The “Underlying Event” in
Hard Scattering Processes
 What happens when a proton and an
antiproton collide with a center-ofmass energy of 2 TeV?
 Most of the time the proton and
antiproton ooze through each other
and fall apart (i.e. no hard scattering).
The outgoing particles continue in
roughly the same direction as initial
proton and antiproton.
“Soft” Collision (no hard scattering)
ProtonProton
“Hard” Scattering
D0 Meeting
September 6, 2002
Outgoing Parton
PT(hard)
Proton
AntiProton
Underlying Event
 Occasionally there will be a “hard”
parton-parton collision resulting in large
transverse momentum outgoing partons.
 The “underlying event” is everything
except the two outgoing hard scattered
“jets”. It is an unavoidable background
to many collider observables.
AntiProton
AntiProton
2 TeV
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Outgoing Parton
“Underlying Event”
Proton
Beam-Beam Remnants
Rick Field - Florida/CDF
AntiProton
Beam-Beam Remnants
Initial-State
Radiation
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
D0 Meeting
September 6, 2002
 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
Studying the “Underlying Event”
at CDF
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
CDF
CDF
Evolution of Charged Jets
Cone Analysis
Valeria Tano
Eve Kovacs
Joey Huston
Anwar Bhatti
Ph.D. Thesis
D0 Meeting
September 6, 2002
Final-State
Radiation
Rick Field - Florida/CDF
Rick Field
David Stuart
Rich Haas
PRD65:092002, 2002
Page 3
Evolution of Charged Jets
“Underlying Event”
Charged Particle  Correlations
PT > 0.5 GeV/c |h| < 1
Charged Jet #1
Direction
2p
Away Region
Toward-side “jet”
(always)
Charged Jet #1
Direction
Perpendicular to the plane of the 
“Toward-Side” Jet

“Toward”
“Transverse”
2-to-2 hard scattering
“Toward”
Transverse
Region

Leading
Jet
Toward Region
“Transverse”
“Away”
“Away-Side” Jet
“Transverse”
“Transverse”
Transverse
Region
“Away”
Very sensitive to the
“underlying event”
Away Region
0
-1
h
+1
Away-side “jet”
 Look at charged particle correlations in the azimuthal angle  relative to the leading charged
(sometimes)
particle jet.
 Define || < 60o as “Toward”, 60o < || < 120o as “Transverse”, and || > 120o as “Away”.
 All three regions have the same size in h- space, hx = 2x120o = 4p/3.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 4
Charged Multiplicity
versus PT(chgjet#1)
Charged Jet #1
Direction

“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)
Factor of 2 more active than
an average Min-Bias event!
 Data on the average number of “toward” (||<60o), “transverse” (60<||<120o), and
“away” (||>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.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 5
“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.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 6
“Max/Min Transverse” Nchg
versus PT(chgjet#1)
Bryan Webber idea!
“TransMAX”
“TransMIN”
“Away”
1.8 TeV |h|<1.0 PT>0.5 GeV
CDF Preliminary
2.5
“Toward”
"Max/Min Transverse" Nchg
3.0

<Nchg> in 1 GeV/c bin
Area h
2x60o = 2p/3
Charged Jet #1
Direction
More sensitive to the “hard
scattering” component
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)
More sensitive to the
“beam-beam remnants”
“TransMIN”
 Define “TransMAX” and “TransMIN” to be the maximum and minimum of the region

60o<<120o (60o<-<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.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 7
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

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).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 8
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

"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
PT (charged jet#1) (GeV/c)
40
45
50
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).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 9
HERWIG has the too steep of a PT
dependence
of the “beam-beam remnant”
"Transverse" Nchg versus PT(charged jet#1)
component of the “underlying event”!
4
"Transverse" PT Distribution (charged)
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
HERWIG: “Transverse”
PT Distribution
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>.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 10
MPI: Multiple Parton
Interactions
“Hard”
Collision
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).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 11
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)
D0 Meeting
September 6, 2002
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 now
HERWIG
!
Herwig MPI
J. M. Butterworth
J. R. Forshaw
M. H. Seymour
Multiple parton
interaction more
likely in a hard
(central) collision!
Hard Core
Page 12
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

"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.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Varying
Impact
Parameter
Page 13
PYTHIA
Multiple Parton Interactions
Charged Jet #1
Direction
“Toward”
“TransMAX”
“TransMIN”
“Away”
Note dependence on PT0.
Larger PT0 means less
multiple parton interactions.
3.5
<PTsum> (GeV/c) in 1 GeV/c bin

"Max/Min Transverse" PTsum
CDF Preliminary
3.0
PYTHIA 6.115 CTEQ4L (3)
data uncorrected
theory corrected
"Max Transverse"
2.5
2.0
1.5
1.8 TeV |h|<1.0 PT>0.5 GeV
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)
 Plots shows data on the “transMAX/MIN” <PTsum> vs 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)=1.6 GeV/c (solid).
PYTHIA 6.115: CTEQ4L, MSTP(82)=3, PT0=PARP(82)=1.8 GeV/c (dashed).
PYTHIA 6.115: CTEQ4L, MSTP(82)=3, PT0=PARP(82)=2.0 GeV/c (dotted).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 14
PYTHIA 6.206 Defaults
PYTHIA default parameters
6.115
6.125
6.158
5
6.206
MSTP(81)
1
1
1
1
MSTP(82)
1
1
1
1
PARP(81)
1.4
1.9
1.9
1.9
PARP(82)
1.55
2.1
2.1
1.9
PARP(89)
1,000
1,000
1,000
PARP(90)
0.16
0.16
0.16
4.0
1.0
1.0
CDF
"Transverse" <Nchg>
Parameter
"Transverse" Nchg versus PT(charged jet#1)
4
3
2
1
1.8 TeV |h|<1.0 PT>0.5 GeV/c
0
0
PARP(67)
4.0
Pythia 6.206 (default)
MSTP(82)=1
PARP(81) = 1.9 GeV/c
data uncorrected
theory corrected
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
CTEQ3L
CTEQ4L
CTEQ5L
CDF Min-Bias
CDF JET20
 Plot shows “Transverse” <Nchg> versus PT(chgjet#1) compared to the QCD
hard scattering predictions of PYTHIA 6.206 (PT(hard) > 0) using the default
parameters for multiple parton interactions and CTEQ3L, CTEQ4L, and
CTEQ5L.
Constant
Note Change
PARP(67) = 4.0 (< 6.138)
PARP(67) = 1.0 (> 6.138)
D0 Meeting
September 6, 2002
Version 6.120
PT0(Ecm) = PT0(Ecm/E0)e
E0 = PARP(89) e = PARP(90)
Rick Field - Florida/CDF
Default parameters give
very poor description of
the “underlying event”!
Probability
Scattering
Page 15
Azimuthal Correlations
b-quark Correlations: Azimuthal  Distribution
b-quark
direction
0.100
1.8 TeV
PT1 > 5 GeV/c
PT2 > 0 GeV/c
|y1| < 1 |y2| < 1
ds/d (mb/deg)

“Toward”
Pythia CTEQ4L
0.010
“Away”
"Away"
"Toward"
bbar-quark
0.001
0
30
60
90
120
150
180
 (degrees)
Pythia Total

Flavor Creation
Flavor Excitation
Shower/Fragmentation
Predictions of PYTHIA 6.158 (CTEQ4L, PARP(67)=1) for the azimuthal angle, , between a bquark with PT1 > 5 GeV/c and |y1| < 1 and a bbar-quark with PT2 > 0 GeV/c and |y2|<1 in protonantiproton collisions at 1.8 TeV. The curves correspond to ds/d (mb/o) for flavor creation, flavor
excitation, shower/fragmentation, and the resulting total.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 16
Azimuthal Correlations
Old PYTHIA default
(more initial-state radiation)
New PYTHIA default
(less initial-state radiation)
b-quark Correlations: Azimuthal  Distribution
b-quark Correlations: Azimuthal  Distribution
0.01000
0.01000
1.8 TeV
PT1 > 15 GeV/c
PT2 > 10 GeV/c
|y1| < 1 |y2| < 1
PYTHIA 6.206
CTEQ5L PARP(67)=1
ds/d (mb/deg)
ds/d (mb/deg)
1.8 TeV
PT1 > 15 GeV/c
PT2 > 10 GeV/c
|y1| < 1 |y2| < 1
0.00100
0.00010
0.00100
0.00010
"Away"
"Toward"
"Away"
"Toward"
PYTHIA 6.206
CTEQ5L PARP(67)=4
0.00001
0.00001
0
30
60
90
120
150
180
0
30
60
 (degrees)
PY62 (67=1) Total
Flavor Creation
Flavor Excitation
90
120
150
180
 (degrees)
Shower/Fragmentation
PY62 (67=4) Total
Flavor Creation
Flavor Excitation
Shower/Fragmentation
b-quark
direction

Predictions of PYTHIA 6.206 (CTEQ5L) with PARP(67)=1
(new default) and PARP(67)=4 (old default) for the azimuthal
angle, , between a b-quark with PT1 > 15 GeV/c, |y1| < 1 and
bbar-quark with PT2 > 10 GeV/c, |y2|<1 in proton-antiproton
collisions at 1.8 TeV. The curves correspond to ds/d (mb/o)
for flavor creation, flavor excitation, shower/fragmentation,
and the resulting total.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF

“Toward”
“Away”
bbar-quark
Page 17
Azimuthal Correlations
Old PYTHIA default
(more initial-state radiation)
b-quark Correlations: Azimuthal  Distribution
b-quark Correlations: Azimuthal  Distribution
0.01000
0.010000
1.8 TeV
PT1 > 15 GeV/c
PT2 > 10 GeV/c
|y1| < 1 |y2| < 1
HERWIG 6.4
CTEQ5L
0.001000
0.00100
ds/d (mb/deg)
ds/d (mb/deg)
1.8 TeV
PT1 > 15 GeV/c
PT2 > 10 GeV/c
|y1| < 1 |y2| < 1
0.00010
"Flavor Creation"
CTEQ5L
HERWIG 6.4
0.000100
PYTHIA 6.206
PARP(67)=4
PYTHIA 6.206
PARP(67)=1
0.000010
"Away"
"Toward"
0.00001
30
60
90
120
150
180
 (degrees)
HW64 Total

"Away"
"Toward"
0
Flavor Creation
Flavor Excitation
0.000001
0
30
60
Predictions of HERWIG 6.4 (CTEQ5L) for the
azimuthal angle, , between a b-quark with
PT1 > 15 GeV/c, |y1| < 1 and bbar-quark with
PT2 > 10 GeV/c, |y2|<1 in proton-antiproton
collisions at 1.8 TeV. The curves correspond to
ds/d (mb/o) for flavor creation, flavor
excitation, shower/fragmentation, and the
resulting total.
90
120
150
180
 (degrees)
Shower/Fragmentation
b-quark
direction
New PYTHIA default
(less initial-state radiation)

“Toward”
“Away”
“Flavor Creation”
bbar-quark
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 18
DiPhoton Correlations
DiPhoton Correlations: Azimuthal  Distribution
Diphoton System Transverse Momentum
0.25
0.14
PYC5 DiPhoton PARP(67)=1
PYC5 DiPhoton PARP(67)=4
PYC5 DiPhoton PARP(67)=4
0.20
PYC5 DiPhoton PARP(67)=1
CDF DiPhoton Data
New Pythia
1.8 TeV PT > 12 GeV |h| < 0.9
1/s ds/d (1/deg)
1/s ds/dPT (1/GeV/c)
0.12
CDF DiPhoton Data
Old Pythia
0.15
0.10
0.10
New Pythia
1.8 TeV PT > 12 GeV |h| < 0.9
Old Pythia
0.08
0.06
0.04
0.05
0.02
0.00
0.00
0
2
4
6
8
10
12
14
16
18
90

105
120
135
150
165
180
 (degrees)
Diphoton System PT (GeV/c)
Predictions of PYTHIA 6.158 (CTEQ5L) with
PARP(67)=1 (new default) and PARP(67)=4 (old default)
for diphoton system PT and the azimuthal angle, ,
between a photon with PT1 > 12 GeV/c, |y1| < 0.9 and
photon with PT2 > 12 GeV/c, |y2|< 0.9 in protonantiproton collisions at 1.8 TeV compared with CDF data.
Photon
direction

“Toward”
“Away”
Photon
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 19
Tuned PYTHIA 6.206
PYTHIA 6.206 CTEQ5L
Tune 1
Tune 2
MSTP(81)
1
MSTP(82)
3
3
PARP(82)
1.6 GeV
1.7 GeV
PARP(85)
1.0
1.0
PARP(86)
1.0
1.0
Bulk of Min-Bias
1
events!
PARP(89)
1.8 TeV
1.8 TeV
PARP(90)
0.16
0.16
PARP(67)
1.0
4.0
"Transverse" <Nchg> in 1 GeV/c bin
Parameter
"Transverse" Nchg versus PT(charged jet#1)
4
Tuned PYTHIA 6.206
PARP(67)=4
CDF
data uncorrected
theory corrected
3
2
Tuned PYTHIA 6.206
PARP(67)=1
1
CTEQ5L
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 two tuned versions of PYTHIA 6.206 (CTEQ5L, PARP(67)=1
and PARP(67)=4).
New
New PYTHIA
PYTHIA default
default
(less
(less initial-state
initial-state radiation)
radiation)
D0 Meeting
September 6, 2002
Old PYTHIA
PYTHIA default
default
Old
(less
initial-state
radiation)
(less initial-state radiation)
Rick Field - Florida/CDF
Can describe transition between
“soft” and “hard” regime!
Page 20
Tuned PYTHIA 6.206
“Transverse” PT Distribution
"Transverse" PT Distribution (charged)
data uncorrected
theory corrected
3
1.0E+01
Tuned PYTHIA 6.206
PARP(67)=4
CDF
PYTHIA 6.206 CTEQ5L (3)
1.0E+00
2
PT(chgjet#1) > 30 GeV/c
Tuned PYTHIA 6.206
PARP(67)=1
1
CTEQ5L
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
50
dNchg/dPT (1/GeV/c)
"Transverse" <Nchg> in 1 GeV/c bin
"Transverse" Nchg versus PT(charged jet#1)
4
1.0E-01
PARP(67)=4
1.0E-02
1.0E-03
CDF
PARP(67)=1
data uncorrected
theory corrected
1.0E-04
PT(chgjet#1) > 5 GeV/c
1.8 TeV |h|<1
PARP(67)=4.0 (old default) is favored
over PARP(67)=1.0 (new default)!
1.0E-05
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 two
tuned versions of PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, PARP(67)=1 and
PARP(67)=4).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 21
Tuned PYTHIA 6.206 vs HERWIG 6.4
“TransMAX/MIN” vs PT(chgjet#1)
Charged Jet #1
Direction
"Max/Min Transverse" Nchg
“Toward”
“TransMAX”
“TransMIN”
“Away”
<PTsum>
"Transverse" <Nchg> in 1 GeV/c bin
<Nchg>

3.0
Tuned PYTHIA 6.206
PARP(67)=1
CDF Preliminary
2.5
data uncorrected
theory corrected
Tuned PYTHIA 6.206
PARP(67)=4
"Max Transverse"
2.0
1.5
CTEQ5L
HERWIG 6.4
1.0
"Min Transverse"
0.5
1.8 TeV |h|<1.0 PT>0.5 GeV
0.0
0
 Plots shows data on the
“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 HERWIG
6.4 (CTEQ5L, PT(hard) > 3 GeV/c)
and two tuned versions of PYTHIA
6.206 (PT(hard) > 0, CTEQ5L,
PARP(67)=1 and PARP(67)=4).
D0 Meeting
September 6, 2002
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
"Max/Min Transverse" PTsum
3.5
<PTsum> (GeV/c) in 1 GeV/c bin

5
Tuned PYTHIA 6.206
PARP(67)=1
CDF Preliminary
3.0
data uncorrected
theory corrected
Tuned PYTHIA 6.206
PARP(67)=4
"Max Transverse"
2.5
2.0
1.5
HERWIG 6.4
CTEQ5L
1.0
"Min Transverse"
0.5
1.8 TeV |h|<1.0 PT>0.5 GeV
0.0
0
5
Rick Field - Florida/CDF
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Page 22
Tuned PYTHIA 6.206 vs HERWIG 6.4
“TransSUM/DIF” vs PT(chgjet#1)
Charged Jet #1
Direction
SUM/DIF "Transverse" Nchg
<Nchg>
“Toward”
“TransMAX”
“TransMIN”
“Away”
<PTsum>
Tuned PYTHIA 6.206
PARP(67)=4
Tuned PYTHIA 6.206
PARP(67)=1
CDF Preliminary
<Nchg> in 1 GeV/c bin

4
data uncorrected
theory corrected
3
"Max+Min Transverse"
2
"Max-Min Transverse"
1
CTEQ5L
HERWIG 6.4
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
 Plots shows data on the
“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 HERWIG
6.4 (CTEQ5L, PT(hard) > 3 GeV/c) and
two tuned versions of PYTHIA 6.206
(PT(hard) > 0, CTEQ5L, PARP(67)=1
and PARP(67)=4).
D0 Meeting
September 6, 2002
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
SUM/DIF "Transverse" PTsum
4
<PTsum> (GeV/c) in 1 GeV/c bin

5
Tuned PYTHIA 6.206
PARP(67)=1
CDF Preliminary
data uncorrected
theory corrected
3
Tuned PYTHIA 6.206
PARP(67)=4
"Max+Min Transverse"
2
"Max-Min Transverse"
1
CTEQ5L
HERWIG 6.4
1.8 TeV |h|<1.0 PT>0.5 GeV
0
0
5
Rick Field - Florida/CDF
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Page 23
Tuned PYTHIA 6.206 vs HERWIG 6.4
“Transverse” PT Distribution
"Transverse" PT Distribution (charged)
"Transverse" PT Distribution (charged)
1.0E+01
1.0E+01
PT(chgjet1) > 30 GeV/c
PT(charged jet#1) > 5 GeV/c
Tuned PYTHIA 6.206
PARP(67)=4
dNchg/dPT (1/GeV/c)
dNchg/dPT (1/GeV/c)
1.0E+00
CTEQ5L
1.0E+00
1.0E-01
Tuned PYTHIA 6.206
PARP(67)=1
1.0E-02
CTEQ5L
1.0E-01
Tuned PYTHIA 6.206
PARP(67)=4
1.0E-02
HERWIG 6.4
1.0E-03
CDF
data uncorrected
theory corrected
1.0E-03
HERWIG 6.4
Tuned PYTHIA 6.206
PARP(67)=1
CDF
1.0E-04
data uncorrected
theory corrected
1.8 TeV |h|<1
1.8 TeV |h|<1
1.0E-04
1.0E-05
0
1
2
3
4
5
6
7
0
PT(charged) GeV/c
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Data on the PT distribution of the “transverse” <Nchg>, dNchg/dPT, compared with the QCD
Monte-Carlo predictions of HERWIG 6.4 (CTEQ5L, PT(hard) > 3 GeV/c) and two tuned
versions of PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, PARP(67)=1 and PARP(67)=4).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 24
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!).
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 25
Multiple Parton Interactions:
Summary & Conclusions
Multiple Parton Interactions
Proton
AntiProton
Energy dependence?
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.
No time to discuss
Multiple parton interactions gives a natural way of explaining the increased
activity in the
this here!
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.
PYTHIA (with varying impact parameter) describes the underlying event data fairly well
and will also fit the min-bias data (must use MSTP(82)=4 “double Gaussian” and tune the
parameters). More work is needed on the energy dependence.
A. Moraes, I. Dawson, and C. Buttar (University of Sheffield) have also been working on
tuning PYTHIA to fit the underlying event using the CDF data with the goal of
extrapolating to the LHC.
D0 Meeting
September 6, 2002
Rick Field - Florida/CDF
Page 26