University of Perugia - Lecture 1 March 30, 2006 Rick Field – Florida/CDF/CMSPage 1 Toward an Understanding of Hadron-Hadron Collisions Lecture 1: From

University of Perugia - Lecture 1 March 30, 2006

Rick Field – Florida/CDF/CMS Page 1

Toward an Understanding of Toward an Understanding of Hadron-Hadron CollisionsHadron-Hadron Collisions

Lecture 1: From Field-Feynman to the Tevatron.

Lecture 2: A Detailed Study of the “Underlying Event” at the Tevatron.

From 7 GeV/c 0’s to 600 GeV/c Jets!

Proton AntiProton

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event Underlying Event

Initial-State Radiation

Final-State Radiation

QCD Monte-Carlo Models (PYTHIA Tune A).

“Min-Bias” Collisions at the Tevatron.

→ extrapolations to the LHC!

QCD Monte-Carlo Models tunes at the Tevatron. → extrapolations to the LHC!

CMS at the LHCUniversity of Perugia

UE&MB@CMSUE&MB@CMS

Florida-Perugia



“Feynman-Field Jet Model”

FeynmanFeynman-Field-Field PhenomenologyPhenomenology

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!



Hadron-Hadron Hadron-Hadron CollisionsCollisions

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-QuarkBlackBlack--Box ModelBox 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)

“Beam-Beam Remnants”

7 GeV/c 0’s!



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!



Feynman Talk at Coral Feynman Talk at Coral Gables in December 1976Gables in December 1976

“Feynman-Field Jet Model”

1st transparency Last transparency



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

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.”



High PHigh PTT Jets Jets

30 GeV/c!

Predictlarge “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 PT discussed here.”



A Parameterization of A Parameterization of the Properties of Jetsthe Properties of Jets

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

Field-Feynman 1978

Original quark with flavor “a” and momentum P0

bb pair

(ba)

Let f()d be the probability that the rank 1 meson leaves fractional momentum to the remaining cascade, leaving quark “b” with momentum P1 = 1P0.

cc pair

(cb) 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 P2 = 2P1 = 21P0.

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!

(bk) (ka)

Rank 1

Secondary Mesons(after decay)



A Parameterization of A Parameterization of the Properties of Jetsthe Properties of Jets

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, ....”



Monte-Carlo SimulationMonte-Carlo Simulationof Hadron-Hadron Collisionsof 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)

PYTHIAtoday

“FF” or “FW” Fragmentationthe past

tomorrow SHERPA PYTHIA 6.3



Monte-Carlo SimulationMonte-Carlo Simulationof Quark and Gluon Jetsof Quark and Gluon Jets

ISAJET: Evolve the parton-shower from Q2 (virtual photon invariant mass) to Qmin ~ 5 GeV. Use a complicated fragmentation model to evolve from Qmin to outgoing hadrons.

Q2

Field-Feynman

hadrons

5 GeV 1 GeV 200 MeV

HERWIG: Evolve the parton-shower from Q2 (virtual photon invariant mass) to Qmin ~ 1 GeV. Form color singlet clusters which “decay” into hadrons according to 2-particle phase space.

MLLA: Evolve the parton-shower from Q2 (virtual photon invariant mass) to Qmin ~ 230 MeV. Assume that the charged particles behave the same as the partons with Nchg/Nparton = 0.56!

CDF Distribution of Particles in Jets MLLA Curve!



Collider CoordinatesCollider Coordinates

The z-axis is defined to be the beam axis with the xy-plane being the “transverse” plane.

Proton AntiProton

z-axis

y-axis

x-axis

Beam Axis

Proton AntiProton z-axis

xz-plane x-axis Center-of-Mass Scattering Angle

cm

P

x-axis

“Transverse” xy-plane y-axis

Azimuthal Scattering Angle

PT

cm is the center-of-mass scattering angle and is the azimuthal angle. The “transverse” momentum of a particle is given by PT = P cos(cm). cm

0 90o

1 40o

2 15o

3 6o

4 2o

Use and to determine the direction of an outgoing particle, where is the “pseudo-rapidity” defined by = -log(tan(cm/2)).

Proton AntiProton

Proton AntiProton 2 TeV

Lots of outgoing hadrons

The “rapidity” is defined by y = log((E+pz)/(E-pz))/2 and is equal to in the limit E >> mc2.



Quark & Gluon JetsQuark & Gluon Jets The CDF calorimeter measures energy deposited

in a cell of size x = 0.11x15o, whch is converted into transverse energy, ET = E cos(cm).

Transverse Energy Grid

Transverse Energy Grid

“Jet” is a cluster of transverse energy within rasius R.

“Jets” are defined to be clusters of transverse energy with a radius R in - space. A “jet” is the representation in the detector of an outgoing parton (quark or gluon).

The sum of the ET of the cells within a “jet” corresponds roughly to the ET of the outgoing parton and the position of the cluster in the grid gives the parton’s direction.

Calorimeter Jets

Charged Particle Jet Can also construct jets from the charged particles!



Proton-AntiProton CollisionsProton-AntiProton Collisionsat the Tevatronat the Tevatron

Elastic Scattering Single Diffraction

M

tot = ELSD DD HC

Double Diffraction

M1 M2

Proton AntiProton

“Soft” Hard Core (no hard scattering)

Proton AntiProton

PT(hard)

Outgoing Parton

Outgoing Parton




“Hard” Hard Core (hard scattering)

Hard Core

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” trigger1 charged particle in forward BBC

AND1 charged particle in backward BBC

tot = ELIN



Proton-AntiProton Collisions Proton-AntiProton Collisions at the Tevatronat the Tevatron

“Hard core” does not imply that a “hard” parton-parton collision has occured?

Proton AntiProton

“Soft” Collision (no hard scattering)

Proton AntiProton

“Hard” Scattering

PT(hard)

Outgoing Parton

Outgoing Parton




90% of “hard core” collisions are “soft hard core” and the proton and antiproton ooze through each other and fall apart (i.e. no hard scattering, PT(hard) < 5 GeV/c). The outgoing particles continue in roughly the same direction as initial proton and antiproton.

10% of the “hard core” collisions arise from a “hard” parton-parton collision (PT(hard) > 5 GeV/c) resulting in large transverse momentum outgoing partons.

About 0.3% of all parton-parton collisions produce a b-bbar quark pair (about 1/1,000 of all interactions).

Proton AntiProton

“Flavor Creation” b-quark

b-quark

q or g q or g

Proton AntiProton

“Flavor Creation” b-quark

b-quark



Hard Core



CDF “Min-Bias” DataCDF “Min-Bias” DataCharged Particle DensityCharged Particle Density

Shows CDF “Min-Bias” data on the number of charged particles per unit pseudo-rapidity at 630 and 1,800 GeV. There are about 4.2 charged particles per unit in “Min-Bias” collisions at 1.8 TeV (|| < 1, all PT).

Charged Particle Pseudo-Rapidity Distribution: dN/d

0

1

2

3

4

5

6

7

-4 -3 -2 -1 0 1 2 3 4

Pseudo-Rapidity

dN

/d

CDF Min-Bias 1.8 TeV

CDF Min-Bias 630 GeV all PT

CDF Published

<dNchg/d> = 4.2

Charged Particle Density: dN/dd

0.0

0.2

0.4

0.6

0.8

1.0

-4 -3 -2 -1 0 1 2 3 4

Pseudo-Rapidity

dN

/d d

CDF Min-Bias 630 GeV

CDF Min-Bias 1.8 TeV all PT

CDF Published

<dNchg/dd> = 0.67

Convert to charged particle density, dNchg/dd by dividing by 2. There are about 0.67 charged particles per unit - in “Min-Bias” collisions at 1.8 TeV (|| < 1, all PT).



-1 +1

2

0

1 charged particle

dNchg/dd = 1/4 = 0.08

Particle DensitiesParticle Densities

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

Charged Particles pT > 0.5 GeV/c || < 1

= 4 = 12.6

1 GeV/c PTsum

dPTsum/dd = 1/4 GeV/c = 0.08 GeV/c

dNchg/dd = 3/4 = 0.24

3 charged particles

dPTsum/dd = 3/4 GeV/c = 0.24 GeV/c

3 GeV/c PTsum

CDF Run 2 “Min-Bias”Observable

AverageAverage Density

per unit -

NchgNumber 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

Divide by 4

CDF Run 2 “Min-Bias”



CDF “Min-Bias” DataCDF “Min-Bias” DataEnergy DependenceEnergy Dependence


0.0

0.2

0.4

0.6

0.8

1.0

-4 -3 -2 -1 0 1 2 3 4

Pseudo-Rapidity

dN

/d d

CDF Min-Bias 630 GeV

CDF Min-Bias 1.8 TeV all PT

CDF Published

Shows the center-of-mass energy dependence of the charged particle density, dNchg/dd for “Min-Bias” collisions at = 0. Also show a log fit (Fit 1) and a (log)2 fit (Fit 2) to the CDF plus UA5 data.


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

10 100 1,000 10,000 100,000

CM Energy W (GeV)

Ch

arg

ed d

ensi

ty d

N/d

d

CDF DataUA5 DataFit 2Fit 1

= 0

<dNchg/dd> = 0.51 = 0 630 GeV

What should we expect for the LHC?

<dNchg/dd> = 0.63 = 0 1.8 TeV

LHC?24% increase



Herwig “Soft” Min-BiasHerwig “Soft” Min-BiasCharged Particle Density: dN/dd

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-6 -4 -2 0 2 4 6

Pseudo-Rapidity

dN

/d d

630 GeV1.8 TeV

Herwig "Soft" Min-Bias 14 TeV

all PT

Shows the center-of-mass energy dependence of the charged particle density, dNchg/dd for “Min-Bias” collisions compared with the HERWIG “Soft” Min-Bias Monte-Carlo model. Note: there is no “hard” scattering in HERWIG “Soft” Min-Bias.

HERWIG “Soft” Min-Bias contains no hard parton-parton interactions and describes fairly well the charged particle density, dNchg/dd, in “Min-Bias” collisions.


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

10 100 1,000 10,000 100,000

CM Energy W (GeV)

Ch

arg

ed d

ensi

ty d

N/d

d

CDF Data

UA5 Data

Fit 2

Fit 1

HW Min-Bias

= 0

HERWIG “Soft” Min-Bias predicts a 45% rise in dNchg/dd at = 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged particles per unit becomes 6.

Can we believe HERWIG “soft” Min-Bias?

Can we believe HERWIG “soft” Min-Bias? No!

LHC?



CDF “Min-Bias” DataCDF “Min-Bias” DataPPTT Dependence Dependence

Charged Particle Density

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

0 2 4 6 8 10 12 14

PT (GeV/c)C

har

ged

Den

sity

dN

/d d

dP

T (

1/G

eV/c

)

||<1CDF Preliminary

CDF Min-Bias Data at 1.8 TeV

HW "Soft" Min-Biasat 630 GeV, 1.8 TeV, and 14 TeV


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-6 -4 -2 0 2 4 6

Pseudo-Rapidity

dN

/d d

630 GeV1.8 TeV

Herwig "Soft" Min-Bias 14 TeV

all PT

Shows the PT dependence of the charged particle density, dNchg/dddPT, for “Min-Bias” collisions at 1.8 TeV collisions compared with HERWIG “Soft” Min-Bias.

HERWIG “Soft” Min-Bias does not describe the “Min-Bias” data! The “Min-Bias” data contain a lot of “hard” parton-parton collisions which results in many more particles at large PT than are produces by any “soft” model.

Shows the energy dependence of the charged particle density, dNchg/dd for “Min-Bias” collisions compared with HERWIG “Soft” Min-Bias.

Lots of “hard” scattering in “Min-Bias”!



Min-Bias: CombiningMin-Bias: Combining“Hard” and “Soft” Collisions“Hard” and “Soft” Collisions


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-4 -3 -2 -1 0 1 2 3 4

Pseudo-Rapidity

dN

/d d

CDF Min-Bias Data Herwig Jet3 Herwig Min-Bias

1.8 TeV all PTHW "Soft" Min-Bias

HW PT(hard) > 3 GeV/c


1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

0 2 4 6 8 10 12 14

PT (GeV/c)C

har

ged

Den

sity

dN

/d

d d

PT

(1/

GeV

/c)

Herwig Jet3Herwig Min-BiasCDF Min-Bias Data

1.8 TeV ||<1

CDF Preliminary

HW PT(hard) > 3 GeV/c

HW "Soft" Min-Bias

HERWIG “hard” QCD with PT(hard) > 3 GeV/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 GeV/c!

One cannot run the HERWIG “hard” QCD Monte-Carlo with PT(hard) < 3 GeV/c because the perturbative 2-to-2 cross-sections diverge like 1/PT(hard)4?

HERWIG “soft” Min-Bias does not fit the “Min-Bias” data!

No easy way to“mix” HERWIG “hard” with HERWIG “soft”.

Hard-Scattering Cross-Section

0.01

0.10

1.00

10.00

100.00

0 2 4 6 8 10 12 14 16 18 20

Hard-Scattering Cut-Off PTmin

Cro

ss

-Se

cti

on

(m

illi

ba

rns

)

PYTHIA

HERWIG

CTEQ5L1.8 TeV

HC

HERWIG diverges!

PYTHIA cuts off the divergence.

Can run PT(hard)>0!



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

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”).

Proton AntiProton

“Hard” Collision

initial-state radiation

final-state radiation outgoing parton

outgoing parton

Clearly? the “underlying event” in a hard scattering process should not look like a “Min-Bias” event because of the “hard” component (i.e. initial & final-state radiation).

+

“Soft?” Component “Hard” Component


final-state radiation outgoing jet

Beam-Beam Remnants

“Soft?” Component

Beam-Beam Remnants

Hadron Hadron

“Min-Bias” Collision

However, perhaps “Min-Bias” collisions are a good model for the “beam-beam remnant” component of the “underlying event”.

Are these the same?

The “beam-beam remnant” component is, however, color connected to the “hard” component so this comparison is (at best) an approximation.

color string

color string

Maybe not all “soft”!



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 6.2: Multiple PartonPYTHIA 6.2: Multiple PartonInteraction ParametersInteraction Parameters

Pythia uses multiple partoninteractions to enhancethe underlying 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

!

Jimmy: MPIJ. M. Butterworth

J. R. ForshawM. H. Seymour

Proton AntiProton

Multiple Parton Interactions

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying EventUnderlying Event

Same parameter that cuts-off the hard 2-to-2 parton cross sections!



Tuning PYTHIA 6.2:Tuning PYTHIA 6.2:Multiple Parton Interaction ParametersMultiple 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


Color String

Color String


Color String

Hard-Scattering Cut-Off PT0

1

2

3

4

5

100 1,000 10,000 100,000

CM Energy W (GeV)

PT

0

(Ge

V/c

)

PYTHIA 6.206

= 0.16 (default)

= 0.25 (Set A))

Take E0 = 1.8 TeV

Reference pointat 1.8 TeV

Determine by comparingwith 630 GeV data!

Affects the amount ofinitial-state radiation!



Old PYTHIA default(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)

Tuned PYTHIA 6.206Tuned PYTHIA 6.206

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)).

PYTHIA 6.206 CTEQ5L"Transverse" Charged Particle Density: dN/dd

0.00

0.25

0.50

0.75

1.00

0 5 10 15 20 25 30 35 40 45 50

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

"Tra

nsv

erse

" C

har

ged

Den

sity

1.8 TeV ||<1.0 PT>0.5 GeV

CDF Preliminarydata uncorrectedtheory corrected

CTEQ5L

PYTHIA 6.206 (Set A)PARP(67)=4

PYTHIA 6.206 (Set B)PARP(67)=1

CDF Default!

Run 1 Analysis



PYTHIA Min-BiasPYTHIA Min-Bias“Soft” + ”Hard”“Soft” + ”Hard”


0.0

0.2

0.4

0.6

0.8

1.0

-4 -3 -2 -1 0 1 2 3 4

Pseudo-Rapidity

dN

/d d

Pythia 6.206 Set A

CDF Min-Bias 1.8 TeV 1.8 TeV all PT

CDF Published

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.

The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned.


1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 2 4 6 8 10 12 14

PT(charged) (GeV/c)C

har

ged

Den

sity

dN

/d d

dP

T (

1/G

eV/c

)

Pythia 6.206 Set A

CDF Min-Bias Data

CDF Preliminary

1.8 TeV ||<1

PT(hard) > 0 GeV/c

Tuned to fit the “underlying event”!

12% of “Min-Bias” events have PT(hard) > 5 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 PT(hard) > 5 GeV/c (1% with PT(hard) > 10 GeV/c)!

Lots of “hard” scattering in “Min-Bias”!

PYTHIA Tune ACDF Run 2 Default



Min-BiasMin-Bias “Associated” “Associated”Charged Particle DensityCharged Particle Density

Use the maximum pT charged particle in the event, PTmax, to define a direction and look at the the “associated” density, dNchg/dd, in “min-bias” collisions (pT > 0.5 GeV/c, || < 1).

PTmax Direction

Correlations in


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CDF Preliminarydata uncorrected

Charged Particles (||<1.0, PT>0.5 GeV/c)

Charge Density

Min-Bias

“Associated” densities do not include PTmax!

Highest pT charged particle!

PTmax Direction

Correlations in

Shows the data on the dependence of the “associated” charged particle density, dNchg/dd, for charged particles (pT > 0.5 GeV/c, || < 1, not including PTmax) relative to PTmax (rotated to 180o) for “min-bias” events. Also shown is the average charged particle density, dNchg/dd, 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!




Associated Particle Density: dN/dd

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PTmax > 1.0 GeV/c

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CDF Preliminarydata uncorrected

PTmaxPTmax not included

Charged Particles (||<1.0, PT>0.5 GeV/c)

Min-Bias

PTmax Direction

Correlations in

Shows the data on the dependence of the “associated” charged particle density, dNchg/dd, for charged particles (pT > 0.5 GeV/c, || < 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.

Transverse Region

Transverse Region

Jet #1

Shows “jet structure” in “min-bias” collisions (i.e. the “birth” of the leading two jets!).

Jet #2

Ave Min-Bias0.25 per unit -

PTmax Direction

“Toward”

“Transverse” “Transverse”

“Away”

PTmax > 0.5 GeV/c

PTmax > 2.0 GeV/c

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




Shows the data on the dependence of the “associated” charged particle density, dNchg/dd, for charged particles (pT > 0.5 GeV/c, || < 1, not including PTmax) relative to PTmax (rotated to 180o) for “min-bias” events with PTmax > 0.5 GeV/c and PTmax > 2.0 GeV/c compared with PYTHIA Tune A (after CDFSIM).

PTmax Direction

Correlations in

Associated Particle Density: dN/dd

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PY Tune A

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PY Tune A

CDF Preliminarydata uncorrectedtheory + CDFSIM

PTmaxPTmax not included (||<1.0, PT>0.5 GeV/c)

PY Tune A 1.96 TeV

PYTHIA Tune A predicts a larger correlation than is seen in the “min-bias” data (i.e. Tune A “min-bias” is a bit too “jetty”).

PTmax > 2.0 GeV/c

PTmax > 0.5 GeV/c

PTmax Direction

“Toward”

“Transverse” “Transverse”

“Away”

Transverse Region Transverse

Region

PY Tune A



PYTHIA Tune APYTHIA Tune ALHC PredictionsLHC Predictions


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PYTHIA was tuned to fit the “underlying event” in hard-scattering processes at 1.8 TeV and 630 GeV.


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Shows the center-of-mass energy dependence of the charged particle density, dNchg/dd for “Min-Bias” collisions compared with PYTHIA Tune A with PT(hard) > 0.

PYTHIA Tune A predicts a 42% rise in dNchg/dd at = 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). Similar to HERWIG “soft” min-bias, 4 charged particles per unit becomes 6.

LHC?



PYTHIA Tune APYTHIA Tune ALHC PredictionsLHC Predictions


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Hard-Scattering in Min-Bias Events

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Pythia 6.206 Set A

Shows the center-of-mass energy dependence of the charged particle density, dNchg/dddPT, for “Min-Bias” collisions compared with PYTHIA Tune A with PT(hard) > 0.

PYTHIA Tune A predicts that 1% of all “Min-Bias” events at 1.8 TeV are a result of a hard 2-to-2 parton-parton scattering with PT(hard) > 10 GeV/c which increases to 12% at 14 TeV!



LHC?



LHC Min-Bias LHC Min-Bias PredictionsPredictions

Tevatron LHC

Both HERWIG and the tuned PYTHIA Tune A predict a 40-45% rise in dNchg/dd at = 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged particles per unit at the Tevatron becomes 6 per unit at the LHC.

Proton AntiProton

PT(hard)

Outgoing Parton

Outgoing Parton




12 times more likely to find a 10 GeV

“jet” in “Min-Bias” at the LHC!

The tuned PYTHIA Tune A predicts that 1% of all “Min-Bias” events at the Tevatron (1.8 TeV) are the result of a hard 2-to-2 parton-parton scattering with PT(hard) > 10 GeV/c which increases to 12% at LHC (14 TeV)!

Documents

University of Perugia - Lecture 1 March 30, 2006 Rick Field – Florida/CDF/CMSPage 1 Toward an Understanding of Hadron-Hadron Collisions Lecture 1: From