Toward an Understanding of the Overall Event Structure of Hard Collisions

Feynman Festival August 23, 2002

Rick Field - Florida/CDF

Toward an Understanding ofToward an Understanding ofthe Overall Event Structurethe Overall Event Structure

of Hard Collisionsof Hard Collisions

The Past: Feynman-Field Fenomenology (1973-1980).

The Present: Studying the “Underlying Event” at CDF.

Outline of Talk

7 GeV 0’sto

400 GeV “jets”



CDF ColliderCDF ColliderPhenomenologyPhenomenology

I am a theorist working in the CDF experimental collaboration.

Proton

AntiProton

1 mile CDF

Proton AntiProton 2 TeV

I work on collider phenomenology related to CDF.

Only by working in the experimental collaboration am I able to have access to data and do the kind of phenomenology I enjoy (i.e. the kind of phenomenology I did with Feynman many years ago).

Theorist!

In addition to being an exceptional theoretical physicist (and very good at math!), Feynman was

a great phenomenologist and he enjoyed very much talking with experimenters.



“Feynman-Field Jet Model”

Feynman-FieldFeynman-FieldFenomenologyFenomenology

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).

1973-1980

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

My 1st graduate student!



“Feynman-Field Jet Model”

Feynman-FieldFeynman-FieldFenomenologyFenomenology

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).

1973-1980

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

My 1st graduate student!

Many people have contributed to our understanding

of hadron-hadron collisions!I will say a few words about

Feynman’s influence on the field.



Hadron-Hadron CollisionsHadron-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!

Hadron Hadron ???

Hadron Hadron

high PT meson

Parton-Parton Scattering

Outgoing Parton

Outgoing Parton

FF1 1977 (preQCD)

“Black-Box Model”



Hadron-Hadron CollisionsHadron-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!

Hadron Hadron ???

Hadron Hadron

“Soft” Collision (no large transverse momentum)

Hadron Hadron

high PT meson

Parton-Parton Scattering

Outgoing Parton

Outgoing Parton

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 thework 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.”

“Black-Box Model”



Quark-QuarkQuark-QuarkBlack-Box ModelBlack-Box ModelFF1 1977 (preQCD)Quark Distribution Functions

determined from deep-inelasticlepton-hadron collisions

Quark Fragmentation Functionsdetermined from e+e- annihilationsQuark-Quark Cross-Section

Unknown! Deteremined fromhadron-hadron collisions.

No gluons!



Quark-QuarkQuark-QuarkBlack-Box ModelBlack-Box ModelFF1 1977 (preQCD)Quark Distribution Functions

determined from deep-inelasticlepton-hadron collisions

Quark Fragmentation Functionsdetermined from e+e- annihilationsQuark-Quark Cross-Section

Unknown! Deteremined fromhadron-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.”



Quark-QuarkQuark-QuarkBlack-Box ModelBlack-Box Model

FF1 1977 (preQCD)Predict

particle ratios

Predictincrease with increasing

CM energy W

Predictoverall event topology

(FFF1 paper 1977)



Telagram from FeynmanTelagram from Feynman

July 1976

SAW CRONIN AM NOW CONVINCED WERE RIGHT TRACK QUICK WRITEFEYNMAN



Letter from FeynmanLetter from FeynmanJuly 1976



Letter from Feynman:Letter from Feynman:page 1page 1

Spelling?



Letter from Feynman:Letter from Feynman:page 3page 3

It is fun!

Onward!



Napkin from FeynmanNapkin from Feynman



QCD ApproachQCD ApproachQuarks & GluonsQuarks & Gluons

FFF2 1978

Parton Distribution FunctionsQ2 dependence predicted from

QCD

Quark & Gluon Fragmentation Functions

Q2 dependence predicted from QCD

Quark & Gluon Cross-SectionsCalculated from QCD




FFF2 1978

Parton Distribution FunctionsQ2 dependence predicted from

QCD

Quark & Gluon Fragmentation Functions

Q2 dependence predicted from QCD

Quark & Gluon Cross-SectionsCalculated 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.”




FFF2 1978

30 GeV!

Predictlarge “jet”

cross-section

Feynman quote from FFF2:“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.”



CDF Run II DiJet EventCDF Run II DiJet EventJuly 2002July 2002

ETjet1 = 403 GeV

ETjet2 = 322 GeV

Raw ET values!!



Monte-Carlo SimulationMonte-Carlo Simulationof Hadron-Hadron Collisionsof Hadron-Hadron Collisions

Color singlet proton collides with a color singlet antiproton.

Proton AntiProton

Proton AntiProton

color string

color string

Beam Remnants

Beam Remnants

color string

color string

Beam Remnants

Beam Remnants

quark-antiquark pairs

quark-antiquark pairs

Beam Remnants

Beam Remnants

Beam Remnants

Beam Remnants

Jet

Jet

A red quark gets knocked out of the proton and a blue antiquark gets knocked out of the antiproton.

At short times (small distances) the color forces are weak and the outgoing partons move away from the beam-beam remnants.

At long times (large distances) the color forces become strong and quark-antiquark pairs are pulled out of the vacuum and hadrons are formed.

The resulting event consists of hadrons and leptons in the form of two large transverse momentum outgoing jets plus the beam-beam remnants.




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)

PYTHIAtoday

“FF” or “FW” Fragmentation




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)

PYTHIAtoday

“FF” or “FW” Fragmentation

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, ....”



The “Underlying Event” inThe “Underlying Event” inHard Scattering ProcessesHard Scattering Processes

What happens when a proton and an antiproton collide with a center-of-mass energy of 2 TeV?

Proton AntiProton

“Soft” Collision (no hard scattering)

Proton AntiProton

“Hard” Scattering

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event Underlying Event

Initial-State Radiation

Final-State Radiation

Proton AntiProton 2 TeV

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.

Occasionally there will be a “hard” parton-parton collision resulting in large transverse momentum outgoing partons.

Proton AntiProton

“Underlying Event”

Beam-Beam Remnants Beam-Beam Remnants


The “underlying event” is everything except the two outgoing hard scattered “jets”. It is an unavoidable background to many collider observables.



Min-Bias?

Beam-Beam RemnantsBeam-Beam Remnants

The underlying event in a hard scattering process has a “hard” component (particles that arise from initial & final-state radiation and from the outgoing hard scattered partons) and a “soft” component (beam-beam remnants).

However the “soft” component is color connected to the “hard” component so this separation is (at best) an approximation.

Proton AntiProton

“Hard” Collision

initial-state radiation

final-state radiation outgoing parton

outgoing parton

color string

color string

+

“Soft” Component “Hard” Component


final-state radiation outgoing jet

Beam-Beam Remnants

For ISAJET (no color flow) the “soft” and “hard” components are completely independent and the model for the beam-beam remnant component is the same as for min-bias (“cut pomeron”) but with a larger <PT>.

HERWIG breaks the color connection with a soft q-qbar pair and then models the beam-beam remnant component the same as HERWIG min-bias (cluster decay).



Studying the “Underlying Event”Studying the “Underlying Event”

at CDFat CDF

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 between perturbative and non-perturbative physics.

There are two CDF analyses which quantitatively study the underlying event and compare with the QCD Monte-Carlo models.

It is important to model this region well since it is an unavoidable background to all collider observables. Also, we need a good model of min-bias (zero-bias) collisions.

The Underlying Event:beam-beam remnantsinitial-state radiation

multiple-parton interactions

Proton AntiProton

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event Underlying Event


Final-State Radiation

CDFCone AnalysisValeria TanoEve KovacsJoey Huston

Anwar Bhatti

CDFEvolution of Charged Jets

Rick FieldDavid Stuart

Rich Haas

Ph.D. Thesis PRD65:092002, 2002



Evolution of Charged JetsEvolution of Charged Jets“Underlying Event”“Underlying Event”

Charged Jet #1Direction

“Transverse” “Transverse”

“Toward”

“Away”

“Toward-Side” Jet

“Away-Side” Jet

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.


“Toward”


“Away”

-1 +1

2

0

Leading Jet

Toward Region

Transverse Region

Transverse Region

Away Region

Away Region

Charged Particle Correlations PT > 0.5 GeV/c || < 1



Charged Multiplicity Charged Multiplicity versus Pversus PTT(chgjet#1)(chgjet#1)

Data on the average number of “toward” (||<60o), “transverse” (60<||<120o), and “away” (||>120o) charged particles (PT > 0.5 GeV, || < 1, including jet#1) as a function of the transverse momentum of the leading charged particle jet. Each point corresponds to the <Nchg> in a 1 GeV bin. The solid (open) points are the Min-Bias (JET20) data. The errors on the (uncorrected) data include both statistical and correlated systematic uncertainties.


“Toward”


“Away”

Underlying Event“plateau”

Nchg versus PT(charged jet#1)

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40 45 50

PT(charged jet#1) (GeV/c)

<N

ch

g>

in

1 G

eV

/c b

in

1.8 TeV ||<1.0 PT>0.5 GeV

"Toward"

"Away"

"Transverse"

CDF Preliminarydata uncorrected

Factor of 2 more active than an average Min-Bias event!



ISAJET: ISAJET: “Transverse” Nchg “Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)

Plot shows the “transverse” <Nchg> vs PT(chgjet#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).

Beam-BeamRemnants

ISAJETCharged Jet #1

Direction

“Toward”


“Away”

"Transverse" Nchg versus PT(charged jet#1)

0

1

2

3

4

0 5 10 15 20 25 30 35 40 45 50


"Tra

ns

ve

rse

" <

Nc

hg

> i

n 1

Ge

V/c

bin

1.8 TeV ||<1.0 PT>0.5 GeV

CDF Preliminarydata uncorrectedtheory corrected

Beam-Beam Remnants

Isajet Total

Hard Component

Outgoing Jetsplus

Initial & Final-StateRadiation



HERWIG: “Transverse” Nchg HERWIG: “Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)

Plot shows the “transverse” <Nchg> vs PT(chgjet#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).

Beam-BeamRemnants

Outgoing Jetsplus

Initial & Final-StateRadiation

HERWIG


“Toward”


“Away”


0

1

2

3

4

0 5 10 15 20 25 30 35 40 45 50

PT (charged jet#1) (GeV/c)

"Tra

ns

ve

rse

" <

Nc

hg

> i

n 1

Ge

V/c

bin

1.8 TeV ||<1.0 PT>0.5 GeV Beam-Beam Remnants

Hard Component

CDF Preliminarydata uncorrectedtheory corrected

Herwig Total



MPI: Multiple PartonMPI: Multiple PartonInteractionsInteractions

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”.

Proton AntiProton

Multiple Parton Interaction



outgoing parton

color string

color string

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).

+

“Semi-Hard” MPI “Hard” Component


final-state radiation outgoing jet Beam-Beam Remnants

or

“Soft” Component

Proton AntiProton

“Hard” Collision



outgoing parton



PYTHIA: Multiple PartonPYTHIA: Multiple PartonInteractionsInteractions

Pythia uses multiple partoninteractions to enhacethe underlying event.

Proton AntiProton

Multiple Parton Interactions

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying EventUnderlying Event

Parameter Value

Description

MSTP(81) 0 Multiple-Parton Scattering off

1 Multiple-Parton Scattering on

MSTP(82) 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 turn-off PT0=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 PT0=PARP(82)

Hard Core

Multiple parton interaction more likely in a hard

(central) collision!

and now HERWIG

!

Herwig MPIJ. M. Butterworth

J. R. ForshawM. H. Seymour



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

PYTHIA 6.206 DefaultsPYTHIA 6.206 Defaults

PYTHIA default parameters

ConstantProbabilityScattering


0

1

2

3

4

5

0 5 10 15 20 25 30 35 40 45 50

PT(charged jet#1) (GeV/c)"T

ran

sver

se"

<N

chg

>

CTEQ3L CTEQ4L CTEQ5L CDF Min-Bias CDF JET20

CDFdata uncorrectedtheory corrected

1.8 TeV ||<1.0 PT>0.5 GeV/c

Pythia 6.206 (default)MSTP(82)=1

PARP(81) = 1.9 GeV/c

Default parameters give very poor description of the “underlying event”!

Plot shows “Transverse” <Nchg> 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.



Parameter Tune 1 Tune 2

MSTP(81) 1 1

MSTP(82) 3 3

PARP(82) 1.6 GeV 1.7 GeV

PARP(85) 1.0 1.0

PARP(86) 1.0 1.0

PARP(89) 1.8 TeV 1.8 TeV

PARP(90) 0.16 0.16

PARP(67) 1.0 4.0

Old PYTHIA default(less initial-state radiation)

New PYTHIA default(less initial-state radiation)

Tuned PYTHIA 6.206Tuned PYTHIA 6.206


0

1

2

3

4

0 5 10 15 20 25 30 35 40 45 50


"Tra

nsv

erse

" <

Nch

g>

in

1 G

eV/c

bin CDF

data uncorrectedtheory corrected

1.8 TeV ||<1.0 PT>0.5 GeV CTEQ5L

Tuned PYTHIA 6.206PARP(67)=1

Tuned PYTHIA 6.206PARP(67)=4

Can we distinguish between PARP(67)=1 and PARP(67)=4?

Plot shows “Transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard scattering predictions of two tuned versions of PYTHIA 6.206 (CTEQ5L, PARP(67)=1 and PARP(67)=4).

PYTHIA 6.206 CTEQ5L

Old PYTHIA default(less initial-state radiation)

New PYTHIA default(less initial-state radiation)



Collider PhenomenologyCollider PhenomenologyFrom 7 GeV/c From 7 GeV/c oo’s to 400 GeV “Jets”’s to 400 GeV “Jets”

FF1 (1977) 7 GeV/c 0’s

NLO QCD (2002)400 GeV “jets”



Collider PhenomenologyCollider PhenomenologyFrom 7 GeV/c From 7 GeV/c oo’s to 400 GeV “Jets”’s to 400 GeV “Jets”

FF1 (1977) 7 GeV/c 0’s

NLO QCD (2002)400 GeV “jets”

Rick Field (Feynman Festival):“At the time of this writing,

there is still no sharp quantitative test of QCD.

We believe it is the correcttheory of strong interactions

because it qualitatively describes an enormous variety and amount of data over many decades of Q2.”

Feynman played an enormous role in our understanding of hadron-hadron collisions

and his influence is still being felt!

Toward an Understanding of the Overall Event Structure of Hard Collisions

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