# Presentation: 712905

## Toward an Understanding ofHadron-Hadron Collisions

**Rick Field**
**University of Florida**

_**Outline of Talk**_

***LBNL January 15, 2009***

**CMS at the LHC**

**CDF Run 2**

**The early days of Feynman-Field Phenomenology.**

**Studying “min-bias” collisions and the “underlying event” at CDF.**

**Extrapolations to the LHC.**

**From Feynman-Field to the LHC**

**Before Feynman-Field Phenomenology.**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Before Feynman-Field

**Rick Field 1964**

_**R. D. Field**_
**University of California, Berkeley, 1962-66 (*****undergraduate*****)**
**University of California, Berkeley, 1966-71 (*****graduate student*****)**

**me**

**My sister Sally!**

**The very first “Berkeley Physics Course”!**

**My Ph.D. advisor!**

**J.D.J**

**Bob Cahn**

**Chris Quigg**

**me**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Before Feynman-Field

**Rick & Jimmie**
**1968**

**Rick & Jimmie**
**1970**

**Rick & Jimmie**
**1972 (*****pregnant!*****)**

**Rick & Jimmie at CALTECH 1973**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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**

**and**

**Field**

**1****st**** hat!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## The Feynman-Field Days

**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, 2590-2616 (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, 997-1000 (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, 3320-3343 (1978).**

**1973-1983**

**FW1: “A QCD Model for e****+****e****-**** Annihilation”****, R. D. Field and S. Wolfram,  Nucl. Phys. B213, 65-84 (1983).**

**My 1****st**** graduate student!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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!  **

**FF1 1977 (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”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Quark-Quark Black-Box Model

**Quark****-****Quark Black****-****Box Model**

**FF1 1977 (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.”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Quark-Quark Black-Box Model

**FF1 1977 (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!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Telagram from Feynman

**July 1976**

**SAW CRONIN AM NOW CONVINCED WERE RIGHT TRACK QUICK WRITE**
**FEYNMAN**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Letter from Feynman

**July 1976**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Letter from Feynman Page 1

- Spelling?

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Letter from Feynman Page 3

**It is fun!**

**Onward!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Feynman Talk at Coral Gables (December 1976)

**“****Feynman-Field **
**Jet Model”**

**1****st**** transparency**

**Last transparency**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## QCD Approach: Quarks & Gluons

**FFF2 1978**

_**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.”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## A Parameterization of the Properties of Jets

**Assumed that jets could be analyzed on a “recursive” principle.**

**Field-Feynman 1978**

**Original quark with flavor “a” and momentum P****0**

**Let f(********)d******** be the probability that the rank 1 meson *****leaves***** fractional momentum ******** to the remaining cascade, leaving quark “b” with momentum P****1**** = ********1****P****0****.**

**Primary Mesons**

**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(********)), leaving quark “c” with momentum P****2**** = ********2****P****1 ****= ********2********1****P****0****.**

**Add in flavor dependence by letting ********u**** = probabliity of producing u-ubar pair, ********d**** = probability of producing d-dbar pair, etc.**

**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.**

**Rank 2**

**continue**

**Calculate F(z) from f(********) and ********i****!**

**Rank 1**

**Secondary Mesons**
**(after decay)**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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, ....”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Monte-Carlo Simulationof Hadron-Hadron Collisions

_**FF1-FFF1 (1977)**_** “Black-Box” Model**

_**F1-FFF2 (1978)**_** QCD Approach**

_**FF2 (1978)**_** **
**Monte-Carlo simulation of “jets”**

_**FFFW “FieldJet” (1980)**_** **
**QCD “leading-log order” simulation **
**of hadron-hadron collisions**

_**ISAJET**_
**(“FF” Fragmentation)**

_**HERWIG**_
**(“FW” Fragmentation)**

_**PYTHIA**_

**today**

**“****FF” or “FW” Fragmentation**

**the past**

**tomorrow**

_**SHERPA**_

_**PYTHIA 6.4**_

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## High PT Jets

**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.”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## CDF DiJet Event: M(jj) ≈ 1.4 TeV 

**E****T****jet1**** = 666 GeV  E****T****jet2**** = 633 GeV **
**E****sum**** = 1,299 GeV M(jj) = 1,364 GeV**

**M(jj)/E****cm**** ****≈ 70%!!**** **

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## The Fermilab Tevatron

**I joined CDF in January 1998.**

**CDF “SciCo” Shift December 12-19, 2008**

**Acquired 4728 nb****-1**** during 8 hour “owl” shift!**

**My wife Jimmie on shift with me!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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**** **

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## QCD Monte-Carlo Models:High Transverse Momentum Jets

**Start with the perturbative 2-to-2 (or sometimes 2-to-3) parton-parton scattering and add initial and final-state gluon radiation (in the leading log approximation or modified leading log approximation). **

**“****Underlying Event”**

**The “underlying event” consists of the “beam-beam remnants” and from particles arising from soft or semi-soft multiple parton interactions (MPI).**

**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!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Study the charged particles (pT > 0.5 GeV/c, || < 1) and form the charged particle density, dNchg/dd, and the charged scalar pT sum density, dPTsum/dd. 

**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**

|  | CDF Run 2 “Min-Bias”
Observable | Average | Average Density
per unit - |
| --- | --- | --- | --- |
| Nchg | Number of Charged Particles
(pT > 0.5 GeV/c, || < 1) | 3.17 +/- 0.31 | 0.252 +/- 0.025 |
| PTsum (GeV/c) | Scalar pT sum of Charged Particles
(pT > 0.5 GeV/c, || < 1) | 2.97 +/- 0.23 | 0.236 +/- 0.018 |

**CDF Run 2 “Min-Bias”**

**Particle Densities**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## CDF Run 1 Min-Bias “Associated”Charged Particle Density

**“****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 1 Min-Bias “Associated”****Charged Particle Density**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## CDF Run 1 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, |********| < 1, *****not including PTmax*****) relative to PTmax (rotated to 180****o****) for ****“min-bias” events**** ****with PTmax > 0.5, 1.0, and 2.0 GeV/c.**

**Transverse Region**

**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 1 Min-Bias “Associated”****Charged Particle Density**

**Rapid rise in the particle density in the “transverse” region as PTmax increases!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Look at charged particle correlations in the azimuthal angle relative to the leading charged particle jet.
Define || < 60o as “Toward”, 60o < || < 120o as “Transverse”, and || > 120o as “Away”.
All three regions have the same size in - space, x = 2x120o = 4/3.

**Look at charged particle correlations in the azimuthal angle ********relative to the leading charged particle jet.**
**Define ****|********| < 60****o**** as “Toward”****, ****60****o**** < |********| < 120****o**** as “Transverse”****, and ****|********| > 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”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Compares the average “transverse” charge particle density with the average “Min-Bias” charge particle density (||<1, pT>0.5 GeV).  Shows how the “transverse” charge particle density and the Min-Bias charge particle density is distributed in pT.

**Compares the average “transverse” charge particle density with the average “Min-Bias” charge particle density (|********|<1, p****T****>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**
**<dN****chg****/d********d********> = 0.25**

**P****T****(charged jet#1) > 30 GeV/c**
**“****Transverse” <dN****chg****/d********d********> = 0.56**

**“****Min-Bias”**

**Run 1 Charged Particle Density**** ****“Transverse” p****T**** Distribution**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Plot shows average “transverse” charge particle density (||<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).

**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! **

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Plot shows average “transverse” charge particle density (||<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).

**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**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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****).**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Tuning PYTHIA:Multiple Parton Interaction Parameters

| Parameter  | 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)  with           = 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**

**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**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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 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)**

| Parameter | 6.115 | 6.125 | 6.158 | 6.206 |
| --- | --- | --- | --- | --- |
| MSTP(81) | 1 | 1 | 1 | 1 |
| MSTP(82) | 1 | 1 | 1 | 1 |
| PARP(81) | 1.4 | 1.9 | 1.9 | 1.9 |
| PARP(82) | 1.55 | 2.1 | 2.1 | 1.9 |
| PARP(89) |  | 1,000 | 1,000 | 1,000 |
| PARP(90) |  | 0.16 | 0.16 | 0.16 |
| PARP(67) | 4.0 | 4.0 | 1.0 | 1.0 |

**Plot shows the ****“Transverse” charged particle density versus P****T****(chgjet#1)**** compared to the QCD hard scattering predictions of ****PYTHIA 6.206**** **** (P****T****(hard) > 0) using the ****default**** parameters for multiple parton interactions and CTEQ3L, CTEQ4L, and CTEQ5L.**

**PYTHIA default parameters**

**PYTHIA 6.206 Defaults**

**MPI constant**
**probability**
**scattering**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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  (CTEQ5L, Set B (PARP(67)=1) and Set A (PARP(67)=4)).

**(more initial-state radiation)**

**New PYTHIA default**
**(less initial-state radiation)**

| Parameter | Tune B | Tune A |
| --- | --- | --- |
| MSTP(81) | 1 | 1 |
| MSTP(82) | 4 | 4 |
| PARP(82) | 1.9 GeV | 2.0 GeV |
| PARP(83) | 0.5 | 0.5 |
| PARP(84) | 0.4 | 0.4 |
| PARP(85) | 1.0 | 0.9 |
| PARP(86) | 1.0 | 0.95 |
| PARP(89) | 1.8 TeV | 1.8 TeV |
| PARP(90) | 0.25 | 0.25 |
| PARP(67) | 1.0 | 4.0 |

**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 6.206**** **** (CTEQ5L, ****Set B**** (PARP(67)=1)**** ****and**** ****Set A**** ****(PARP(67)=4)).**

**Run 1 Analysis**

**Run 1 PYTHIA Tune A**

**PYTHIA 6.206 CTEQ5L**

**CDF Default!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Shows the data on the average “transverse” charge particle density (||<1, pT>0.5 GeV) as a function of the transverse momentum of the leading charged particle jet from Run 1.

**Shows the data on the average “transverse” charge particle density (|********|<1, p****T****>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”**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## 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**

**Tune A**

**Tune AW**

**Tune B**

**Tune BW**

**Tune D**

**Tune DW**

**Tune D6**

**Tune D6T**

**These are “old” PYTHIA 6.2 tunes!  **
**There are new 6.4 tunes by**
**Arthur Moraes (ATLAS)**
**Hendrik Hoeth (MCnet)**
**Peter Skands (Tune S0)**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Min-Bias Correlations

**Min-Bias Correlations**

**Data at 1.96 TeV on the ****average p****T**** of charged particles versus the number of charged particles**** (p****T**** > 0.4 GeV/c, |********| < 1) for “min-bias” collisions at CDF Run 2.  The data are corrected to the particle level and are compared with PYTHIA Tune A at the particle level (*****i.e.***** generator level).**

- New

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Min-Bias: Average PT versus Nchg

- =

- +

- +

**Beam-beam remnants (*****i.e.***** soft hard core) produces low multiplicity and small <p****T****> with ****<p****T****> independent of the multiplicity****.**

**Hard scattering (with no MPI) produces large multiplicity and large <p****T****>. **

**Hard scattering (with MPI) produces large multiplicity and medium <p****T****>. **

**The CDF “min-bias” trigger picks up most of the “hard core” component!**

**This observable is sensitive to the MPI tuning!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Average PT versus Nchg

**Average PT versus Nchg**

**Data at 1.96 TeV on the ****average p****T**** of charged particles versus the number of charged particles**** (p****T**** > 0.4 GeV/c, |********| < 1) for ****“min-bias” collisions**** at CDF Run 2.  The data are corrected to the particle leveland are compared with PYTHIA Tune A, Tune DW, and the ATLAS tune at the particle level (*****i.e.***** generator level).**

**Particle level predictions for the ****average p****T**** of charged particles versus the number of charged particles**** (p****T**** > 0.5 GeV/c, |********| < 1, excluding the lepton-pair) for for ****Drell-Yan production**** (70 < M(pair) < 110 GeV) at CDF Run 2.  **

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Average PT versus Nchg

**Average PT versus Nchg**

- =

- +

**Z-boson production (with low p****T****(Z) and no MPI) produces low multiplicity and small <p****T****>.**

- +

**High p****T**** Z-boson production produces large multiplicity and high <p****T****>.**

**Z-boson production (with MPI) produces large multiplicity and medium <p****T****>.**

**No MPI!**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Average PT(Z) versus Nchg

**Average PT(Z) versus Nchg**

**Data on the ****average p****T**** of charged particles versus the number of charged particles**** (p****T**** > 0.5 GeV/c, |********| < 1, excluding the lepton-pair) for for ****Drell-Yan production**** (70 < M(pair) < 110 GeV) at CDF Run 2. The data are corrected to the particle level and are compared with various Monte-Carlo tunes at the particle level (*****i.e.***** generator level).**

**No MPI!**

**Predictions for the ****average P****T****(Z-Boson) versus the number of charged particles**** (p****T**** > 0.5 GeV/c, |********| < 1, excluding the lepton-pair) for for ****Drell-Yan production**** (70 < M(pair) < 110 GeV) at CDF Run 2. **

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## Average PT versus Nchg

**Average PT versus Nchg**

**Predictions for the ****average p****T**** of charged particles versus the number of charged particles**** (p****T**** > 0.5 GeV/c, |********| < 1, excluding the lepton-pair) for for ****Drell-Yan production**** (70 < M(pair) < 110 GeV, ****P****T****(pair) < 10 GeV/c****) at CDF Run 2. **

**Data the ****average p****T**** of charged particles versus the number of charged particles**** (p****T**** > 0.5 GeV/c, |********| < 1, excluding the lepton-pair) for for ****Drell-Yan production**** (70 < M(pair) < 110 GeV, ****P****T****(pair) < 10 GeV/c****) at CDF Run 2. The data are corrected to the particle level and are compared with various Monte-Carlo tunes at the particle level (*****i.e.***** generator level).**

**P****T****(Z) < 10 GeV/c**

**No MPI!**

**Remarkably similar behavior! Perhaps indicating that MPI playing an important role in both processes.**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009

## UE&MB@CMS

**UE&MB@CMS**

**“****Underlying Event” Studies:**** The “transverse region” in ****“leading Jet”**** and ****“back-to-back”**** charged particle jet production and the “central region” in ****Drell-Yan production****.  (requires ****charged tracks**** and**** ****muons**** for Drell-Yan)**

**Drell-Yan Studies:**** Transverse momentum distribution of the lepton-pair versus the mass of the lepton-pair, <p****T****(pair)>, <p****T****2****(pair)>, d********/dp****T****(pair) (****only requires muons****).  Event structure for large lepton-pair p****T**** (*****i.e.***** ******** +jets, requires ****muons****).  **

**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****)**

**UE&MB@CMS**

**Study the “underlying event” by **
**using charged particles and muons!**
**(*****start as soon as possible*****)**

**<p****T****(********+********-****)> is much larger at the LHC!**

**Shapes of the p****T****(********+********-****) distribution at the Z-boson mass.**

- Lawrence Berkeley Laboratory                                                                   January 15, 2009