Toward an Understanding of
Hadron-Hadron Collisions
 Lecture 1: From Field-Feynman to the Tevatron.
 From 7 GeV/c p0’s to 600 GeV/c Jets!
 QCD Monte-Carlo Models (PYTHIA Tune A).
 “Min-Bias” Collisions at the Tevatron. → extrapolations to the LHC!
 Lecture 2: A Detailed Study of the “Underlying Event” at
the Tevatron.
Outgoing Parton
PT(hard)
Initial-State Radiation
 QCD Monte-Carlo Models tunes at the
Tevatron. → extrapolations to the LHC!
Proton
AntiProton
Underlying Event
Outgoing Parton
Underlying Event
Final-State
Radiation
UE&MB@CMS
Florida-Perugia
University of Perugia
University of Perugia - Lecture 1
March 30, 2006
CMS at the LHC
Rick Field – Florida/CDF/CMS
Page 1
Feynman-Field
Phenomenology
1973-1980
“Feynman-Field
Jet Model”
 FF1: “Quark Elastic Scattering as a Source of High Transverse Momentum
Mesons”, R. D. Field and R. P. Feynman, Phys. Rev. D15, 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).
 FW1: “A QCD Model for e+e- Annihilation”, R. D. Field and S. Wolfram, Nucl.
Phys. B213, 65-84 (1983).
My 1st graduate
student!
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 3
Hadron-Hadron
Collisions
FF1 1977 (preQCD)
 What happens when two hadrons
collide at high energy?
Hadron
???
Hadron
Feynman quote from FF1
“The model
we shall choose is not a popular one,
 Most of the time the hadrons
ooze
thatapart
we will
not duplicate too much of the
through each other andsofall
(i.e.
work of others who are similarly analyzing
no hard scattering). The outgoing
various models (e.g. constituent interchange
particles continue in roughly
the same
Parton-Parton
Scattering Outgoing Parton
model, multiperipheral
models,
etc.). We shall
direction as initial proton
andthat the high PT particles arise from
assume
“Soft” Collision (no large transverse momentum)
antiproton.
direct hard collisions between constituent
in the incoming
particles, which
Hadron
 Occasionally there will bequarks
a large
Hadron
fragment or cascade down into several hadrons.”
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!
Outgoing Parton
high PT meson
“Black-Box Model”
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 4
Quark-Quark
Black-Box Model
No gluons!
Quark Distribution Functions
determined from deep-inelastic
lepton-hadron collisions
FF1 1977 (preQCD)
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-Quark Cross-Section
Unknown! Deteremined from
hadron-hadron collisions.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Quark Fragmentation Functions
determined from e+e- annihilations
Page 5
Predict
particle ratios
Quark-Quark
Black-Box Model
FF1 1977 (preQCD)
Predict
increase with increasing
CM energy W
“Beam-Beam
Remnants”
Predict
overall event topology
(FFF1 paper 1977)
7 GeV/c p0’s!
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 6
Telagram from Feynman
July 1976
SAW CRONIN AM NOW CONVINCED WERE RIGHT TRACK QUICK WRITE
FEYNMAN
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 7
Letter from Feynman
July 1976
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 8
Letter from Feynman:
page 1
Spelling?
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 9
Letter from Feynman:
page 3
It is fun!
Onward!
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 10
Feynman Talk at Coral
Gables in December 1976
1st transparency
Last transparency
“Feynman-Field
Jet Model”
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 11
QCD Approach
Quarks & Gluons
Quark & Gluon Fragmentation
Functions
2
Q dependence predicted from QCD
Parton Distribution Functions
Q2 dependence predicted from
QCD
FFF2 1978
Feynman quote from FFF2
“We investigate whether the present
experimental behavior of mesons with
large transverse momentum in hadron-hadron
collisions is consistent with the theory of
quantum-chromodynamics (QCD) with
asymptotic freedom, at least as the theory
is now partially understood.”
Quark & Gluon Cross-Sections
Calculated from QCD
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 12
High PT Jets
CDF (2006)
Feynman, Field, & Fox (1978)
Predict
large “jet”
cross-section
30 GeV/c!
Feynman quote
from FFF
600 GeV/c
Jets!
“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.”
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 13
A Parameterization of
the Properties of Jets
Secondary Mesons
(after decay)
continue
Field-Feynman 1978
 Assumed that jets could be analyzed on a “recursive”
principle.
(bk) (ka)
 Let f(h)dh be the probability that the rank 1 meson leaves
fractional momentum h to the remaining cascade, leaving
Rank 2
Rank 1
quark “b” with momentum P1 = h1P0.
 Assume that the mesons originating from quark “b” are
distributed in presisely the same way as the mesons which
(cb)
(ba)
Primary Mesons
came from quark a (i.e. same function f(h)), leaving
quark “c” with momentum P2 = h2P1 = h2h1P0.
cc pair bb pair
Calculate F(z)
from f(h) and b i!
Original quark with
flavor “a” and
momentum P0
University of Perugia - Lecture 1
March 30, 2006
 Add in flavor dependence by letting bu = probabliity of
producing u-ubar pair, bd = probability of producing ddbar 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.
Rick Field – Florida/CDF/CMS
Page 14
A Parameterization of
the 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, ....”
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 15
Monte-Carlo Simulation
of Hadron-Hadron Collisions
FF1-FFF1 (1977)
“Black-Box” Model
F1-FFF2 (1978)
QCD Approach
FFFW “FieldJet” (1980)
QCD “leading-log order” simulation
of hadron-hadron collisions
the past
today
FF2 (1978)
Monte-Carlo
simulation of “jets”
ISAJET
HERWIG
(“FF” Fragmentation)
(“FW” Fragmentation)
tomorrow
University of Perugia - Lecture 1
March 30, 2006
SHERPA
“FF” or “FW”
Fragmentation
PYTHIA
PYTHIA 6.3
Rick Field – Florida/CDF/CMS
Page 16
Monte-Carlo Simulation
of Quark and Gluon Jets
hadrons
 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.
 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
Q2 
the charged particles behave the same as the partons

with Nchg/Nparton = 0.56!
CDF Distribution of Particles in Jets
MLLA Curve!
Field-Feynman
5 GeV
1 GeV 200 MeV
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 18
Collider Coordinates
x-axis
xz-plane
Center-of-Mass
Scattering Angle
x-axis
Beam Axis
P
Proton
Proton
Proton
AntiProton
AntiProton
AntiProton
2 TeV
z-axis
Proton
“Transverse”
xy-plane
y-axis
Lots of outgoing hadrons
cm
AntiProton z-axis
 The z-axis is defined to be the beam axis with
the xy-plane being the “transverse” plane.
 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).
 Use h and  to determine the direction of an
outgoing particle, where h is the “pseudo-rapidity”
defined by h= -log(tan(cm/2)).
 The “rapidity” is defined by y = log((E+pz)/(E-pz))/2
and is equal to h in the limit E >> mc2.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Azimuthal
Scattering Angle
y-axis
PT

x-axis
h
cm
0
90o
1
40o
2
15o
3
6o
4
2o
Page 19
Quark & Gluon Jets

The CDF calorimeter measures energy deposited
in a cell of size DhxD = 0.11x15o, whch is
converted into transverse energy, ET = E
cos(cm).
Transverse Energy Grid
 “Jets” are defined to be clusters of transverse
energy with a radius R in h- 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.
Charged Particle Jet
“Jet” is a cluster of
transverse energy
within rasius R.

h
Can also construct jets from
the charged particles!
Calorimeter Jets
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 20
Proton-AntiProton Collisions
at the Tevatron
Elastic Scattering
The CDF “Min-Bias” trigger
picks up most of the “hard
core” cross-section plus a
Double
Diffraction
small
amount of single &
double diffraction.
M2
M1
Single Diffraction
M
stot = sEL +sIN
SD +sDD +sHC
1.8 TeV: 78mb
= 18mb
+ 9mb
+ (4-7)mb + (47-44)mb
CDF “Min-Bias” trigger
1 charged particle in forward BBC
AND
1 charged particle in backward BBC
Hard Core
The “hard core” component
contains both “hard” and
“soft” collisions.
“Hard” Hard Core (hard scattering)
Outgoing Parton
“Soft” Hard Core (no hard scattering)
Proton
AntiProton
PT(hard)
Beam-Beam Counters
3.2 < |h| < 5.9
Proton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Outgoing Parton
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 22
Proton-AntiProton Collisions
at the Tevatron
 “Hard core” does not imply that a “hard”
parton-parton collision has occured?
“Soft” Collision (no hard scattering)
Hard Core
Proton
 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).
AntiProton
“Hard” Scattering
Outgoing Parton
PT(hard)
Proton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
“Flavor Creation”
Outgoing Parton
b-quark
Proton
AntiProton
q or g
Underlying Event
q or g
Underlying Event
Initial-State
Radiation
b-quark
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 23
CDF “Min-Bias” Data
Charged Particle Density
Charged Particle Density: dN/dhd
Charged Particle Pseudo-Rapidity Distribution: dN/dh
1.0
7
CDF Published
CDF Published
6
0.8
dN/dhd
dN/dh
5
4
3
0.6
0.4
2
0.2
CDF Min-Bias 630 GeV
CDF Min-Bias 1.8 TeV
1
CDF Min-Bias 1.8 TeV
all PT
CDF Min-Bias 630 GeV
all PT
0.0
0
-4
-3
-2
-1
0
1
2
3
4
-4
-3
<dNchg/dh> = 4.2
-2
-1
0
1
2
3
4
Pseudo-Rapidity h
Pseudo-Rapidity h
<dNchg/dhd> = 0.67
 Shows CDF “Min-Bias” data on the number of charged particles per unit pseudo-rapidity
at 630 and 1,800 GeV. There are about 4.2 charged particles per unit h in “Min-Bias”
collisions at 1.8 TeV (|h| < 1, all PT).
 Convert to charged particle density, dNchg/dhd, by dividing by 2p. There are about 0.67
charged particles per unit h- in “Min-Bias” collisions at 1.8 TeV (|h| < 1, all PT).
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 24
Particle Densities
DhD = 4p = 12.6
2p

31 charged
charged particles
particle
Charged Particles
pT > 0.5 GeV/c |h| < 1
CDF Run 2 “Min-Bias”
CDF Run 2 “Min-Bias”
Observable
Average
Nchg
Number of Charged Particles
(pT > 0.5 GeV/c, |h| < 1)
3.17 +/- 0.31
0.252 +/- 0.025
PTsum
(GeV/c)
Scalar pT sum of Charged Particles
(pT > 0.5 GeV/c, |h| < 1)
2.97 +/- 0.23
0.236 +/- 0.018
Average Density
per unit h-
chg/dhd = 1/4p
3/4p = 0.08
0.24
dNchg
13 GeV/c PTsum
0
-1
h
+1
Divide by 4p
dPTsum/dhd = 1/4p
3/4p GeV/c = 0.08
0.24 GeV/c
 Study the charged particles (pT > 0.5 GeV/c, |h| < 1) and form the charged
particle density, dNchg/dhd, and the charged scalar pT sum density,
dPTsum/dhd.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 25
CDF “Min-Bias” Data
Energy Dependence
Charged Particle Density: dN/dhd
Charged Particle Density: dN/dhd
1.4
1.0
CDF Published
CDF Data
UA5 Data
Fit 2
Fit 1
1.2
Charged density dN/dhd
dN/dhd
0.8
0.6
0.4
0.2
CDF Min-Bias 630 GeV
CDF Min-Bias 1.8 TeV
all PT
1.0
0.8
0.6
0.4
0.2
h=0
0.0
-4
-3
-2
-1
0
1
2
3
4
0.0
10
Pseudo-Rapidity h
100
1,000
10,000
100,000
CM Energy W (GeV)
<dNchg/dhd> = 0.51
h = 0 630 GeV
24% increase
<dNchg/dhd> = 0.63
h = 0 1.8 TeV
LHC?
 Shows the center-of-mass energy dependence of the charged particle density, dNchg/dhd,
for “Min-Bias” collisions at h = 0. Also show a log fit (Fit 1) and a (log)2 fit (Fit 2) to the
CDF plus UA5 data.
 What should we expect for the LHC?
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 26
Herwig “Soft” Min-Bias
Can
Can we
we believe
believe HERWIG
HERWIG
“soft”
Min-Bias?
“soft”
Min-Bias?
No!
Charged Particle Density: dN/dhd
Charged Particle Density: dN/dhd
1.4
1.4
14 TeV
Herwig "Soft" Min-Bias
1.2
CDF Data
1.2
Charged density dN/dhd
UA5 Data
dN/dhd
1.0
0.8
0.6
0.4
1.8 TeV
0.2
Fit 2
1.0
Fit 1
HW Min-Bias
0.8
0.6
0.4
0.2
630 GeV
all PT
h=0
0.0
0.0
-6
-4
-2
0
2
4
6
10
Pseudo-Rapidity h
100
1,000
10,000
CM Energy W (GeV)
100,000
LHC?
 Shows the center-of-mass energy dependence of the charged particle density, dNchg/dhd,

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/dhd, in “Min-Bias” collisions.
 HERWIG “Soft” Min-Bias predicts a 45% rise in dNchg/dhd at h = 0 in going from the
Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged particles per unit h becomes 6.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 27
CDF “Min-Bias” Data
PT Dependence
Lots of “hard” scattering
in “Min-Bias”!
Charged Particle Density
Charged Particle Density: dN/dhd
1.4
1.0E+01
14 TeV
Herwig "Soft" Min-Bias
1.2
CDF Preliminary
0.6
1.8 TeV
0.2
630 GeV
all PT
0.0
-6
-4
-2
0
2
4
6
Pseudo-Rapidity h
 Shows the energy dependence of the

Charged Density dN/dhddPT (1/GeV/c)
dN/dhd
0.8
0.4

|h|<1
1.0E+00
1.0
1.0E-01
CDF Min-Bias Data at 1.8 TeV
1.0E-02
1.0E-03
HW "Soft" Min-Bias
at 630 GeV, 1.8 TeV, and 14 TeV
1.0E-04
1.0E-05
charged particle density, dNchg/dhd, for
1.0E-06
“Min-Bias” collisions compared with
0
2
4
6
8
10
12
14
PT (GeV/c)
HERWIG “Soft” Min-Bias.
Shows the PT dependence of the charged particle density, dNchg/dhddPT, 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.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 28
Min-Bias: Combining
“Hard” and “Soft” Collisions
No easy way to
“mix” HERWIG “hard”
with HERWIG “soft”.
HERWIG diverges!
sHC
Hard-Scattering Cross-Section
Charged
Particle Density
100.00
Charged Particle Density: dN/dhd
1.6
Cross-Section (millibarns)
1.4
Herwig Jet3
Herwig Min-Bias
CDF Min-Bias Data
10.00
CDF Preliminary
1.2
1.0E+00
1.0
HW PT(hard) > 3 GeV/c
0.8
0.6
0.4
0.2
HW "Soft" Min-Bias
1.8 TeV all PT
0.0
-4
-3
-2
-1
0
1
2
3
4
Pseudo-Rapidity h
CDF Min-Bias Data
Herwig Jet3
Herwig 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!
Charged Density dN/dhddPT (1/GeV/c)
dN/dhd
CTEQ5L
1.8 TeV
1.0E+01
1.0E-01
1.0E-02
1.00
1.8 TeV |h|<1
0.10
PYTHIA
HERWIG
HW PT(hard) > 3 GeV/c
0.01
0
2
4
6
8
10
12
14
16
18
PYTHIA cuts off
the divergence.
HW "Soft" Min-Bias
Can run
PT(hard)>0!
1.0E-03
1.0E-04
1.0E-05
1.0E-06
0
2
4
6
8
10
12
14
PT (GeV/c)
HERWIG “soft”
Min-Bias does not fit
the “Min-Bias” data!
 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?
University of Perugia - Lecture 1
March 30, 2006
20
Hard-Scattering Cut-Off PTmin
Rick Field – Florida/CDF/CMS
Page 29
Monte-Carlo Simulation
of Hadron-Hadron Collisions
Maybe not all “soft”!
“Hard” Collision
outgoing parton
“Hard” Component
“Soft?” Component
AntiProton
Proton
initial-state radiation
initial-state radiation
+
Beam-Beam Remnants
outgoing parton
outgoing jet
final-state radiation

final-state radiation
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”).
 Clearly? the “underlying event” in a hard scattering process should not look like a “MinBias” event because of the “hard” component (i.e. initial & final-state radiation).
 However, perhaps “Min-Bias” collisions are a good model for the “beam-beam remnant”
component of the “underlying event”.
“Min-Bias” Collision
“Soft?” Component
color string
Are these the same?
Hadron
Hadron
color string
Beam-Beam Remnants
 The “beam-beam remnant” component is, however, color connected to the “hard”
component so this comparison is (at best) an approximation.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 30
MPI: Multiple Parton
Interactions
“Hard”
Collision
Multiple
Parton
Interaction
outgoing parton
“Hard” Component
“Semi-Hard” MPI
“Soft” Component
AntiProton
Proton
initial-state radiation
initial-state radiation
outgoing parton
final-state radiation
or
+
outgoing jet
final-state radiation
 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”.
Beam-Beam Remnants
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).
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 31
PYTHIA 6.2: Multiple Parton
Interaction Parameters
Multiple Parton Interactions
Outgoing Parton
PT(hard)
Proton
AntiProton
Underlying Event
Underlying Event
ParameterOutgoingValue
Parton
MSTP(81)
MSTP(82)
Same parameter that
cuts-off the hard 2-to-2
parton cross sections!
Pythia uses multiple parton
interactions to enhance
the underlying event.
Description
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 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)
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
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 32
Tuning PYTHIA 6.2:
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.
Multiple Parton Interaction
Color String
Color String
PARP(86)
0.33
0.66
PARP(89)
1 TeV
PARP(90)
0.16
PARP(67)
1.0
Probability that the MPI produces two gluons
with color connections to the “nearest neighbors.
Multiple PartonDetermine
Interactionby comparing
with 630 GeV 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!
Color String
Hard-Scattering Cut-Off PT0
5
Determines the reference energy E0.
Determines the energy dependence of the cut-off
PT0 as follows PT0(Ecm) = PT0(Ecm/E0)e with
e = PARP(90)
A scale factor that determines the maximum
parton virtuality for space-like showers. The
larger the value of PARP(67) the more initialstate radiation.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
PARP(85)
Take E0 = 1.8 TeV
3
2
e = 0.16 (default)
1
100
1,000
10,000
100,000
CM Energy W (GeV)
Reference point
at 1.8 TeV
Page 33
Tuned PYTHIA 6.206
CDF Default!
PYTHIA 6.206 CTEQ5L
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
New PYTHIA default
(less initial-state radiation)
University of Perugia - Lecture 1
March 30, 2006
1.00
"Transverse" Charged Density
Parameter
"Transverse" Charged Particle Density: dN/dhd
CDF Preliminary
PYTHIA 6.206 (Set A)
PARP(67)=4
data uncorrected
theory corrected
0.75
Run 1 Analysis
0.50
0.25
CTEQ5L
PYTHIA 6.206 (Set B)
PARP(67)=1
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 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)).
Old PYTHIA default
(more initial-state radiation)
Rick Field – Florida/CDF/CMS
Page 34
PYTHIA Min-Bias
“Soft” + ”Hard”
Tuned to fit the
“underlying event”!
PYTHIA Tune A
CDF Run 2 Default
Charged Particle Density
Charged Particle Density: dN/dhd
1.0
1.0E+00
CDF Published
Pythia 6.206 Set A
0.8
CDF Min-Bias Data
0.6
0.4
0.2
Pythia 6.206 Set A
1.8 TeV all PT
CDF Min-Bias 1.8 TeV
0.0
-4
-3
-2
-1
0
1
2
3
4
Pseudo-Rapidity h
 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.
Charged Density dN/dhddPT (1/GeV/c)
dN/dhd
1.0E-01
1.8 TeV |h|<1
1.0E-02
12% of “Min-Bias” events
have PT(hard) > 5 GeV/c!
PT(hard) > 0 GeV/c
1.0E-03
1% of “Min-Bias” events
have PT(hard) > 10 GeV/c!
1.0E-04
1.0E-05
CDF Preliminary
1.0E-06
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned.
 This PYTHIA fit predicts that 12% of all “Min-Bias” events are a result of a hard 2-to-2
parton-parton scattering with PT(hard) > 5 GeV/c (1% with PT(hard) > 10 GeV/c)!
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 35
Min-Bias “Associated”
Highest pT
charged particle!
Charged Particle Density
“Associated” densities do
not include PTmax!
Charged Particle Density: dN/dhd
PTmax Direction
0.5
PTmax
Direction
D
Correlations in 
Charged Particle Density
CDF Preliminary
Associated Density
PTmax not included
data uncorrected
0.4
D
Charge Density
0.3
0.2
0.1
Min-Bias
Correlations
in 
Charged Particles
(|h|<1.0, PT>0.5 GeV/c)
PTmax
0.0
0
30
60
90
120
150
180
210
240
270
300
330
360
D (degrees)
 Use the maximum pT charged particle in the event, PTmax, to define a direction and

is “associated”
more probable
to finddN
a chg
particle
look at the It
the
density,
/dhd, in “min-bias” collisions (pT > 0.5
PTmax than it is to
GeV/c, |h| <accompanying
1).
find a particle in the central region!
Shows the data on the D dependence of the “associated” charged particle density,
dNchg/dhd, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative
to PTmax (rotated to 180o) for “min-bias” events. Also shown is the average charged
particle density, dNchg/dhd, for “min-bias” events.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 36
Min-Bias “Associated”
Rapid rise in the particle
density in the “transverse”
region as PTmax increases!
Charged Particle Density
Associated Particle Density: dN/dhd
PTmaxDirection
Direction
PTmax
D
“Toward”
“Transverse”
“Transverse”
Correlations in 
“Away”
Associated Particle Density
Jet #1
D
PTmax > 2.0 GeV/c
1.0
PTmax > 2.0 GeV/c
PTmax > 1.0 GeV/c
0.8
Charged Particles
(|h|<1.0, PT>0.5 GeV/c)
CDF Preliminary
data uncorrected
PTmax > 0.5 GeV/c
Transverse
Region
0.6
Transverse
Region
0.4
0.2
Jet #2
PTmax
PTmax not included
Min-Bias
0.0
0
30
60
90
120
150
180
210
240
270
300
330
360
D (degrees)
Ave Min-Bias
0.25 per unit h-
PTmax > 0.5 GeV/c
 Shows the data on the D dependence of the “associated” charged particle density,
dNchg/dhd, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative
to PTmax (rotated to 180o) for “min-bias” events with PTmax > 0.5, 1.0, and 2.0 GeV/c.
 Shows “jet structure” in “min-bias” collisions (i.e. the “birth” of the leading two jets!).
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 37
Min-Bias “Associated”
Charged Particle Density
PY Tune A
PTmax > 2.0 GeV/c
PTmax Direction
Direction
PTmax
D
“Toward”
“Transverse”
“Transverse”
Correlations in 
“Away”
PTmax > 2.0 GeV/c
Associated Particle Density
D
Associated Particle Density: dN/dhd
1.0
CDF Preliminary
PY Tune A
0.8
data uncorrected
theory + CDFSIM
PTmax > 0.5 GeV/c
PY Tune A
Transverse
Region
0.6
PY Tune A 1.96 TeV
Transverse
Region
0.4
0.2
PTmax
PTmax not included
(|h|<1.0, PT>0.5 GeV/c)
0.0
0
30
60
90
120
PTmax > 0.5 GeV/c
150
180
210
240
270
300
330
360
D (degrees)
 Shows the data on the D dependence of the “associated” charged particle density,
dNchg/dhd, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative
to PTmax (rotated to 180o) for “min-bias” events with PTmax > 0.5 GeV/c and PTmax >
2.0 GeV/c compared with PYTHIA Tune A (after CDFSIM).
 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”).
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 40
PYTHIA Tune A
LHC Predictions
Charged Particle Density: dN/dhd
Charged Particle Density: dN/dhd
1.4
1.4
Pythia 6.206 Set A
CDF Data
14 TeV
Charged density dN/dhd
1.0
dN/dhd
Pythia 6.206 Set A
CDF Data
UA5 Data
Fit 2
Fit 1
1.2
1.2
1.8 TeV
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
all PT
630 GeV
1.0
h=0
0.0
-6
-4
-2
0
Pseudo-Rapidity h
2
4
6
LHC?
0.0
10
100
1,000
10,000
100,000
CM Energy W (GeV)
 Shows the center-of-mass energy dependence of the charged particle density, dNchg/dhd,
for “Min-Bias” collisions compared with PYTHIA Tune A with PT(hard) > 0.
 PYTHIA was tuned to fit the “underlying event” in hard-scattering processes at 1.8 TeV
and 630 GeV.
 PYTHIA Tune A predicts a 42% rise in dNchg/dhd at h = 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 h becomes 6.
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 42
PYTHIA Tune A
LHC Predictions
Charged Particle Density
12% of “Min-Bias” events
50%
have PT(hard) > 10 GeV/c!
1.0E+00
Hard-Scattering in Min-Bias Events
|h|<1
Pythia 6.206 Set A
Pythia 6.206 Set A
40%
% of Events
Charged Density dN/dhddPT (1/GeV/c)
1.0E-01
1.0E-02
PT(hard) > 5 GeV/c
PT(hard) > 10 GeV/c
30%
20%
1.8 TeV
1.0E-03
10%
14 TeV
1.0E-04
0%
100
630 GeV
100,000
LHC?
 Shows the center-of-mass energy
CDF Data
1.0E-06
2
10,000
CM Energy W (GeV)
1.0E-05
0
1,000
4
6
8
PT(charged) (GeV/c)
1% of “Min-Bias” events
have PT(hard) > 10 GeV/c!
10
12
14
dependence of the charged particle density,
dNchg/dhddPT, 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!
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 43
LHC Min-Bias
Predictions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event
Outgoing Parton
Tevatron
LHC
Underlying Event
12 times more likely
to find a 10 GeV
“jet” in “Min-Bias”
at the LHC!
Final-State
Radiation
 Both HERWIG and the tuned PYTHIA Tune A predict a 40-45% rise in dNchg/dhd at h
= 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged particles per
unit h at the Tevatron becomes 6 per unit h 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)!
University of Perugia - Lecture 1
March 30, 2006
Rick Field – Florida/CDF/CMS
Page 44