MinBias and the Underlying Event at CDF Outline

“Min-Bias” and the “Underlying Event” at CDF Outline of Talk Æ Review what we learned at CDF in Run 1 about “min-bias”, the “underlying event”, and “initial-state radiation”. Æ Compare the CDF Run 1 analysis which used the leading “charged particle jet” to define the “underlying event” with CDF Run 2 data. Tuned to fit the charged particle component of the “underlying event” in Run 1 Æ Study the “underlying event” in CDF Run 2 as defined by the leading “calorimeter jet” and compare with the “charged particle jet” analysis. Æ Discuss PYTHIA Tune A and extrapolations to the LHC. Jet. Clu R = 0. 7 MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 1

The “Underlying Event” in Hard Scattering Processes “Min-Bias” Æ What happens when a high energy proton and an antiproton collide? Æ Most of the time the proton and antiproton 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. A “Min-Bias” collision. Æ Occasionally there will be a “hard” parton-parton collision resulting in large transverse momentum outgoing partons. Also a “Min-Bias” collision. Æ The “underlying event” is everything except the two outgoing hard scattered “jets”. It is an unavoidable background to many collider observables. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF Are these the same? No! “underlying event” has initial-state radiation! 2

Beam-Beam Remnants Maybe not all “soft”! Æ The underlying event in a hard. Ifscattering process we are going to lookhas at a “hard” component (particles that arise from initial & final-state “Min-Bias” radiation and fromasthe collisions a outgoing hard scattered partons) guide to understanding the and a “soft? ” component (“beam-beam remnants”). “beam-beam remnants”, Æ Clearly? the “underlying event” then in a hard scattering we better study process should not look like a “Min. Bias” event because of the “hard” component (i. e. initial & final-state radiation). carefully the “Min-Bias” Æ However, perhaps “Min-Bias” collisions are data! a good model for the “beam-beam remnant” component of the “underlying event”. Are these the same? Æ The “beam-beam remnant” component is, however, color connected to the “hard” component so this comparison is (at best) an approximation. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 3

CDF Run 1 “Min-Bias” Data Charged Particle Density Æ <d. Nchg /dh> = 4. 2 Shows CDF Æ Æ <d. Nchg/dhdf> = 0. 67 particles per unit pseudo-rapidity “Min-Bias” data on the number of charged at 630 and 1, 800 Ge. V. There about 4. 2 charged particles per unit h in “Min-Bias” collisions at 1. 8 Te. V (|h| < 1, all PT). Convert to charged particle density, d. Nchg/dhdf, by dividing by 2 p. There about 0. 67 charged particles per unit h-f in “Min-Bias” collisions at 1. 8 Te. V (|h| < 1, all PT). 0. 67 0. 25 There about 0. 25 charged particles per unit h-f in “Min-Bias” collisions at 1. 8 Te. V (|h| < 1, PT > 0. 5 Ge. V/c). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 4

CDF Run 1 “Min-Bias” Data PT Dependence Lots of “hard” scattering in “Min-Bias”! Æ Shows the energy dependence of the Æ Æ charged particle density, d. Nchg/dhdf, for “Min-Bias” collisions compared with HERWIG “Soft” Min-Bias. Shows the PT dependence of the charged particle density, d. Nchg/dhdfd. PT, for “Min-Bias” collisions at 1. 8 Te. V collisions compared with 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 PT than are produces by any “soft” model. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 5

Min-Bias: Combining “Hard” and “Soft” Collisions No easy way to “mix” HERWIG “hard” with HERWIG “soft”. HERWIG diverges! s. HC PYTHIA cuts off the divergence. Can run PT(hard)>0! Æ HERWIG “hard” QCD with PT(hard) > 3 Ge. V/c describes well the high PT tail but produces too many charged particles overall. Not all of the “Min-Bias” collisions have a hard scattering with PT(hard) > 3 Ge. V/c! HERWIG “soft” Min -Bias does not fit the “Min-Bias” data! Æ One cannot run the HERWIG “hard” QCD Monte-Carlo with PT(hard) < 3 Ge. V/c because the perturbative 2 -to-2 cross-sections diverge like 1/PT(hard)4? MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 6

PYTHIA Min-Bias PYTHIA Tune A “Soft” + ”Hard” CDF Run 2 Default Tuned to fit the “underlying event”! 12% of “Min-Bias” events have PT(hard) > 5 Ge. V/c! 1% of “Min-Bias” events have PT(hard) > 10 Ge. V/c! Æ PYTHIA regulates the perturbative 2 -to-2 parton-parton cross sections with cut-off parameters which allows one to run with Lots of “hard” scattering PT(hard)in>“Min-Bias”! 0. One can simulate both “hard” and “soft” collisions in one program. Æ The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned. Æ 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 Ge. V/c (1% with PT(hard) > 10 Ge. V/c)! MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 7

Studying the “Underlying Event” at CDF The Underlying Event: beam-beam remnants initial-state radiation multiple-parton interactions Æ The underlying event in a hard scattering process is a complicated and not very well understood object. It is an interesting region since it probes the interface Compares 630 Ge. V between perturbative and nonwith 1. 8 Te. V! perturbative physics. Æ There are now four CDF analyses which quantitatively study the underlying event Run I CDF and compare with the QCD Monte-Carlo “Cone Analysis” models (2 Run I and 2 Run II). Valeria Tano Eve Kovacs Æ It is important to model this region well since it is an unavoidable background to Joey Huston all collider observables. Also, we need a Anwar Bhatti good model of “min-bias” collisions. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF Run II I CDF Run CDF “Evolution of Charged “Evolution of Jets” Rick. Particle Field Jets Charged Stuart and. David Calorimeter Jets” Rich Field Haas Rick Run II CDF “Jet Shapes & Energy Flow” Mario Martinez 8

“Underlying Event” as defined by “Charged particle Jets” “Transverse” region is very sensitive to the “underlying event”! Away-side “jet” (sometimes) Charged Particle Df Correlations PT > 0. 5 Ge. V/c |h| < 1 Look at the charged particle density in the “transverse” region! Toward-side “jet” (always) Perpendicular to the plane of the 2 -to-2 hard scattering Æ Look at charged particle correlations in the azimuthal angle Df relative to the leading Æ Æ charged particle jet. Define |Df| < 60 o as “Toward”, 60 o < |Df| < 120 o as “Transverse”, and |Df| > 120 o as “Away”. All three regions have the same size in h-f space, Dhx. Df = 2 x 120 o = 4 p/3. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 9

Run 1 Charged Particle Density “Transverse” PT Distribution Factor of 2! PT(charged jet#1) > 30 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 56 “Min-Bias” CDF Run 1 Min-Bias data <d. Nchg/dhdf> = 0. 25 Æ Compares the average “transverse” charge particle density with the average “Min-Bias” charge particle density (|h|<1, PT>0. 5 Ge. V). Shows how the “transverse” charge particle density and the Min-Bias charge particle density is distributed in PT. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 10

ISAJET 7. 32 “Transverse” Density ISAJET uses a naïve leading-log parton shower-model which does not agree with the data! ISAJET Beam-Beam Remnants Initial-State Radiation Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged Æ jet#1) and the PT distribution of the “transverse” density, d. Nchg/dhdfd. PT with the QCD hard scattering predictions of ISAJET 7. 32 (default parameters with PT(hard)>3 Ge. V/c). The predictions of ISAJET are divided into three categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants), charged particles that arise initial-state radiation, and charged particles that arise from the outgoing jets plus final-state radiation. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 11

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 Æ Plot shows average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged Æ jet#1) compared to the QCD hard scattering predictions of ISAJET 7. 32 (default parameters with PT(hard)>3 Ge. V/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). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 12

HERWIG 6. 4 “Transverse” Density HERWIG uses a modified leadinglog parton shower-model which does agrees better with the data! HERWIG Beam-Beam Remnants “Hard” Component Æ Plot shows average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged Æ jet#1) compared to the QCD hard scattering predictions of HERWIG 5. 9 (default parameters with PT(hard)>3 Ge. V/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). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 13

HERWIG 6. 4 “Transverse” PT Distribution HERWIG has the too steep of a PT dependence of the “beam-beam remnant” component of the “underlying event”! Herwig PT(chgjet#1) > 30 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 51 Herwig PT(chgjet#1) > 5 Ge. V/c <d. Nchg/dhdf> = 0. 40 Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1) and the PT distribution of the “transverse” density, d. Nchg/dhdfd. PT with the QCD hard scattering predictions of HERWIG 6. 4 (default parameters with PT(hard)>3 Ge. V/c. Shows how the “transverse” charge particle density is distributed in PT. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 14

MPI: Multiple Parton Interactions Æ 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”. Æ 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. Æ 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). Æ One can adjust the color connections and flavor of the MPI (singlet or nearest neighbor, q-qbar or glue-glue). Æ Also, one can adjust how the probability of a MPI depends on PT(hard) (single or double Gaussian matter distribution). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 15

PYTHIA: Multiple Parton Interaction Parameters Pythia uses multiple parton interactions to enhance the underlying event. Parameter Value MSTP(81) 0 Multiple-Parton Scattering off 1 Multiple-Parton Scattering on 1 Multiple interactions assuming the same probability, with an abrupt cut-off PTmin=PARP(81) 3 Multiple interactions assuming a varying impact parameter and a hadronic matter overlap consistent with a single Gaussian matter distribution, with a smooth turnoff PT 0=PARP(82) 4 Multiple interactions assuming a varying impact parameter and a hadronic matter overlap consistent with a double Gaussian matter distribution (governed by PARP(83) and PARP(84)), with a smooth turn-off PT 0=PARP(82) MSTP(82) Same parameter that cuts-off the hard 2 -to-2 parton cross sections! MC Tools for the LHC CERN July 31, 2003 Description Rick Field - Florida/CDF and now HERWIG ! Jimmy: MPI J. M. Butterworth J. R. Forshaw M. H. Seymour Multiple parton interaction more likely in a hard (central) collision! Hard Core 16

Tuning PYTHIA: Multiple Parton Interaction Parameters Parameter Default PARP(83) 0. 5 Description Double-Gaussian: Fraction of total hadronic matter within 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 PARP(89) 1 Te. V PARP(90) 0. 16 PARP(67) 1. 0 MC Tools for the LHC CERN July 31, 2003 d Har Core Determine by comparing with 630 Ge. V data! Probability that the MPI produces two gluons either as described by PARP(85) or as a closed gluon loop. remaining fraction consists of Affects the The amount of quark-antiquark pairs. initial-state radiation! Determines the reference energy E 0. Determines the energy dependence of the cut-off PT 0 as follows PT 0(Ecm) = PT 0(Ecm/E 0)e with e = PARP(90) Take E 0 = 1. 8 Te. V A scale factor that determines the maximum parton virtuality for space-like showers. The larger the value of PARP(67) the more initialstate radiation. Rick Field - Florida/CDF Reference point at 1. 8 Te. V 17

PYTHIA 6. 206 Defaults MPI constant probability scattering PYTHIA default parameters Parameter 6. 115 6. 125 6. 158 6. 206 MSTP(81) 1 1 MSTP(82) 1 1 PARP(81) 1. 4 1. 9 PARP(82) 1. 55 2. 1 1. 9 PARP(89) 1, 000 PARP(90) 0. 16 4. 0 1. 0 PARP(67) 4. 0 Æ Plot shows the “Transverse” charged particle density versus PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA 6. 206 (PT(hard) > 0) using the default parameters for multiple parton interactions and CTEQ 3 L, CTEQ 4 L, and CTEQ 5 L. Note Change PARP(67) = 4. 0 (< 6. 138) PARP(67) = 1. 0 (> 6. 138) MC Tools for the LHC CERN July 31, 2003 Default parameters give very poor description of the “underlying event”! Rick Field - Florida/CDF 20

Tuned PYTHIA 6. 206 Tune A CDF Double Gaussian Run 2 Default! PYTHIA 6. 206 CTEQ 5 L Parameter Tune B Tune A MSTP(81) 1 1 MSTP(82) 4 4 PARP(82) 1. 9 Ge. V 2. 0 Ge. V PARP(83) 0. 5 PARP(84) 0. 4 PARP(85) 1. 0 0. 9 PARP(86) 1. 0 0. 95 PARP(89) 1. 8 Te. V PARP(90) 0. 25 PARP(67) 1. 0 4. 0 New PYTHIA default (less initial-state radiation) MC Tools for the LHC CERN July 31, 2003 Æ Plot shows the “Transverse” charged particle density versus PT(chgjet#1) compared to the QCD hard scattering predictions of two tuned versions of PYTHIA 6. 206 (CTEQ 5 L, Set B (PARP(67)=1) and Set A (PARP(67)=4)). Old PYTHIA default (more initial-state radiation) Rick Field - Florida/CDF 21

Azimuthal Correlations PYTHIA Tune B (less initial-state radiation) PYTHIA Tune A (more initial-state radiation) Æ Predictions of PYTHIA 6. 206 (CTEQ 5 L) with PARP(67)=1 (new default) and PARP(67)=4 (old default) for the azimuthal angle, Df, between a b-quark with PT 1 > 15 Ge. V/c, |y 1| < 1 and bbar-quark with PT 2 > 10 Ge. V/c, |y 2|<1 in proton-antiproton collisions at 1. 8 Te. V. The curves correspond to ds/d. Df (mb/o) for flavor creation, flavor excitation, shower/fragmentation, and the resulting total. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 22

Azimuthal Correlations PYTHIA Tune A (more initial-state radiation) Æ Predictions of HERWIG 6. 4 (CTEQ 5 L) for the azimuthal angle, Df, between a b-quark with PT 1 > 15 Ge. V/c, |y 1| < 1 and bbar-quark with PT 2 > 10 Ge. V/c, |y 2|<1 in proton-antiproton collisions at 1. 8 Te. V. The curves correspond to ds/d. Df (mb/o) for flavor creation, flavor excitation, shower/fragmentation, and the resulting total. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF PYTHIA Tune B (less initial-state radiation) “Flavor Creation” 23

Di. Photon Correlations Æ Predictions of PYTHIA 6. 158 (CTEQ 5 L) with PARP(67)=1 (new default) and PARP(67)=4 (old default) for diphoton system PT and the azimuthal angle, Df, between a photon with PT 1 > 12 Ge. V/c, |y 1| < 0. 9 and photon with PT 2 > 12 Ge. V/c, |y 2|< 0. 9 in protonantiproton collisions at 1. 8 Te. V compared with CDF data. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 24

Tuned PYTHIA 6. 206 “Transverse” PT Distribution PT(charged jet#1) > 30 Ge. V/c PARP(67)=4. 0 (old default) is favored over PARP(67)=1. 0 (new default)! Can we distinguish between PARP(67)=1 and PARP(67)=4? No way! Right! Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1) and the PT distribution of the “transverse” density, d. Nchg/dhdfd. PT with the QCD Monte-Carlo predictions of two tuned versions of PYTHIA 6. 206 (PT(hard) > 0, CTEQ 5 L, Set B (PARP(67)=1) and Set A (PARP(67)=4)). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 25

Tuned PYTHIA 6. 206 Run 1 Tune A Describes the rise from “Min-Bias” to “underlying event”! Set A PT(charged jet#1) > 30 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 60 “Min-Bias” Set A Min-Bias <d. Nchg/dhdf> = 0. 24 Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1) and the PT distribution of the “transverse” and “Min-Bias” densities with the QCD Monte-Carlo predictions of a tuned version of PYTHIA 6. 206 (PT(hard) > 0, CTEQ 5 L, Set A). Describes “Min-Bias” collisions! Describes the “underlying event”! MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 27

“Transverse” Charged Particle Density “Transverse” region as defined by the leading “charged particle jet” Excellent agreement between Run 1 and 2! Æ Shows the data on the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) as a function of the transverse momentum of the leading charged particle jet from Run 1. Æ Compares the Run 2 data (Min-Bias, JET 20, JET 50, JET 70, JET 100) with Run 1. Æ The errors on the (uncorrected) Run 2 data include both statistical and Tune correlated PYTHIA A was tuned to fit the “underlying event” in Run I! systematic uncertainties. Shows the prediction of PYTHIA Tune A at 1. 96 Te. V after detector simulation (i. e. after CDFSIM). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 30

“Transverse” Charged PTsum Density “Transverse” region as defined by the leading “charged particle jet” Æ Shows the Excellent agreement between Run average 1 and 2! data on the “transverse” charged PTsum density (|h|<1, PT>0. 5 Ge. V) as a function of the transverse momentum of the leading charged particle jet from Run 1. Æ Compares the Run 2 data (Min-Bias, JET 20, JET 50, JET 70, JET 100) with Run 1. The errors on the (uncorrected) Run 2 data include both statistical and correlated systematic PYTHIA Tune A was tuned to fit the “underlying event” in Run I! uncertainties. Æ Shows the prediction of PYTHIA Tune A at 1. 96 Te. V after detector simulation (i. e. after CDFSIM). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 31

Charged Particle Density “Transverse” PT Distribution Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1)agreement with the Excellent PT distribution of the between “transverse” Run 1 and 2! density, d. Nchg/dhdfd. PT. Shows how the “transverse” charge particle density is distributed in PT. MC Tools for the LHC CERN July 31, 2003 Æ Compares the Run 2 data (Min-Bias, JET 20, JET 50, JET 70, JET 100) with Run 1. Rick Field - Florida/CDF 32

Charged Particle Density “Transverse” PT Distribution 70 < PT(charged jet#1) > 95 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 62 30 < PT(charged jet#1) < 50 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 59 Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1) with the PT distribution of the “transverse” density, d. Nchg/dhdfd. PT. Æ Shows the prediction of PYTHIA Tune A at 1. 96 Te. V after detector simulation (i. e. after CDFSIM). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 33

“Underlying Event” as defined by “Calorimeter Jets” “Transverse” region is very sensitive to the “underlying event”! Away-side “jet” (sometimes) Charged Particle Df Correlations PT > 0. 5 Ge. V/c |h| < 1 Look at the charged particle density in the “transverse” region! Perpendicular to the plane of the 2 -to-2 hard scattering Æ Look at charged particle correlations in the azimuthal angle Df relative to the leading Æ Æ Jet. Clu jet. Define |Df| < 60 o as “Toward”, 60 o < |Df| < 120 o as “Transverse”, and |Df| > 120 o as “Away”. All three regions have the same size in h-f space, Dhx. Df = 2 x 120 o = 4 p/3. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 34

“Transverse” Charged Particle Density “Transverse” region as defined by the leading “calorimeter jet” Æ Shows the data on the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) as a function of the transverse energy of the leading Jet. Clu jet (R = 0. 7, |h(jet)| < 2) from Run 2. , compared with PYTHIA Tune A after CDFSIM. Æ Compares the “transverse” region of the leading “charged particle jet”, chgjet#1, with the “transverse” region of the leading “calorimeter jet” (Jet. Clu R = 0. 7), jet#1. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 35

“Transverse” Charged PTsum Density “Transverse” region as defined by the leading “calorimeter jet” Æ Shows the data on the average “transverse” charged PTsum density (|h|<1, PT>0. 5 Ge. V) as a function of the transverse energy of the leading Jet. Clu jet (R = 0. 7, |h(jet)| < 2) from Run 2. , compared with PYTHIA Tune A after CDFSIM. Æ Compares the “transverse” region of the leading “charged particle jet”, chgjet#1, with the “transverse” region of the leading “calorimeter jet” (Jet. Clu R = 0. 7), jet#1. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 36

Charged Particle Density “Transverse” PT Distribution 95 < ET(jet#1) > 130 Ge. V “Transverse” <d. Nchg/dhdf> = 0. 65 30 < ET(jet#1) < 70 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 61 Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus ET(jet#1) with the PT distribution of the “transverse” density, d. Nchg/dhdfd. PT. Æ Shows the prediction of PYTHIA Tune A at 1. 96 Te. V after detector simulation (i. e. after CDFSIM). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 37

Charged Particle Density “Transverse” PT Distribution 30 < PT(charged jet#1) < 50 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 59 30 < ET(jet#1) < 70 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 61 Æ Compares the average “transverse” as defined by “calorimeter jets” (Jet. Clu R = 0. 7) with the “transverse” region defined by “charged particle jets”. Æ Shows the prediction of PYTHIA Tune A at 1. 96 Te. V after detector simulation (i. e. after CDFSIM). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 38

Relationship Between “Calorimeter” and “Charged Particle” Jets Æ Shows the “matched” Jet. Clu jet ET versus the transverse momentum of the leading “charged particle jet” (closest jet within R = 0. 7 of the leading chgjet). Æ Shows the ratio of PT(chgjet#1) to the “matched” Jet. Clu jet ET versus PT(chgjet#1). The leading chgjet from a Æ Shows the EM fraction of comes the “matched” Jet. Clu jet that is, on the average, Jet. Clu jet and the EM fraction about 90% charged!of a typical Jet. Clu jet. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 39

Tuned PYTHIA (Set A) LHC Predictions Factor of 2! Æ Shows the average “transverse” charge particle and PTsum density (|h|<1, PT>0) versus PT(charged jet#1) predicted by HERWIG 6. 4 (PT(hard) > 3 Ge. V/c, CTEQ 5 L). and a tuned versions of PYTHIA 6. 206 (PT(hard) > 0, CTEQ 5 L, Set A) at 1. 8 Te. V and 14 Te. V. Æ At 14 Te. V tuned PYTHIA (Set A) predicts roughly 2. 3 charged particles per unit h-f (PT > 0) Æ in the “transverse” region (14 charged particles per unit h) which is larger than the HERWIG prediction. At 14 Te. V tuned PYTHIA (Set A) predicts roughly 2 Ge. V/c charged PTsum per unit h-f (PT > 0) in the “transverse” region at PT(chgjet#1) = 40 Ge. V/c which is a factor of 2 larger than at 1. 8 Te. V and much larger than the HERWIG prediction. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 40

Tuned PYTHIA (Set A) LHC Predictions Big difference! Æ Shows the average “transverse” charge particle and PTsum density (|h|<1, PT>0) versus Æ PT(charged jet#1) predicted by HERWIG 6. 4 (PT(hard) > 3 Ge. V/c, CTEQ 5 L). and a tuned versions of PYTHIA 6. 206 (PT(hard) > 0, CTEQ 5 L, Set A) at 1. 8 Te. V and 14 Te. V. Also shown is the 14 Te. V prediction of PYTHIA 6. 206 with the default value e = 0. 16. Tuned PYTHIA (Set A) predicts roughly 2. 5 Ge. V/c per unit h-f (PT > 0) from charged particles in the “transverse” region for 3. 8 Ge. V/c (charged) PT(chgjet#1) = 100 Ge. V/c. Note, however, that the “transverse” in cone of radius R=0. 7 charged PTsum density increases rapidly as PT(chgjet#1) at 14 Te. V increases. MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 42

Tuned PYTHIA (Set A) LHC Predictions LHC? Æ Shows the center-of-mass energy dependence of the charged particle density, d. Nchg/dhdf, for “Min-Bias” collisions compared with the a tuned version of PYTHIA 6. 206 (Set A) with PT(hard) > 0. Æ PYTHIA was tuned to fit the “underlying event” in hard-scattering processes at 1. 8 Te. V and 630 Ge. V. Æ PYTHIA (Set A) predicts a 42% rise in d. Nchg/dhdf at h = 0 in going from the Tevatron (1. 8 Te. V) to the LHC (14 Te. V). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 43

Tuned PYTHIA (Set A) LHC Predictions 12% of “Min-Bias” events have PT(hard) > 10 Ge. V/c! LHC? Æ Shows the center-of-mass energy dependence 1% of “Min-Bias” events have PT(hard) > 10 Ge. V/c! of the charged particle density, d. Nchg/dhdfd. PT, for “Min-Bias” collisions compared with the a tuned version of PYTHIA 6. 206 (Set A) with PT(hard) > 0. Æ This PYTHIA fit predicts that 1% of all “Min-Bias” events at 1. 8 Te. V are a result of a hard 2 -to-2 parton-parton scattering with PT(hard) > 10 Ge. V/c which increases to 12% at 14 Te. V! MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 44

Summary & Conclusions The “Underlying Event” Æ There is excellent agreement between the Run 1 and the Run 2. The Æ Æ “underlying event” is the same in Run 2 as in Run 1 but now we can study the evolution out to much higher energies! Also see Mario’s Runin 2 the PYTHIA Tune A does a good job of describing the “underlying event” flow” analysis! Run 2 data as defined by “charged particle jets” and as“energy defined by “calorimeter jets”. HERWIG Run 2 comparisons will be coming soon! Lots more CDF Run 2 data to come including MAX/MIN “transverse” and MAX/MIN “cones”. Also, more to come on the energy in the “underlying event”! MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 46

LHC Predictions Summary & Conclusions Tevatron LHC 12 times more likely Æ Æ “Min-Bias” at the LHC contains to find a 10 Ge. V much more hard (Set collisions than at the Both HERWIG and the tuned PYTHIA A) predict a 40 -45% rise inind. N “jet” “Min-Bias” chg/dhdf at h Tevatron! At the at theparticles LHC! = 0 in going from the Tevatron (1. 8 Te. V) to Tevatron the LHCthe (14 Te. V). 4 charged per “underlying event” factor of 2 unit h at the Tevatron becomes 6 per unit h is atathe LHC. more active than “Tevaron Min-Bias”. Twice as much The tuned PYTHIA (Set. At A)the predicts that 1% of allevent” “Min-Bias” events at the Tevatron LHC the “underlying will activity in the (1. 8 Te. V) are the result of a hard > 10 be at 2 -to-2 least a parton-parton factor of 2 more scattering with PT(hard) “underlying event” Ge. V/c which increases to 12% at LHC (14 Te. V)! active than “LHC Min-Bias”! at the LHC! Æ For the “underlying event” in hard scattering processes the predictions of HERWIG and Æ the tuned PYTHIA (Set A) differ greatly (factor of 2!). HERWIG predicts a smaller increase in the activity of the “underlying event” in going from the Tevatron to the LHC. The tuned PYTHIA (Set A) predicts about a factor of two increase at the LHC in the charged PTsum density of the “underlying event” at the same PT(jet#1) (the “transverse” charged PTsum density increases rapidly as PT(jet#1) increases). MC Tools for the LHC CERN July 31, 2003 Rick Field - Florida/CDF 47