The Underlying Event in
Hard Scattering Processes
Outgoing Parton
The Underlying Event:
beam-beam remnants
initial-state radiation
multiple-parton interactions
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event
 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.
Underlying Event
Outgoing Parton
Final-State
Radiation
CDF
QFL+Cones
Valeria Tano
Eve Kovacs
Joey Huston
Anwar Bhatti
CDF
WYSIWYG+Df
Rick Field
David Stuart
Rich Haas
Ph.D. Thesis
Different but related problem!
Les Houches 2001
Rick Field - Florida/CDF
Page 1
Beam-Beam Remnants
“Hard” Collision
outgoing parton
“Hard” Component
“Soft” Component
AntiProton
Proton
initial-state radiation
initial-state radiation
+
Beam-Beam Remnants
outgoing jet
outgoing parton
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).
 However the “soft” component is color connected to the “hard” component so this
separation is (at best) an approximation.
Min-Bias?
color string
color string
Les Houches 2001
 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).
Rick Field - Florida/CDF
Page 2
Multiple Parton Interactions
Multiple Parton Interaction
outgoing parton
“Hard” Component
“Semi-Hard” MPI
“Soft” Component
AntiProton
Proton
initial-state radiation
outgoing parton
final-state radiation
or
+
initial-state radiation
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).
Les Houches 2001
Rick Field - Florida/CDF
Page 3
WYSIWYG: Comparing Data
with QCD Monte-Carlo Models
Charged Particle
Data
Select
“clean”
region





WYSIWYG
What you see is
what you get.
Almost!
Make
efficiency
corrections
Look only at the charged
particles measured by
the CTC.
Zero or one vertex
|zc-zv| < 2 cm, |CTC d0| < 1 cm
Require PT > 0.5 GeV, |h| < 1
Assume a uniform track finding
efficiency of 92%
Errors include both statistical
and correlated systematic
uncertainties
 Require PT > 0.5 GeV, |h| < 1
 Make an 8% correction for the
compare
Uncorrected data
Les Houches 2001
QCD
Monte-Carlo

track finding efficiency
Errors (statistical plus
systematic) of around 5%
Corrected theory
Rick Field - Florida/CDF
Small
Corrections!
Page 4
Charged Particle Df
Correlations
Charged Jet #1
Direction
2p
Away Region
“Toward-Side” Jet
Df
“Toward”
“Transverse”
“Transverse”
“Away”
Charged Jet #1
Direction
Df
“Toward”
“Transverse”
Transverse
Region
f
Leading
Jet
“Transverse”
Toward Region
Transverse
Region
“Away”
Away Region
0
-1
“Away-Side” Jet
+1
h
 Look at charged particle correlations in the azimuthal angle Df relative to the leading charged
particle jet.
 Define |Df| < 60o as “Toward”, 60o < |Df| < 120o as “Transverse”, and |Df| > 120o as “Away”.
 All three regions have the same size in h-f space, DhxDf = 2x120o = 4p/3.
Les Houches 2001
Rick Field - Florida/CDF
Page 5
Charged Multiplicity
versus PT(chgjet#1)
Charged Jet #1
Direction
Df
“Transverse”
“Transverse”
“Away”
12
CDF Preliminary
<Nchg> in 1 GeV/c bin
“Toward”
Nchg versus PT(charged jet#1)
"Toward"
data uncorrected
10
8
"Away"
6
4
"Transverse"
2
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
Underlying Event
“plateau”
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Data on the average number of “toward” (|Df|<60o), “transverse” (60<|Df|<120o), and “away”
(|Df|>120o) charged particles (PT > 0.5 GeV, |h| < 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.
Les Houches 2001
Rick Field - Florida/CDF
Page 6
Shape of an Average
Event with PT(chgjet#1) = 20 GeV/c
Nchg versus PT(charged jet#1)
Includes Jet#1
12
<Nchg> in 1 GeV/c bin
CDF Preliminary
"Toward"
data uncorrected
10
Underlying event
“plateau”
8
"Away"
6
4
"Transverse"
Remember
|h| < 1 PT > 0.5 GeV
2
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Shape in Nchg
<Nchg> = 1.2
<Nchg> = 4.5
<Nchg> = 8.2
PT(charged jet#1) = 20 GeV
<Nchg> = 1.2
Les Houches 2001
Rick Field - Florida/CDF
Page 7
“Height” of the Underlying
Event “Plateau”
<Nchg> = 1.2
<Nchg> = 4.5
<Nchg> = 8.2
PT(jet#1) = 20 GeV
<Nchg> = 1.2
Implies 1.09*3(2.4)/2 = 3.9
charged particles per unit h
with PT > 0.5 GeV/c.
Charged Particle Pseudo-Rapidity Distribution
dNchg/dh
10
Hard
8
6
Soft
4 per
unit h
4
2
1.8 TeV Proton-Antiproton Collisions
0
-10
-8
-6
-4
-2
0
2
4
6
8
Pseudo-Rapidity
Herwig "Soft" MB
Les Houches 2001
Isajet "Soft" MB
MBR
10
Implies 2.3*3.9 = 9 charged
particles per unit h
with PT > 0 GeV/c which is
a factor of 2 larger
than “soft” collisions.
CDF DATA
Rick Field - Florida/CDF
Page 8
“Transverse” PT Distribution
Charged Jet #1
Direction
"Transverse" PT Distribution (charged)
Df
1.0E+01
CDF Preliminary
data uncorrected
PT(chgjet1) > 5 GeV/c
“Toward”
1.0E+00
“Transverse”
“Away”
 Plot shows the PT distribution of

the “Transverse” <Nchg>,
dNchg/dPT. The integral of
dNchg/dPT is the “Transverse”
<Nchg>.
The triangle and circle (square)
points are the Min-Bias (JET20)
data. The errors on the
(uncorrected) data include both
statistical and correlated
systematic uncertainties.
Les Houches 2001
dNchg/dPT (1/GeV/c)
“Transverse”
1.8 TeV |h|<1 PT>0.5 GeV/c
1.0E-01
1.0E-02
1.0E-03
PT(chgjet1) > 2 GeV/c
1.0E-04
PT(chgjet1) > 30 GeV/c
1.0E-05
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
Rick Field - Florida/CDF
Page 9
“Transverse” PT Distribution
"Transverse" Nchg versus PT(charged jet#1)
CDF JET20
data uncorrected
4
1.0E+01
CDF Min-Bias
CDF Preliminary
CDF Preliminary
data uncorrected
PT(chgjet1) > 5 GeV/c
1.0E+00
3
2
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
PT(charged jet#1) > 30 GeV/c
“Transverse” <Nchg> = 2.3
50
dNchg/dPT (1/GeV/c)
"Transverse" <Nchg> in 1 GeV/c bin
5
"Transverse" PT Distribution (charged)
1.8 TeV |h|<1 PT>0.5 GeV/c
1.0E-01
1.0E-02
1.0E-03
PT(chgjet1) > 2 GeV/c
1.0E-04
PT(chgjet1) > 30 GeV/c
1.0E-05
0
PT(charged jet#1) > 5 GeV/c
“Transverse” <Nchg> = 2.2
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Comparison of the “transverse” <Nchg> versus PT(charged jet#1) with the PT
distribution of the “transverse” <Nchg>, dNchg/dPT. The integral of dNchg/dPT is the
“transverse” <Nchg>. Shows how the “transverse” <Nchg> is distributed in PT.
Les Houches 2001
Rick Field - Florida/CDF
Page 10
“Max/Min Transverse” Nchg
versus PT(chgjet#1)
"Max/Min Transverse" Nchg
3.0
Df
“Toward”
“TransMAX”
“TransMIN”
“Away”
1.8 TeV |h|<1.0 PT>0.5 GeV
CDF Preliminary
2.5
<Nchg> in 1 GeV/c bin
Area DhDf
2x60o = 2p/3
Charged Jet #1
Direction
data uncorrected
"Max Transverse"
2.0
1.5
1.0
"Min Transverse"
0.5
“TransMAX”
0.0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
“TransMIN”
 Define “TransMAX” and “TransMIN” to be the maximum and minimum of the region

60o<Df<120o (60o<-Df<120o) on an event by event basis. The overall “transverse” region
is the sum of “TransMAX” and “TransMIN”. The plot shows the average “TransMAX”
Nchg and “TransMIN” Nchg versus PT(charged jet#1).
The solid (open) points are the Min-Bias (JET20) data. The errors on the (uncorrected)
data include both statistical and correlated systematic uncertainties.
Les Houches 2001
Rick Field - Florida/CDF
Page 11
“TransSUM/DIF” Nchg
versus PT(chgjet#1)
Charged Jet #1
Direction
Area DhDf
2x60o = 2p/3
SUM/DIF "Transverse" Nchg
Df
4
“Toward”
“TransMAX”
“TransMIN”
“Away”
<Nchg> in 1 GeV/c bin
CDF Preliminary
data uncorrected
3
"Max+Min Transverse"
2
"Max-Min Transverse"
1
1.8 TeV |h|<1.0 PT>0.5 GeV
“TransSUM”
“TransDIF”
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Define “TransMAX” and “TransMIN” to be the maximum and minimum of the region

60o<Df<120o (60o<-Df<120o) on an event by event basis. The plot shows the average sum
and difference of the “TransMAX” Nchg and the “TransMIN” Nchg versus PT(charged
jet#1). The overall “transverse” region is the sum of “TransMAX” and “TransMIN”.
The solid (open) points are the Min-Bias (JET20) data. The errors on the (uncorrected)
data include both statistical and correlated systematic uncertainties.
Les Houches 2001
Rick Field - Florida/CDF
Page 12
“Max/Min Transverse” PTsum
versus PT(chgjet#1)
Df
“Toward”
“TransMAX”
“TransMIN”
“Away”
“TransMAX”
"Max/Min Transverse" PTsum
3.5
<PTsum> (GeV/c) in 1 GeV/c bin
Area DhDf
2x60o = 2p/3
Charged Jet #1
Direction
1.8 TeV |h|<1.0 PT>0.5 GeV
CDF Preliminary
3.0
"Max Transverse"
data uncorrected
2.5
2.0
1.5
1.0
"Min Transverse"
0.5
0.0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
“TransMIN”
 Define “TransMAX” and “TransMIN” to be the maximum and minimum of the region

60o<Df<120o (60o<-Df<120o) on an event by event basis. The overall “transverse” region
is the sum of “TransMAX” and “TransMIN”. The plot shows the average “TransMAX”
PTsum and “TransMIN” PTsum versus PT(charged jet#1)..
The solid (open) points are the Min-Bias (JET20) data. The errors on the (uncorrected)
data include both statistical and correlated systematic uncertainties.
Les Houches 2001
Rick Field - Florida/CDF
Page 13
“TransSUM/DIF” PTsum
versus PT(chgjet#1)
Charged Jet #1
Direction
Area DhDf
2x60o = 2p/3
SUM/DIF "Transverse" PTsum
5
“Toward”
“TransMAX”
“TransMIN”
“Away”
“TransSUM”
<PTsum> (GeV/c) in 1 GeV/c bin
Df
CDF Preliminary
4
data uncorrected
"Max+Min Transverse"
3
2
"Max-Min Transverse"
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
“TransDIF”
 Define “TransMAX” and “TransMIN” to be the maximum and minimum of the region

60o<Df<120o (60o<-Df<120o) on an event by event basis. The plot shows the average sum
and difference of the “TransMAX” PTsum and the “TransMIN” PTsum versus PT(charged
jet#1). The overall “transverse” region is the sum of “TransMAX” and “TransMIN”.
The solid (open) points are the Min-Bias (JET20) data. The errors on the (uncorrected)
data include both statistical and correlated systematic uncertainties.
Les Houches 2001
Rick Field - Florida/CDF
Page 14
QFL: Comparing Data
with QCD Monte-Carlo Models
Charged Particle
And Calorimeter
Data
Select
region
QCD
Look only at both the
Monte-Carlo
charged particles
measured by the CTC
and the calorimeter data.
Tano-Kovacs-Huston-Bhatti
QFL
detector
simulation
 Calorimeter: tower threshold = 50
MeV, Etot < 1800 GeV, |hlj| < 0.7,
|zvtx| < 60 cm, 1 and only 1 class 10,
11, or 12 vertex
 Tracks: |zc-zv| < 5 cm, |CTC d0| < 0.5
cm, PT > 0.4 GeV, |h| < 1, correct for
track finding efficiency
Les Houches 2001
compare 
Rick Field - Florida/CDF
Require PT > 0.4 GeV, |h| < 1
Page 15
“Transverse” Cones
2p
2p
Transverse
Cone:
p(0.7)2=0.49p
Away Region
Transverse
Region
f
1.36
Leading
Jet
Tano-Kovacs-Huston-Bhatti
Cone 1
f
Leading
Jet
Toward Region
Transverse
Region:
2p/3=0.67p
Transverse
Region
Cone 2
Away Region
0
0
-1
h
+1
-1
h
+1
 Sum the PT of charged particles (or the energy)
in two cones of radius 0.7 at the same h as the
leading jet but with |DF| = 90o.
 Plot the cone with the maximum and minimum
PTsum versus the ET of the leading (calorimeter)
jet..
Les Houches 2001
Rick Field - Florida/CDF
Page 16
Transverse Regions
vs Transverse Cones
Field-Stuart-Haas
"Max/Min Transverse" PTsum
<PTsum> (GeV/c) in 1 GeV/c bin
3.5
1.8 TeV |h|<1.0 PT>0.5 GeV
CDF Preliminary
3.0
2.9 GeV/c
"Max Transverse"
data uncorrected
2.5
2.0
1.5
1.0
2.1 GeV/c
"Min Transverse"
0.5
0.5 GeV/c
0.0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
0.4 GeV/c
0 < PT(chgjet#1) < 50 GeV/c
 Multiply by ratio of the areas:
Max=(2.1 GeV/c)(1.36) = 2.9 GeV/c
Min=(0.4 GeV/c)(1.36) = 0.5 GeV/c.
 This comparison is only qualitative!
50 < ET(jet#1) < 300 GeV/c
Tano-Kovacs-Huston-Bhatti
Can study the “underlying event” over a wide range!
Les Houches 2001
Rick Field - Florida/CDF
Page 17
“Transverse” Nchg
versus PT(chgjet#1)
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Herwig 5.9
"Transverse" <Nchg> in 1 GeV/c bin
Df
Isajet 7.32
"Transverse" Nchg versus PT(charged jet#1)
5
CDF Preliminary
4
data uncorrected
theory corrected
3
2
Pythia 6.115
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Herwig
Isajet
Pythia 6.115
CDF Min-Bias
CDF JET20
 Plot shows the “transverse” <Nchg> versus PT(chgjet#1) compared to the the QCD hard

scattering predictions of HERWIG 5.9, ISAJET 7.32, and PYTHIA 6.115 (default
parameters with PT(hard)>3 GeV/c).
Only charged particles with |h| < 1 and PT > 0.5 GeV are included and the QCD MonteCarlo predictions have been corrected for efficiency.
Les Houches 2001
Rick Field - Florida/CDF
Page 18
“Transverse” PTsum
versus PT(chgjet#1)
Charged Jet #1
Direction
Df
“Transverse”
“Transverse”
“Away”
5
<Ptsum> (GeV/c) in 1 GeV/c bin
“Toward”
Isajet 7.32
"Transverse" PTsum versus PT(charged jet#1)
CDF Preliminary
4
data uncorrected
theory corrected
3
2
Pythia 6.115
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
Herwig 5.9
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Herwig
Isajet
Pythia 6.115
CDF Min-Bias
CDF JET20
 Plot shows the “transverse” <PTsum> versus PT(chgjet#1) compared to the the QCD

hard scattering predictions of HERWIG 5.9, ISAJET 7.32, and PYTHIA 6.115 (default
parameters with PT(hard)>3 GeV/c).
Only charged particles with |h| < 1 and PT > 0.5 GeV are included and the QCD MonteCarlo predictions have been corrected for efficiency.
Les Houches 2001
Rick Field - Florida/CDF
Page 19
ISAJET: “Transverse” Nchg
versus PT(chgjet#1)
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Beam-Beam
Remnants
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
ISAJET
4
Isajet Total
CDF Preliminary
data uncorrected
theory corrected
3
Initial-State
Radiation
2
Initial-State Radiation
1
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
Outgoing Jets +
Final-State Radiation
0
Outgoing Jets
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 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 three categories: charged particles that
arise from the break-up of the beam and target (beam-beam remnants), charged
particles that arise from initial-state radiation, and charged particles that result from
the outgoing jets plus final-state radiation.
Les Houches 2001
Rick Field - Florida/CDF
Page 20
ISAJET: “Transverse” Nchg
versus PT(chgjet#1)
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Beam-Beam
Remnants
"Transverse" <Nchg> in 1 GeV/c bin
Df
ISAJET
"Transverse" Nchg versus PT(charged jet#1)
4
Isajet Total
CDF Preliminary
data uncorrected
theory corrected
3
Hard Component
Outgoing Jets
plus
Initial &
Final-State
Radiation
2
1
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 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).
Les Houches 2001
Rick Field - Florida/CDF
Page 21
HERWIG: “Transverse” Nchg
versus PT(chgjet#1)
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Beam-Beam
Remnants
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
4
HERWIG
CDF Preliminary
Herwig Total
data uncorrected
theory corrected
3
2
Hard Component
1
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT (charged jet#1) (GeV/c)
Outgoing Jets
plus
Initial &
Final-State
Radiation
 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).
Les Houches 2001
Rick Field - Florida/CDF
Page 22
PYTHIA: “Transverse” Nchg
versus PT(chgjet#1)
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Beam-Beam Remnants
plus
Multiple Parton Interactions
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
PYTHIA
4
CDF Preliminary
Pythia 6.115 Total
data uncorrected
theory corrected
3
2
Beam-Beam Remnants
1
Hard Component
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Outgoing Jets
plus
Initial &
Final-State
Radiation
 Plot shows the “transverse” <Nchg> vs PT(chgjet#1) compared to the QCD hard

scattering predictions of PYTHIA 6.115 (default parameters with PT(hard)>3 GeV/c).
The predictions of PYTHIA are divided into two categories: charged particles that arise
from the break-up of the beam and target (beam-beam remnants including multiple
parton interactions); and charged particles that arise from the outgoing jet plus initial
and final-state radiation (hard scattering component).
Les Houches 2001
Rick Field - Florida/CDF
Page 23
Hard Scattering Component:
“Transverse” Nchg vs PT(chgjet#1)
Df
“Toward”
“Transverse”
“Transverse”
“Away”
ISAJET
"Transverse" Nchg versus PT(charged jet#1)
"Transverse" <Nchg> in 1 GeV/c bin
Charged Jet #1
Direction
3
Hard Scattering Component
Isajet
theory corrected
PYTHIA
2
Pythia 6.115
1
HERWIG
Herwig
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 QCD hard scattering predictions of HERWIG 5.9, ISAJET 7.32, and PYTHIA 6.115.
 Plot shows the “transverse” <Nchg> vs PT(chgjet#1) arising from the outgoing jets plus

initial and finial-state radiation (hard scattering component).
HERWIG and PYTHIA modify the leading-log picture to include “color coherence
effects” which leads to “angle ordering” within the parton shower. Angle ordering
produces less high PT radiation within a parton shower.
Les Houches 2001
Rick Field - Florida/CDF
Page 24
ISAJET: “Transverse”
PT Distribution
"Transverse" PT Distribution (charged)
4
1.0E+01
Isajet Total
CDF Preliminary
data uncorrected
theory corrected
3
CDF Preliminary
Hard Component
data uncorrected
theory corrected
PT(chgjet1) > 5 GeV/c
1.0E+00
2
Isajet 7.32
1
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
PT(charged jet#1) > 30 GeV/c
“Transverse” <Nchg> = 3.7
50
dNchg/dPT (1/GeV/c)
"Transverse" <Nchg> in 1 GeV/c bin
"Transverse" Nchg versus PT(charged jet#1)
1.0E-01
1.8 TeV |h|<1
1.0E-02
1.0E-03
PT(chgjet1) > 2 GeV/c
1.0E-04
PT(chgjet1) > 30 GeV/c
1.0E-05
0
PT(charged jet#1) > 5 GeV/c
“Transverse” <Nchg> = 2.0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Data on the “transverse” <Nchg> versus PT(charged jet#1) and the PT distribution of
the “transverse” <Nchg>, dNchg/dPT, compared with the QCD Monte-Carlo predictions
of ISAJET 7.32 (default parameters with with PT(hard) > 3 GeV/c). The integral of
dNchg/dPT is the “transverse” <Nchg>.
Les Houches 2001
Rick Field - Florida/CDF
Page 25
ISAJET: “Transverse”
PT Distribution
"Transverse" PT Distribution (charged)
"Transverse" PT Distribution (charged)
1.0E+01
1.0E+01
PT(charged jet#1) > 5 GeV/c
PT(charged jet#1) > 30 GeV/c
1.0E+00
Isajet
Total
1.0E-01
1.0E-02
CDF Preliminary
1.0E-03
data uncorrected
theory corrected
same
1.8 TeV |h|<1
1.0E-04
data uncorrected
theory corrected
1.0E+00
dNchg/dPT (1/GeV/c)
dNchg/dPT (1/GeV/c)
exp(-2pT)
CDF Preliminary
1.8 TeV |eta|<1
Isajet
Total
1.0E-01
1.0E-02
1.0E-03
Beam-Beam
Remnants
Beam-Beam
Remnants
1.0E-04
1.0E-05
0
1
2
3
4
5
6
7
0
2
4
6
8
10
12
14
PT(charged) GeV/c
PT(charged) GeV/c
 Data on the PT distribution of the “transverse” <Nchg>, dNchg/dPT, compared with the
QCD Monte-Carlo predictions of ISAJET 7.32 (default parameters with with PT(hard)
> 3 GeV/c). The dashed curve is the beam-beam remnant component and the solid
curve is the total (beam-beam remnants plus hard component).
Les Houches 2001
Rick Field - Florida/CDF
Page 26
HERWIG: “Transverse”
PT Distribution
"Transverse" PT Distribution (charged)
4
1.0E+01
CDF Preliminary
Herwig Total
CDF Preliminary
data uncorrected
theory corrected
3
data uncorrected
theory corrected
PT(chgjet1) > 5 GeV/c
1.0E+00
2
Herwig 5.9
Hard Component
1
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
PT (charged jet#1) (GeV/c)
PT(charged jet#1) > 30 GeV/c
“Transverse” <Nchg> = 2.2
50
dNchg/dPT (1/GeV/c)
"Transverse" <Nchg> in 1 GeV/c bin
"Transverse" Nchg versus PT(charged jet#1)
1.0E-01
1.8 TeV |h|<1
1.0E-02
1.0E-03
PT(chgjet1) > 2 GeV/c
1.0E-04
PT(chgjet1) > 30 GeV/c
1.0E-05
PT(charged jet#1) > 5 GeV/c
“Transverse” <Nchg> = 1.7
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Data on the “transverse” <Nchg> versus PT(charged jet#1) and the PT distribution of
the “transverse” <Nchg>, dNchg/dPT, compared with the QCD Monte-Carlo predictions
of HERWIG 5.9 (default parameters with with PT(hard) > 3 GeV/c). The integral of
dNchg/dPT is the “transverse” <Nchg>.
Les Houches 2001
Rick Field - Florida/CDF
Page 27
HERWIG: “Transverse”
PT Distribution
"Transverse" PT Distribution (charged)
"Transverse" PT Distribution (charged)
1.0E+01
1.0E+01
PT(charged jet#1) > 30 GeV/c
PT(charged jet#1) > 5 GeV/c
CDF Preliminary
data uncorrected
theory corrected
1.0E+00
exp(-2pT)
1.0E+00
1.0E-01
dNchg/dPT (1/GeV/c)
dNchg/dPT (1/GeV/c)
1.8 TeV |h|<1
Herwig
Total
1.0E-02
1.0E-03
Herwig
Total
1.0E-01
1.0E-02
CDF Preliminary
data uncorrected
theory corrected
1.0E-04
same
1.0E-03
Beam-Beam
Remnants
Beam-Beam
Remnants
1.8 TeV |h|<1
1.0E-04
1.0E-05
0
1
2
3
4
5
6
7
0
2
4
6
8
10
12
14
PT(charged) GeV/c
PT(charged) GeV/c
 Data on the PT distribution of the “transverse” <Nchg>, dNchg/dPT, compared with the
QCD Monte-Carlo predictions of HERWIG 5.9 (default parameters with with
PT(hard) > 3 GeV/c). The dashed curve is the beam-beam remnant component and the
solid curve is the total (beam-beam remnants plus hard component).
Les Houches 2001
Rick Field - Florida/CDF
Page 28
PYTHIA: “Transverse”
PT Distribution
"Transverse" PT Distribution (charged)
4
1.0E+01
CDF Preliminary
Pythia 6.115 Total
CDF Preliminary
data uncorrected
theory corrected
3
data uncorrected
theory corrected
PT(chgjet1) > 5 GeV/c
1.0E+00
2
Pythia 6.115
Beam-Beam Remnants
1
Hard Component
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
Includes
Multiple
Parton
Interactions
PT(charged jet#1) > 30 GeV/c
“Transverse” <Nchg> = 2.9
50
dNchg/dPT (1/GeV/c)
"Transverse" <Nchg> in 1 GeV/c bin
"Transverse" Nchg versus PT(charged jet#1)
1.8 TeV |h|<1
1.0E-01
1.0E-02
1.0E-03
PT(chgjet1) > 2 GeV/c
1.0E-04
PT(chgjet1) > 30 GeV/c
1.0E-05
PT(charged jet#1) > 5 GeV/c
“Transverse” <Nchg> = 2.3
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Data on the “transverse” <Nchg> versus PT(charged jet#1) and the PT distribution of
the “transverse” <Nchg>, dNchg/dPT, compared with the QCD Monte-Carlo predictions
of PYTHIA 6.115 (default parameters with with PT(hard) > 3 GeV/c). The integral of
dNchg/dPT is the “transverse” <Nchg>.
Les Houches 2001
Rick Field - Florida/CDF
Page 29
PYTHIA: Multiple Parton
Interactions
Multiple Parton Interactions
Outgoing Parton
PT(hard)
Proton
AntiProton
Underlying Event
Parameter
Underlying Event
Value
Outgoing Parton
MSTP(81)
MSTP(82)
Les Houches 2001
Pythia uses multiple parton
interactions to enhace
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)
Rick Field - Florida/CDF
and new
HERWIG
!
Multiple parton
interaction more
likely in a hard
(central) collision!
Hard Core
Page 30
PYTHIA
Multiple Parton Interactions
Parameter
6.115
MSTP(81)
1
MSTP(82)
PARP(81)
PARP(82)
6.125
1
1
1
1.4
GeV/c
1.9
GeV/c
1.55
GeV/c
2.1
GeV/c
"Transverse" <Nchg> in 1 GeV/c bin
PYTHIA default parameters
"Transverse" Nchg versus PT(charged jet#1)
5
6.115
CDF Preliminary
data uncorrected
theory corrected
4
3
2
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
PT(charged jet#1) (GeV/c)
No multiple
scattering
Pythia 6.115
Pythia 6.125
Pythia No MS
CDF Min-Bias
45
6.125
50
CDF JET20
 Plot shows “Transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard



scattering predictions of PYTHIA with PT(hard) > 3 GeV.
PYTHIA 6.115: GRV94L, MSTP(82)=1, PTmin=PARP(81)=1.4 GeV/c.
PYTHIA 6.125: GRV94L, MSTP(82)=1, PTmin=PARP(81)=1.9 GeV/c.
PYTHIA 6.115: GRV94L, MSTP(81)=0, no multiple parton interactions.
Les Houches 2001
Rick Field - Florida/CDF
Constant
Probability
Scattering
Page 31
PYTHIA
Multiple Parton Interactions
Charged Jet #1
Direction
"Transverse" Nchg versus PT(charged jet#1)
Df
“Transverse”
“Transverse”
“Away”
Note: Multiple parton
interactions depend
sensitively on the
PDF’s!
"Transverse" <Nchg> in 1 GeV/c bin
“Toward”
5
CDF Preliminary
4
GRV94L MSTP(82)=1
PARP(81) = 1.4 GeV/c
data uncorrected
theory corrected
CTEQ3L MSTP(82)=1
PARP(81) = 0.9 GeV/c
3
2
1
CTEQ3L MSTP(82)=1
PARP(81) = 1.4 GeV/c
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Plot shows “transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard



scattering predictions of PYTHIA with PT(hard) > 0 GeV/c.
PYTHIA 6.115: GRV94L, MSTP(82)=1, PTmin=PARP(81)=1.4 GeV/c.
PYTHIA 6.115: CTEQ3L, MSTP(82)=1, PTmin =PARP(81)=1.4 GeV/c.
PYTHIA 6.115: CTEQ3L, MSTP(82)=1, PTmin =PARP(81)=0.9 GeV/c.
Les Houches 2001
Rick Field - Florida/CDF
Constant
Probability
Scattering
Page 32
PYTHIA
Multiple Parton Interactions
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Note: Multiple parton
interactions depend
sensitively on the
PDF’s!
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
5
GRV94L MSTP(82)=3
PARP(82) = 1.55 GeV/c
CDF Preliminary
data uncorrected
theory corrected
4
CTEQ3L MSTP(82)=3
PARP(82) = 1.35 GeV/c
CTEQ4L MSTP(82)=3
PARP(82) = 1.8 GeV/c
3
2
1
CTEQ3L MSTP(82)=3
PARP(82) = 1.55 GeV/c
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Plot shows “transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard




scattering predictions of PYTHIA with PT(hard) > 0 GeV/c.
PYTHIA 6.115: GRV94L, MSTP(82)=3, PT0=PARP(82)=1.55 GeV/c.
PYTHIA 6.115: CTEQ3L, MSTP(82)=3, PT0=PARP(82)=1.55 GeV/c.
PYTHIA 6.115: CTEQ3L, MSTP(82)=3, PT0=PARP(82)=1.35 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=3, PT0=PARP(82)=1.8 GeV/c.
Les Houches 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 33
PYTHIA
Multiple Parton Interactions
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Note: Multiple parton
interactions depend
sensitively on the
PDF’s!
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
6
CDF Preliminary
5
data uncorrected
theory corrected
CTEQ4L MSTP(82)=4
PARP(82) = 1.55 GeV/c
4
CTEQ3L MSTP(82)=4
PARP(82) = 1.55 GeV/c
CTEQ4L MSTP(82)=4
PARP(82) = 2.4 GeV/c
3
2
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Plot shows “transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard



scattering predictions of PYTHIA with PT(hard) > 0 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=4, PT0=PARP(82)=1.55 GeV/c.
PYTHIA 6.115: CTEQ3L, MSTP(82)=4, PT0=PARP(82)=1.55 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
Les Houches 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Hard Core
Page 34
Tuned PYTHIA:
“Transverse” Nchg vs PT(chgjet#1)
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Describes correctly the
rise from soft-collisions
to hard-collisions!
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
5
CDF Preliminary
4
data uncorrected
theory corrected
CTEQ4L MSTP(82)=4
PARP(82) = 2.4 GeV/c
3
2
CTEQ4L MSTP(82)=3
PARP(82) = 1.8 GeV/c
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Plot shows “transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard


scattering predictions of PYTHIA with PT(hard) > 0 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=3, PT0=PARP(82)=1.8 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
Les Houches 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 35
Tuned PYTHIA:
“Transverse” PTsum vs PT(chgjet#1)
Charged Jet #1
Direction
"Transverse" PTsum versus PT(charged jet#1)
Df
“Transverse”
“Transverse”
“Away”
Describes correctly the
rise from soft-collisions
to hard-collisions!
<PTsum> (GeV/c) in 1 GeV/c bin
“Toward”
5
CTEQ4L MSTP(82)=4
PARP(82) = 2.4 GeV/c
CDF Preliminary
4
data uncorrected
theory corrected
3
2
CTEQ4L MSTP(82)=3
PARP(82) = 1.8 GeV/c
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Plot shows “transverse” <PTsum> versus PT(chgjet#1) compared to the QCD hard


scattering predictions of PYTHIA with PT(hard) > 0 GeV.
PYTHIA 6.115: CTEQ4L, MSTP(82)=3, PT0=PARP(82)=1.8 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
Les Houches 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 36
Tuned PYTHIA:
“Transverse” PT Distribution
"Transverse" PT Distribution (charged)
4
1.0E+01
CDF Preliminary
Pythia CTEQ4L (4, 2.4 GeV/c)
data uncorrected
theory corrected
3
CDF Preliminary
PT(chgjet1) > 5 GeV/c
data uncorrected
theory corrected
1.0E+00
2
Pythia CTEQ4L (4, 2.4 GeV/c)
1
Hard Component
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
Includes
Multiple
Parton
Interactions
PT(charged jet#1) > 30 GeV/c
“Transverse” <Nchg> = 2.7
50
dNchg/dPT (1/GeV/c)
"Transverse" <Nchg> in 1 GeV/c bin
"Transverse" Nchg versus PT(charged jet#1)
1.0E-01
1.8 TeV |h|<1
1.0E-02
1.0E-03
PT(chgjet1) > 2 GeV/c
1.0E-04
PT(chgjet1) > 30 GeV/c
1.0E-05
PT(charged jet#1) > 5 GeV/c
“Transverse” <Nchg> = 2.3
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Data on the “transverse” <Nchg> versus PT(charged jet#1) and the PT distribution of
the “transverse” <Nchg>, dNchg/dPT, compared with the QCD Monte-Carlo predictions
of PYTHIA 6.115 with PT(hard) > 0 GeV/c, CTEQ4L, MSTP(82)=4, PT0=PARP(82)=2.4
GeV/c. The integral of dNchg/dPT is the “transverse” <Nchg>.
Les Houches 2001
Rick Field - Florida/CDF
Page 37
Tuned PYTHIA:
“Transverse” PT Distribution
"Transverse" PT Distribution (charged)
"Transverse" PT Distribution (charged)
1.0E+01
1.0E+01
PT(charged jet#1) > 30 GeV/c
PT(charged jet#1) > 5 GeV/c
CDF Preliminary
data uncorrected
theory corrected
1.0E+00
1.0E+00
1.0E-01
1.8 TeV |h|<1
Includes
Multiple
Parton
Interactions
1.0E-02
1.0E-03
CDF Preliminary
Beam-Beam
Remnants
data uncorrected
theory corrected
1.0E-04
dNchg/dPT (1/GeV/c)
dNchg/dPT (1/GeV/c)
Pythia CTEQ4L (4, 2.4 GeV/c)
Pythia CTEQ4L (4, 2.4 GeV/c)
1.0E-01
1.0E-02
1.0E-03
Beam-Beam Remnants
1.8 TeV |h|<1
1.0E-04
1.0E-05
0
1
2
3
4
5
6
7
0
2
4
6
8
10
12
14
PT(charged) GeV/c
PT(charged) GeV/c
 Data on the PT distribution of the “transverse” <Nchg>, dNchg/dPT, compared with the
QCD Monte-Carlo predictions of PYTHIA 6.115 with PT(hard) > 0, CTEQ4L, MSTP(82)=4,
PT0=PARP(82)=2.4 GeV/c. The dashed curve is the beam-beam remnant component and
the solid curve is the total (beam-beam remnants plus hard component).
Les Houches 2001
Rick Field - Florida/CDF
Page 38
Tuned PYTHIA:
“TransMAX/MIN” vs PT(chgjet#1)
Charged Jet #1
Direction
"Max/Min Transverse" Nchg
"Transverse" <Nchg> in 1 GeV/c bin
<Nchg>
Df
“Toward”
“TransMAX”
“TransMIN”
“Away”
<PTsum>
3.0
data uncorrected
theory corrected
"Max Transverse"
2.0
1.5
1.0
"Min Transverse"
0.5
1.8 TeV |h|<1.0 PT>0.5 GeV
0.0
0
 Plots shows data on the
5
10
15
20
25
30
35
40
45
50
45
50
PT(charged jet#1) (GeV/c)
“transMAX/MIN” <Nchg> and
“transMAX/MIN” <PTsum> vs
PT(chgjet#1). The solid (open) points are
the Min-Bias (JET20) data.
The data are compared with the QCD
Monte-Carlo predictions of PYTHIA
6.115 with PT(hard) > 0, CTEQ4L,
MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
"Max/Min Transverse" PTsum
3.5
<PTsum> (GeV/c) in 1 GeV/c bin

Pythia CTEQ4L (4, 2.4 GeV/c)
CDF Preliminary
2.5
CDF Preliminary
3.0
Pythia CTEQ4L (4, 2.4 GeV/c)
data uncorrected
theory corrected
2.5
"Max Transverse"
2.0
1.5
1.0
"Min Transverse"
0.5
1.8 TeV |h|<1.0 PT>0.5 GeV
0.0
0
5
10
15
20
25
30
35
40
PT(charged jet#1) (GeV/c)
Les Houches 2001
Rick Field - Florida/CDF
Page 39
Tuned PYTHIA:
“TransSUM/DIF” vs PT(chgjet#1)
Charged Jet #1
Direction
SUM/DIF "Transverse" Nchg
<Nchg>
4
CDF Preliminary
<Nchg> in 1 GeV/c bin
Df
“Toward”
“TransMAX”
“TransMIN”
“Away”
<PTsum>
Pythia CTEQ4L (4, 2.4 GeV/c)
data uncorrected
theory corrected
3
"Max+Min Transverse"
2
"Max-Min Transverse"
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
 Plots shows data on the
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15
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40
45
50
PT(charged jet#1) (GeV/c)
“transSUM/DIF” <Nchg> and
“transSUM/DIF” <PTsum> vs
PT(chgjet#1). The solid (open) points are
the Min-Bias (JET20) data.
The data are compared with the QCD
Monte-Carlo predictions of PYTHIA
6.115 with PT(hard) > 0, CTEQ4L,
MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
SUM/DIF "Transverse" PTsum
5
<PTsum> (GeV/c) in 1 GeV/c bin

5
CDF Preliminary
4
Pythia CTEQ4L (4, 2.4 GeV/c)
data uncorrected
theory corrected
"Max+Min Transverse"
3
2
"Max-Min Transverse"
1
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
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PT(charged jet#1) (GeV/c)
Les Houches 2001
Rick Field - Florida/CDF
Page 40
Tuned PYTHIA:
“TransMAX/MIN” vs PT(chgjet#1)
“Toward”
“TransMAX”
“TransMIN”
“Away”
“TransMIN” <Nchg>
Includes
Multiple
Parton
Interactions
 Data on the “transMAX/MIN” Nchg vs

PT(chgjet#1) comared with the QCD
Monte-Carlo predictions of PYTHIA
6.115 with PT(hard) > 0, CTEQ4L,
MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
The predictions of PYTHIA 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 jets plus initial and final-state
radiation (hard scattering component).
Les Houches 2001
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Max Transverse" Nchg versus PT(charged jet#1)
“TransMAX” <Nchg>
3.0
CDF Preliminary
2.5
Pythia CTEQ4L (4, 2.4 GeV/c)
data uncorrected
theory corrected
2.0
1.5
Hard Component
1.0
0.5
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0.0
0
5
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25
30
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50
45
50
PT(charged jet#1) (GeV/c)
"Min Transverse" Nchg versus PT(charged jet#1)
"Transverse" <Nchg> in 1 GeV/c bin
Charged Jet #1
Direction
1.2
CDF Preliminary
1.8 TeV |h|<1.0 PT>0.5 GeV
data uncorrected
theory corrected
Pythia CTEQ4L (4, 2.4 GeV/c)
0.8
0.4
Hard Component
Beam-Beam Remnants
0.0
0
5
Rick Field - Florida/CDF
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PT(charged jet#1) (GeV/c)
Page 41
Tuned PYTHIA:
“TransSUM/DIF” vs PT(chgjet#1)
“Toward”
“TransMAX”
“TransMIN”
“Away”
“TransDIF” <Nchg>
Includes
Multiple
Parton
Interactions
 Data on the “transSUM/DIF” Nchg vs

PT(chgjet#1) comared with the QCD
Monte-Carlo predictions of PYTHIA
6.115 with PT(hard) > 0, CTEQ4L,
MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
The predictions of PYTHIA 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).
Les Houches 2001
"Transverse" <Nchg> in 1 GeV/c bin
Df
"Transverse" Nchg versus PT(charged jet#1)
“TransSUM” <Nchg>
4
CDF Preliminary
Pythia CTEQ4L (4, 2.4 GeV/c)
data uncorrected
theory corrected
3
2
1
Hard Component
Beam-Beam Remnants
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
10
15
20
25
30
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45
50
45
50
PT(charged jet#1) (GeV/c)
"Max-Min Transverse" Nchg
2.5
CDF Preliminary
<Nchg> in 1 GeV/c bin
Charged Jet #1
Direction
1.8 TeV |h|<1.0 PT>0.5 GeV
data uncorrected
theory corrected
2.0
Pythia CTEQ4L (4, 2.4 GeV/c)
1.5
1.0
Hard Component
0.5
Beam-Beam Remnants
0.0
0
5
Rick Field - Florida/CDF
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PT(charged jet#1) (GeV/c)
Page 42
The Underlying Event:
Summary & Conclusions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event
Underlying Event
The “Underlying Event”
Final-State
Radiation
 Combining the two CDF analyses gives a quantitative study of the underlying event from
Outgoing Parton




very soft collisions to very hard collisions.
ISAJET (with independent fragmentation) produces too many (soft) particles in the
underlying event with the wrong dependence on PT(jet#1). HERWIG and PYTHIA
modify the leading-log picture to include “color coherence effects” which leads to “angle
ordering” within the parton shower and do a better job describing the underlying event.
Both ISAJET and HERWIG have the too steep of a PT dependence of the beam-beam
remnant component of the underlying event and hence do not have enough beam-beam
remnants with PT > 0.5 GeV/c.
PYTHIA (with multiple parton interactions) does the best job in describing the
underlying event.
Perhaps the multiple parton interaction approach is correct or maybe we simply need to
improve the way the Monte-Carlo models handle the beam-beam remnants (or both!).
Les Houches 2001
Rick Field - Florida/CDF
Page 43
Multiple Parton Interactions:
Summary & Conclusions
Multiple Parton Interactions
Proton
AntiProton
Hard Core
Hard Core
 The increased activity in the underlying event in a hard scattering over a soft collision



cannot be explained by initial-state radiation.
Multiple parton interactions gives a natural way of explaining the increased activity in
the underlying event in a hard scattering. A hard scattering is more likely to occur when
the hard cores overlap and this is also when the probability of a multiple parton
interaction is greatest. For a soft grazing collision the probability of a multiple parton
interaction is small.
Slow!
PYTHIA (with varying impact parameter) describes the underlying event data fairly
well. However, there are problems in fitting min-bias events with this approach.
Multiple parton interactions are very sensitive to the parton structure functions. You
must first decide on a particular PDF and then tune the multiple parton interactions to
fit the data.
Les Houches 2001
Rick Field - Florida/CDF
Page 44