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). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Charged Particle Density Df Dependence
Log Scale! Leading Jet Min-Bias 0.25 per unit h-f 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 “leading jet” events 30 < ET(jet#1) < 70 GeV. Also shows charged particle density, dNchg/dhdf, for charged particles in the range pT > 0.5 GeV/c and |h| < 1 for “min-bias” collisions. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

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. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

“Transverse” PTsum Density PYTHIA Tune A vs HERWIG
“Leading Jet” “Back-to-Back” Now look in detail at “back-to-back” events in the region 30 < ET(jet#1) < 70 GeV! 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 after CDFSIM. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Charged PTsum Density PYTHIA Tune A vs HERWIG
HERWIG (without multiple parton interactions) does not produces enough PTsum in the “transverse” region for 30 < ET(jet#1) < 70 GeV! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

The “Underlying Event” in High PT Jet Production (LHC)
Charged particle density versus PT(jet#1) The “Underlying Event” “Underlying event” much more active at the LHC! 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 in the “Transverse” region versus PT(jet#1) at 14 TeV for PY Tune AW and HERWIG (without MPI). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

“Transverse” PTmax versus ET(jet#1)
“Leading Jet” Highest pT particle in the “transverse” region! “Back-to-Back” Min-Bias Use the leading jet to define the “transverse” region and look at the maximum pT charged particle in the “transverse” region, PTmaxT. Shows the average PTmaxT, in the “transverse” region (pT > 0.5 GeV/c, |h| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events compared with the average maximum pT particle, PTmax, in “min-bias” collisions (pT > 0.5 GeV/c, |h| < 1). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Back-to-Back “Associated” Charged Particle Densities
Maximum pT particle in the “transverse” region! “Associated” densities do not include PTmaxT! Use the leading jet in “back-to-back” events to define the “transverse” region and look at the maximum pT charged particle in the “transverse” region, PTmaxT. Look at the Df dependence of the “associated” charged particle and PTsum densities, dNchg/dhdf and dPTsum/dhdf for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmaxT) relative to PTmaxT. Rotate so that PTmaxT is at the center of the plot (i.e. 180o). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Back-to-Back “Associated” Charged Particle Densities
“Associated” densities do not include PTmaxT! Jet#2 Region ?? Log Scale! Look at the Df dependence of the “associated” charged particle density, dNchg/dhdf for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmaxT) relative to PTmaxT (rotated to 180o) for PTmaxT > 0.5 GeV/c, PTmaxT > 1.0 GeV/c and PTmaxT > 2.0 GeV/c, for “back-to-back” events with 30 < ET(jet#1) < 70 GeV. Shows “jet structure” in the “transverse” region (i.e. the “birth” of the 3rd & 4th jet). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

“Back-to-Back” vs “Min-Bias” “Associated” Charge Density
“Birth” of jet#3 in the “transverse” region! “Back-to-Back” “Associated” Density “Min-Bias” “Associated” Density Log Scale! “Birth” of jet#1 in “min-bias” collisions! Shows the Df dependence of the “associated” charged particle density, dNchg/dhdf for pT > 0.5 GeV/c, |h| < 1 (not including PTmaxT) relative to PTmaxT (rotated to 180o) for PTmaxT > 2.0 GeV/c, for “back-to-back” events with 30 < ET(jet#1) < 70 GeV. Shows the data on the Df dependence of the “associated” charged particle density, dNchg/dhdf, pT > 0.5 GeV/c, |h| < 1 (not including PTmax) relative to PTmax (rotated to 180o) for “min-bias” events with PTmax > 2.0 GeV/c. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

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. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

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. TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

The “Underlying Event” in Drell-Yan Production
Charged particle density versus M(pair) 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). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Summary and Conclusions
“Min-Bias” is not well defined. What you see depends on what you trigger on! Every trigger produces some biases. We have learned a lot about “Min-Bias” at the Tevatron, but we do not know what to expect at the LHC. This will depend on the Min-Bias Trigger! We are making good progress in understanding and modeling the “underlying event”. However, we do not yet have a perfect fit to all the features of the CDF “underlying event” data! Need to measure “Min-Bias” and the “underlying event” at the LHC as soon as possible and tune the Monte-Carlo modles and compare with CDF! TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Study charged particles and muons using the CMS detector at the LHC (as soon as possible)! 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). TRIUMF Laboratory September 27, 2007 Rick Field – Florida/CDF/CMS

Toward an Understanding of Hadron-Hadron Collisions

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Toward an Understanding of Hadron-Hadron Collisions

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