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
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.
 It is important to model this region well
since it is an unavoidable background to all
collider observables.
 I will report on two CDF analyses which
quantitatively study the underlying event
and compare with the QCD Monte-Carlo
models.
Run 2 Monte-Carlo
Workshop April 20, 2001
AntiProton
Underlying Event
Outgoing Parton
Final-State
Radiation
CDF
QFL+Cones
Valeria Tano
Eve Kovacs
Joey Huston
Anwar Bhatti
Rick Field - Florida/CDF
CDF
WYSIWYG+Df
Rick Field
David Stuart
Rich Haas
Ph.D. Thesis
Page 1
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
Run 2 Monte-Carlo
Workshop April 20, 2001
QCD
Monte-Carlo

track finding efficiency
Errors (statistical plus
systematic) of around 5%
Corrected theory
Rick Field - Florida/CDF
Small
Corrections!
Page 2
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 3
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 4
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
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 5
“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.
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
Run 2 Monte-Carlo
Workshop April 20, 2001
Isajet "Soft" MB
MBR
10
Implies 2.3*3.9 = 9 charged
particles per unit h
with PT > 0 GeV which is
a factor of 2 larger
than “soft” collisions.
CDF DATA
Rick Field - Florida/CDF
Page 6
“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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 7
“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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 8
The Underlying Event:
DiJet vs Z-Jet
Charged Jet #1
Direction
“Toward-Side” Jet
Charged Jet #1
or
Z-Boson
Direction
Df
Z-Boson
Direction
Df
“Toward”
Df
“Toward”
“Transverse”
“Transverse”
“Toward”
“Transverse”
“Transverse”
“Transverse”
“Transverse”
“Away”
“Away”
“Away-Side” Jet
“Away”
PT > 0.5 GeV |h| < 1
“Away-Side” Jet
 Look at charged particle correlations in the azimuthal angle Df relative to the leading charged
particle jet or the Z-boson.
 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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 9
Z-boson: Charged Multiplicity
versus PT(Z)
Z-boson
Direction
10
"Away"
Df
CDF Preliminary
data uncorrected
8
“Toward”
“Transverse”
Nchg versus PT(Z-boson)
<Nchg>
“Transverse”
6
4
2
"Toward"
“Away”
"Transverse"
1.8 TeV |eta|<1.0 PT>0.5 GeV
0
0
10
20
30
40
50
60
70
80
90
100
PT(Z-boson) (GeV)
 Z-boson 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, excluding decay products of the Zboson) as a function of the transverse momentum of the Z-boson. The errors on the (uncorrected)
data include both statistical and correlated systematic uncertainties.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 10
DiJet vs Z-Jet
“Transverse” Nchg
Charged Jet #1
or
Z-boson
Direction
Df
“Toward”
“Transverse”
“Transverse”
"Transverse" Nchg vs PT(charged jet#1 or Z-boson)
<Nchg>
PYTHIA
5
CDF Preliminary
data uncorrected
data corrected
4
3
2
1
“Away”
DiJet
1.8 TeV |eta|<1.0 PT>0.5 GeV
0
0
Z-boson
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1 or Z-boson) (GeV)
Pythia Z-jet
Pythia DiJet
CDF Z-boson
CDF Min-Bias
CDF JET20
 Comparison of the dijet and the Z-boson data on the average number of charged

particles (PT > 0.5 GeV, |h| <1) for the “transverse” region.
The plot shows the QCD Monte-Carlo predictions of PYTHIA 6.115 (default
parameters with PT(hard)>3 GeV/c) for dijet (dashed) and “Z-jet” (solid) production.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 11
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.
QFL
Tano-Kovacs-Huston-Bhatti
 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
compare
 Require PT > 0.4 GeV, |h| < 1
 Correct for track finding
efficiency
Uncorrected data
Run 2 Monte-Carlo
Workshop April 20, 2001
detector
simulation
Corrected theory
Rick Field - Florida/CDF
Page 12
“Transverse” Cones
2p
2p
Transverse
Cone:
p(0.7)2=0.49p
Away Region
Transverse
Region
f
Leading
Jet
Toward Region
Transverse
Region:
2(p/3)=0.66p
Tano-Kovacs-Huston-Bhatti
Cone 1
f
Leading
Jet
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..
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 13
Transverse Region
vs Transverse Cones
Field-Stuart-Haas
"Transverse" PTsum versus PT(charged jet#1)
<Ptsum> (GeV/c) in 1 GeV/c bin
5
CDF Preliminary
4
data uncorrected
theory corrected
3.4 GeV/c
3
2
1
2.1 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)
Herwig
Isajet
Pythia 6.115
CDF Min-Bias
CDF JET20
0.4 GeV/c
0 < PT(chgjet#1) < 50 GeV/c
 Add max and min cone:
2.1 GeV/c + 0.4 GeV/c = 2.5 GeV/c.
 Multiply by ratio of the areas:
(2.5 GeV/c)(1.36) = 3.4 GeV/c.
 The two analyses are consistent!
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
0 < ET(jet#1) < 50 GeV/c
Tano-Kovacs-Huston-Bhatti
Page 14
Max/Min Cones
at 630 GeV/c
 HERWIG+QFL slightly lower at 1,800 GeV/c
agrees at 630 GeV/c.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Tano-Kovacs-Huston-Bhatti
Page 15
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
Outgoing Jets
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 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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 16
PYTHIA: “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)
PYTHIA
4
CDF Preliminary
Pythia 6.115 Total
data uncorrected
theory corrected
3
2
Beam-Beam Remnants
1
Outgoing Jets +
Initial & Final-State Radiation
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); and charged
particles that arise from the outgoing jet plus initial and final-state radiation (hard
scattering component).
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 17
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
Outgoing Jets +
Initial & Final-State Radiation
PYTHIA
Isajet
theory corrected
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 dijet “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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 18
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)
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)
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
and new
HERWIG
!
Multiple parton
interaction more
likely in a hard
(central) collision!
Hard Core
Page 19
PYTHIA
Multiple Parton Interactions
Parameter
MSTP(81)
MSTP(82)
PARP(81)
PARP(82)
6.115
6.125
1
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Constant
Probability
Scattering
Page 20
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.
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Constant
Probability
Scattering
Page 21
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
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.
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 22
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.
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Hard Core
Page 23
PYTHIA
Multiple Parton Interactions
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
CTEQ3L MSTP(82)=3
PARP(82) = 1.35 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.
PYTHIA 6.115: CTEQ3L, MSTP(82)=3, PT0=PARP(82)=1.35 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 24
PYTHIA
Multiple Parton Interactions
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
CDF Preliminary
4
CTEQ4L MSTP(82)=4
PARP(82) = 2.4 GeV/c
data uncorrected
theory corrected
3
2
CTEQ3L MSTP(82)=3
PARP(82) = 1.35 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: CTEQ3L, MSTP(82)=3, PT0=PARP(82)=1.35 GeV/c.
PYTHIA 6.115: CTEQ4L, MSTP(82)=4, PT0=PARP(82)=2.4 GeV/c.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 25
The Underlying Event:
Summary & Conclusions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event
Underlying Event
The “Underlying Event”
Final-State
Radiation
 The underlying event is very similar in dijet and the Z-boson production as predicted by
Outgoing Parton




the QCD Monte-Carlo models.
The number of charged particles per unit rapidity (height of the “plateau”) is at least
twice that observed in “soft” collisions at the same corresponding energy.
ISAJET (with independent fragmentation) produces too many (soft) particles in the
underlying event with the wrong dependence on PT(jet#1) or PT(Z). 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. HERWIG 5.9 does not have enough activity in the underlying event.
PYTHIA (with multiple parton interactions) does the best job in describing the
underlying event.
Combining the two CDF analyses gives a quantitative study of the underlying event from
very soft collisions to very hard collisions.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 26
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 data very nicely! I need to
check out the new version of HERWIG.
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.
Run 2 Monte-Carlo
Workshop April 20, 2001
Rick Field - Florida/CDF
Page 27