“Min-Bias” & “Underlying Event”
at the Tevatron and the LHC
 What happens when a high energy
proton and an antiproton collide?
“Min-Bias”
“Soft” Collision (no hard scattering)
Proton
AntiProton
 Most of the time the proton and
antiproton ooze through each other
and fall apart (i.e. no hard scattering).
“Hard” Scattering Outgoing Parton
The outgoing particles continue in
PT(hard)
roughly the same direction as initial
proton and antiproton. A “Min-Bias” Proton
AntiProton
collision.
Underlying Event
Underlying Event
Initial-State
 Occasionally there will be a “hard”
Radiation
parton-parton collision resulting in large
Final-State
Radiation
transverse momentum outgoing partons.
Outgoing Parton
Also a “Min-Bias” collision.
 The “underlying event” is everything
except the two outgoing hard scattered
“jets”. It is an unavoidable background
to many collider observables.
Fermilab MC Workshop
October 4, 2002
“Underlying Event”
Proton
Beam-Beam Remnants
Rick Field - Florida/CDF
AntiProton
Beam-Beam Remnants
Initial-State
Radiation
Page 1
“Min-Bias” & “Underlying Event”
at the Tevatron and the LHC
 What happens when a high energy
proton and an antiproton collide?
“Min-Bias”
“Soft” Collision (no hard scattering)
Proton
AntiProton
 Most of the time the proton and
antiproton ooze through each other
and fall apart (i.e. no hard scattering).
“Hard” Scattering Outgoing Parton
The outgoing particles continue in
PT(hard)
roughly the same direction as initial
Are
proton and antiproton. A “Min-Bias” Proton
these
AntiProton
collision.
the
Underlying Event
Underlying Event
Initial-State
 Occasionally there will be a “hard”
same?
Radiation
parton-parton collision resulting in large
Final-State
Radiation
transverse momentum outgoing partons.
Outgoing Parton
Also a “Min-Bias” collision.
 The “underlying event” is everything
except the two outgoing hard scattered
“jets”. It is an unavoidable background
to many collider observables.
Fermilab MC Workshop
October 4, 2002
“Underlying Event”
Proton
Beam-Beam Remnants
Rick Field - Florida/CDF
AntiProton
Beam-Beam Remnants
Initial-State
Radiation
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
Underlying Event
AntiProton
Underlying Event
 The underlying event in a hard scattering
Final-State
process is a complicated and not very well
Radiation
Outgoing Parton
understood object. It is an interesting
Run I CDF
region since it probes the interface
“Evolution of Charged Jets”
between perturbative and nonRick Field
perturbative physics.
David Stuart
 There are three CDF analyses which
Rich Haas
quantitatively study the underlying event Run I CDF
and compare with the QCD Monte-Carlo “Cone Analysis”
models (2 Run I and 1 Run II).
Valeria Tano
Run II CDF
Eve Kovacs
 It is important to model this region well
“Jet Shapes & Energy Flow”
since it is an unavoidable background to
Joey Huston
all collider observables. Also, we need a
Mario Martinez
Anwar Bhatti
good model of “min-bias” collisions.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 3
The Proton-Antiproton
Total Cross-Section
Elastic Scattering
Single Diffraction
Double Diffraction
M2
M
M1
stot = sEL + sSD +sDD +sHC
1.8 TeV: 78mb
= 18mb
+ 9mb
+ (4-7)mb + (47-44)mb
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
PT(hard)
Proton
AntiProton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Outgoing Parton
Page 4
The Proton-Antiproton
Total Cross-Section
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 + sSD +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
Beam-Beam Counters
3.2 < |h| < 5.9
PT(hard)
Proton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Outgoing Parton
Page 5
Beam-Beam Remnants
“Hard” Collision
outgoing parton
“Hard” Component
Maybe not all “soft”!
“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
Are these the same?
Hadron
Hadron
Beam-Beam Remnants
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 6
Beam-Beam Remnants
“Hard” Collision
outgoing parton
“Hard” Component
Maybe not all “soft”!
“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 hardIfscattering
process
we are going
to lookhas
at a “hard” component (particles that
arise from initial & final-state “Min-Bias”
radiation and
fromasthe
collisions
a outgoing hard scattered partons)
guide to understanding
the
and a “soft?” component (“beam-beam
remnants”).
“beam-beam remnants”,
 Clearly? the “underlying event” then
in a hard
scattering
we better
study process should not look like a “MinBias” event because of the “hard” component
(i.e. initial & final-state radiation).
carefully the
“Min-Bias”
 However, perhaps “Min-Bias” collisions
are adata!
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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 7
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).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 8
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?
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 9
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
100,000
CM Energy W (GeV)
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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 10
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
contains a lot of “hard” parton-parton collisions which results in many more particles at
large PT than are produces by any “soft” model.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 11
CDF “Min-Bias” Data
PT Dependence
“Tano Analysis”
Charged Particle Density: dN/dhd
(Tano, Kovacs, Huston, 1.0E+01
Bhatti)
1.4
14 TeV
Herwig "Soft" Min-Bias
1.2
CDF Preliminary
0.6
Charged Density dN/dhddPT (1/GeV/c)
dN/dhd
0.8
1.0E-01
CDF Min-Bias Data at 1.8 TeV
HERWIG
“Soft” Min-Bias
1.0E-02
0.4
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

|h|<1
1.0E+00
1.0

Lots of “hard” scattering
in “Min-Bias”!
Charged Particle Density
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
contains a lot of “hard” parton-parton collisions which results in many more particles at
large PT than are produces by any “soft” model.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 12
Min-Bias: Combining
“Hard” and “Soft” Collisions
Charged Particle Density
Charged Particle Density: dN/dhd
1.6
1.0E+01
1.4
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!
Fermilab MC Workshop
October 4, 2002
Charged Density dN/dhddPT (1/GeV/c)
dN/dhd
Herwig Jet3
Herwig Min-Bias
CDF Min-Bias Data
CDF Preliminary
1.8 TeV |h|<1
1.0E-01
HW PT(hard) > 3 GeV/c
1.0E-02
1.0E-03
HW "Soft" Min-Bias
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!
Rick Field - Florida/CDF
Page 13
Min-Bias: Combining
“Hard” and “Soft” Collisions
No easy way to
“mix” HERWIG “hard”
with HERWIG “soft”.
sHC
Hard-Scattering Cross-Section
Charged
Particle Density
100.00
Charged Particle Density: dN/dhd
1.6
CTEQ5L
1.8 TeV
Cross-Section (millibarns)
1.0E+01
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
HERWIG diverges!
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?
Fermilab MC Workshop
October 4, 2002
20
Hard-Scattering Cut-Off PTmin
Rick Field - Florida/CDF
Page 14
PYTHIA Min-Bias
“Soft” + ”Hard”
Tuned to fit the
“underlying event”!
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 of 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)!
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 15
Evolution of Charged Jets
“Underlying Event”
“Transverse” region
Charged

very
sensitive toParticle
the
“underlying event”!
PT > 0.5 GeV/c
Charged Jet #1
Direction
“Toward-Side” Jet

“Toward”
“Transverse”
Look at the charged
particle density in the
“transverse” region!
Correlations
2p
|h| < 1
Away Region
Charged Jet #1
Direction

“Toward”
Transverse
Region

Leading
Jet
Toward Region
“Transverse”
“Away”
“Transverse”
“Transverse”
Transverse
Region
“Away”
Away Region
0
-1
“Away-Side” Jet
h
+1
 Look at charged particle correlations in the azimuthal angle  relative to the leading charged
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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 16
“Transverse”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
CDF “Min-Bias” data
(|h|<1, PT>0.5 GeV)
<dNchg/dhd> = 0.25
"Transverse" Charged Density

1.25
CDF Min-Bias
CDF Data
CDF JET20
data uncorrected
1.00
0.75
0.50
Factor of 2!
0.25
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)
 Data on the average charge particle density (PT > 0.5 GeV, |h| < 1) in the “transverse”
(60<||<120o) region as a function of the transverse momentum of the leading charged
particle jet. Each point corresponds to the <dNchg/dhd> 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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 17
“Transverse”
Charged PTsum Density
"Transverse" Charged PTsum Density: dPTsum/dhd
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
"Transverse" PTsum Density (GeV)

1.25
CDF JET20
CDF Data
CDF Min-Bias
data uncorrected
1.00
0.75
0.50
Increases with
PT(jet1)!
> factor of 2!
0.25
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
CDF “Min-Bias” data
(|h|<1, PT>0.5 GeV)
<dPTsum/dhd> = 0.23 GeV/c
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Data on the average charge scalar PTsum density (PT > 0.5 GeV, |h| < 1) in the “transverse”
(60<||<120o) region as a function of the transverse momentum of the leading charged
particle jet. Each point corresponds to the <dPTsum/dhd> 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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 18
“Max/Min Transverse”
Charged Particle Density
Bryan Webber idea!
1.25
CDF Preliminary

data uncorrected
"Max Transverse"
1.00
“Toward”
“TransMAX”
“TransMIN”
“Away”
Charged Density
Area h
2x60o = 2p/3
Charged Jet #1
Direction
More sensitive to the “hard
scattering” component
"Max/Min Transverse" Charged Density: dN/dhd
0.75
0.50
"Min Transverse"
0.25
1.8 TeV |h|<1.0 PT>0.5 GeV
CDF “Min-Bias” data
(|h|<1, PT>0.5 GeV)
<dNchg/dhd> = 0.25
0.00
0
5
10
15
20
25
30
PT(charged jet#1) (GeV/c)
35
40
45
50
More sensitive to the
“beam-beam remnants”
 Define “MAX” and “MIN” to be the maximum and minimum charge particle density of

the two “transverse” regions 60o<<120o (60o<-<120o) on an event by event basis.
The overall “transverse” region is the sum of the “MAX” and “MIN”.
The plot shows the average “MAX” and “MIN” charge particle density 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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 19
“Max/Min Transverse”
Charged PTsum Density
Charged Jet #1
Direction

“Toward”
“TransMAX”
“TransMIN”
“Away”
CDF “Min-Bias” data
(|h|<1, PT>0.5 GeV)
<dPTsum/dhd> = 0.23 GeV/c
1.50
CDF Preliminary
Charged PTsum Density (GeV)
Area h
2x60o = 2p/3
More sensitive to the “hard
scattering” component
"Max/Min Transverse" PTsum Density: dPTsum/dhd
"Max Transverse"
data uncorrected
1.25
1.00
0.75
0.50
"Min Transverse"
0.25
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
15
20
25
30
35
PT(charged jet#1) (GeV/c)
40
45
50
More sensitive to the
“beam-beam remnants”
 Define “MAX” and “MIN” to be the maximum and minimum charge particle density of
the two “transverse” regions 60o<<120o (60o<-<120o) on an event by event basis.
The overall “transverse” region is the sum of the “MAX” and “MIN”.
 The plot shows the average “MAX” and “MIN” charge particle density 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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 20
Charged Particle Density
“Transverse” PT Distribution
"Transverse" Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
CDF JET20
data uncorrected
1.00
1.0E+00
CDF Min-Bias
CDF Preliminary
CDF Preliminary
PT(chgjet#1) > 5 GeV/c
0.75
0.50
0.25
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
PT(charged jet#1) > 30 GeV/c
“Transverse” <dNchg/dhd> = 0.56
PT(charged jet#1) > 5 GeV/c
“Transverse” <dNchg/dhd> = 0.52
50
Charged Density dN/dhddPT (1/GeV/c)
"Transverse" Charged Density
1.25
data uncorrected
1.0E-01
1.8 TeV |h|<1 PT>0.5 GeV/c
1.0E-02
1.0E-03
1.0E-04
PT(chgjet#1) > 2 GeV/c
1.0E-05
PT(chgjet#1) > 30 GeV/c
1.0E-06
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Compares the average “transverse” charge particle density (|h|<1, PT>0.5 GeV) versus
PT(charged jet#1) with the PT distribution of the “transverse” density, dNchg/dhddPT.
Shows how the “transverse” charge particle density is distributed in PT.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 21
Charged Particle Density
“Transverse” PT Distribution
"Transverse" Charged Particle Density: dN/dhd
Charged Particle Density
CDF JET20
data uncorrected
1.00
1.0E+00
CDF Min-Bias
CDF Preliminary
0.75
0.50
Factor of 2!
0.25
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
CDF Preliminary
"Transverse"
PT(chgjet#1) > 5 GeV/c
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
PT(charged jet#1) > 30 GeV/c
“Transverse” <dNchg/dhd> = 0.56
50
Charged Density dN/dhddPT (1/GeV/c)
"Transverse" Charged Density
1.25
data uncorrected
1.0E-01
"Transverse"
PT(chgjet#1) > 30 GeV/c
1.0E-02
1.0E-03
1.0E-04
Min-Bias
1.0E-05
1.8 TeV |h|<1 PT>0.5 GeV/c
CDF “Min-Bias” data
<dNchg/dhd> = 0.25
1.0E-06
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Compares the average “transverse” charge particle density with the average “Min-Bias”
charge particle density (|h|<1, PT>0.5 GeV). Shows how the “transverse” charge particle
density and the Min-Bias charge particle density is distributed in PT.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 22
HERWIG 6.4 (PT(hard) > 3 GeV/c)
“Transverse” Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
Charged Jet #1
Direction
“Toward”
“Transverse”
“Transverse”
“Away”
Herwig “Soft” Min-Bias
(|h|<1, PT>0.5 GeV)
<dNchg/dhd> = 0.18
"Transverse" Charged Density

1.00
CDF Data
"Hard"
Total
data uncorrected
theory corrected
0.75
Herwig 6.4 CTEQ5L
PT(hard) > 3 GeV/c
0.50
0.25
"Remnants"
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 vs PT(chgjet#1) compared to the

QCD hard scattering predictions of HERWIG 6.4 (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”).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 23
HERWIG 6.4 (PT(hard) > 3 GeV/c)
“Min-Transverse” Charged Particle Density
Charged Jet #1
Direction
"Min Transverse" Charged Density: dN/dhd
0.5
“Toward”
“TransMAX”
“TransMIN”
“Away”
CDF Min-Bias
(|h|<1, PT>0.5 GeV)
<dNchg/dhd> = 0.25
"Transverse" Charged Density

CDF Preliminary
data uncorrected
theory corrected
0.4
Herwig 6.4 CTEQ5L
PT(hard) > 3 GeV/c
Total
"Remnants"
0.3
0.2
0.1
1.8 TeV |h|<1.0 PT>0.5 GeV
"Hard"
0.0
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Plot shows the “min-transverse” charged particle density vs PT(chgjet#1) compared to

the QCD hard scattering predictions of HERWIG 6.4 (default parameters with
Equal “hard” and
PT(hard)>3 GeV/c).
“beam-beam” remnants!
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”).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 24
HERWIG 6.4
“Transverse” PT Distribution
"Transverse" Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
CDF Data
"Hard"
Total
data uncorrected
theory corrected
0.75
1.0E+00
Herwig 6.4 CTEQ5L
PT(hard) > 3 GeV/c
CDF Data
PT(chgjet#1) > 5 GeV/c
0.50
0.25
"Remnants"
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
Herwig PT(chgjet#1) > 30 GeV/c
“Transverse” <dNchg/dhd> = 0.51
50
Charged Density dN/dhddPT (1/GeV/c)
"Transverse" Charged Density
1.00
data uncorrected
theory corrected
1.0E-01
1.8 TeV |h|<1 PT>0.5 GeV/c
1.0E-02
1.0E-03
1.0E-04
1.0E-05
PT(chgjet#1) > 30 GeV/c
Herwig 6.4 CTEQ5L
1.0E-06
Herwig PT(chgjet#1) > 5 GeV/c
<dNchg/dhd> = 0.40
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Compares the average “transverse” charge particle density (|h|<1, PT>0.5 GeV) versus
PT(charged jet#1) and the PT distribution of the “transverse” density, dNchg/dhddPT with
the QCD hard scattering predictions of HERWIG 6.4 (default parameters with PT(hard)>3
GeV/c. Shows how the “transverse” charge particle density is distributed in PT.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 25
HERWIG 6.4
“Transverse” PT Distribution
 Plot shows the PT dependence of the
"Transverse" Charged Particle Density
1.0E+00
PT(charged Jet#1) > 5 GeV/c
Charged Density dN/dhddPT (1/GeV/c)
CDF Data
data uncorrected
theory corrected
Herwig Total
1.0E-01

Herwig 6.4
"Hard"
1.0E-02
1.0E-03
1.0E-04

"Remnants"
1.8 TeV |h|<1 PT>0.5 GeV/c
CDF Min-Bias Data
1.0E-05
0
1
3
2
4
5
6
7
8
“transverse” charged particle density compared
to the QCD hard scattering predictions of
HERWIG 6.4 (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 (“beambeam remnants”); and charged particles that
arise from the outgoing jet plus initial and finalstate radiation (“hard scattering component”).
Both HERWIG’s “soft” Min-Bias model and
HERWIG’s model for the “beam-beam
remnants” do not produce enough high PT
hadrons (i.e. they are both too “soft”).
PT(charged) (GeV/c)
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 26
HERWIG 6.4
“Transverse” PT Distribution

Density
Particle Density
Charged Particle
"Transverse"
"Transverse" Charged
1.0E+00
1.0E+00
Data
CDF Data
CDF
GeV/c
Jet#1) >> 55 GeV/c
PT(charged
PT(charged Jet#1)
data uncorrected
data uncorrected
theory corrected
theory corrected
Charged
Charged Density
(1/GeV/c)
Density dN/dhddPT
dN/dhddPT (1/GeV/c)
Herwig Total
1.0E-01
1.0E-01
Herwig "Hard" +
CDF Min-Bias Data

6.4
Herwig 6.4
Herwig
"Hard"
1.0E-02
1.0E-02
1.0E-03
1.0E-03

Herwig "Hard"
1.0E-04
1.0E-04
"Remnants"
1.8 TeV |h|<1 PT>0.5 GeV/c
Data
Min-BiasData
CDF
CDFMin-Bias
1.8 TeV |h|<1 PT>0.5 GeV/c
1.0E-05
1.0E-05
00
11
33
22
44
55
66
77
88
HERWIG “hard” component (i.e.
& final
+
Plotinitial
shows
thestate
PT radiation)
dependence
CDF “Min-Bias” data.
of the
“transverse” charged particle density compared
to the QCD hard scattering predictions of
HERWIG 6.4 (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 (“beambeam remnants”); and charged particles that
Adding
theoutgoing
two assumesjet
noplus
correlations,
arise from
the
initial and finalbut
if
there
is
one
hard
scattering
the
state radiation (“hard scattering incomponent”).
event then maybe it is more probable that
Both HERWIG’s
Min-Bias
model and
there“soft”
is a second
one!
HERWIG’s model for the “beam-beam
remnants” do not produce enough high PT
hadrons (i.e. they are both too “soft”).
(GeV/c)
PT(charged)
PT(charged) (GeV/c)
 The CDF “Min-Bias” data describe the “beam-beam” remnants better than HERWIG! But the
CDF “Min-Bias” data contain a hard scattering component and hence maybe the “beam-beam
remnants” have a hard scattering component (i.e. multiple parton interactions).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 27
PYTHIA: 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)
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
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
Page 28
PYTHIA: Multiple Parton
Interaction Parameters
Multiple Parton Interactions
Outgoing Parton
PT(hard)
Proton
AntiProton
Underlying Event
Underlying Event
ParameterOutgoingValue
Parton
MSTP(81)
0
1
MSTP(82)
1
Pythia uses multiple parton
interactions to enhance
the underlying event.
Note thatDescription
since the same cut-off
parameters govern both the primary
Multiple-Parton Scattering off
hard scattering and the secondary MPI
interaction,
changing
the amount of MPI
Multiple-Parton
Scattering
on
also changes the amount of hard primary
Multiple interactions assuming the same probability, with
in PYTHIA Min-Bias events!
an abruptscattering
cut-off P min=PARP(81)
and now
HERWIG
!
Jimmy: MPI
J. M. Butterworth
J. R. Forshaw
M. H. Seymour
Multiple parton
interaction more
likely in a hard
(central) collision!
T
Same parameter that
cuts-off the hard 2-to-2
parton cross sections!
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)
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Hard Core
Page 29
PYTHIA: 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 Parton Interaction
Probability that the MPI produces two gluons
either as described by PARP(85) or as a closed
gluon loop. The remaining fraction consists of
quark-antiquark pairs.
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.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
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 30
PYTHIA 6.206 Defaults
MPI constant
probability
scattering
PYTHIA default parameters
6.115
6.125
6.158
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
PARP(67)
4.0
1.00
"Transverse" Charged Density
Parameter
"Transverse" Charged Particle Density: dN/dhd
CDF Data
Pythia 6.206 (default)
MSTP(82)=1
PARP(81) = 1.9 GeV/c
data uncorrected
theory corrected
0.75
0.50
0.25
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)
CTEQ3L
CTEQ4L
CTEQ5L
CDF Min-Bias
CDF JET20
 Plot shows the “Transverse” charged particle density 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.
Note Change
PARP(67) = 4.0 (< 6.138)
PARP(67) = 1.0 (> 6.138)
Fermilab MC Workshop
October 4, 2002
Default parameters give
very poor description of
the “underlying event”!
Rick Field - Florida/CDF
Page 31
Tuned PYTHIA 6.206
Single Gaussian
PYTHIA 6.206 CTEQ5L
Tune D
1.00
Tune C
MSTP(81)
1
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
PARP(89)
1.8 TeV
1.8 TeV
PARP(90)
0.25
0.25
PARP(67)
1.0
4.0
"Transverse" Charged Density
Parameter
"Transverse" Charged Particle Density: dN/dhd
CDF Preliminary
PYTHIA 6.206 (Set C)
PARP(67)=4
data uncorrected
theory corrected
0.75
0.50
0.25
CTEQ5L
PYTHIA 6.206 (Set D)
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 D (PARP(67)=1) and Set C (PARP(67)=4)).
New
New PYTHIA
PYTHIA default
default
(less
(less initial-state
initial-state radiation)
radiation)
Fermilab MC Workshop
October 4, 2002
Old PYTHIA default
(more initial-state radiation)
Rick Field - Florida/CDF
Page 32
Tuned PYTHIA 6.206
Double Gaussian
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
Corrected 11/4/02
New PYTHIA default
(less initial-state radiation)
Fermilab MC Workshop
October 4, 2002
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
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
Page 33
Tuned PYTHIA 6.206 vs HERWIG 6.4
“Transverse” Densities

"Transverse" Charged Particle Density: dN/dhd
Charged Particle
Density
“Toward”
“Transverse”
“Transverse”
“Away”
Charged PTsum
Density
1.00
CDF Preliminary
"Transverse" Charged Density
Charged Jet #1
Direction
PYTHIA 6.206 (Set A)
PARP(67)=4
data uncorrected
theory corrected
0.75
0.50
HERWIG 6.4
0.25
PYTHIA 6.206 (Set B)
PARP(67)=1
CTEQ5L
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)
"Transverse" Charged PTsum Density: dPTsum/dhd
 Plots shows CDF data on the charge
"Transverse" PTsum Density (GeV)

particle density and the charged PTsum
density in the “transverse” region.
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).
1.00
PYTHIA 6.206 (Set A)
PARP(67)=4
CDF Preliminary
data uncorrected
theory corrected
0.75
0.50
HERWIG 6.4
0.25
PYTHIA 6.206 (Set B)
PARP(67)=1
CTEQ5L
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)
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 34
Tuned PYTHIA 6.206 vs HERWIG 6.4
“MAX/MIN Transverse” Densities

"Max/Min Transverse" Charged Density: dN/dhd
Charged Particle
Density
“Toward”
“TransMAX”
“TransMIN”
“Away”
1.25
CDF Preliminary
Charged PTsum
Density
PY Set A
data uncorrected
theory corrected
1.00
Charged Density
Charged Jet #1
Direction
"Max Transverse"
0.75
HERWIG 6.4
0.50
PY Set B
"Min Transverse"
0.25
CTEQ5L
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)
 Plots shows CDF data on the charge
1.50
CDF Preliminary
Charged PTsum Density (GeV)

particle density and the charged PTsum
density in the MAX/MIN “transverse”
region.
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).
"Max/Min Transverse" PTsum Density: dPTsum/dhd
PY Set A
"Max Transverse"
data uncorrected
theory corrected
1.25
1.00
0.75
HERWIG 6.4
PY Set B
0.50
"Min Transverse"
0.25
CTEQ5L
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)
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 35
Tuned PYTHIA 6.206
“Transverse” PT Distribution
"Transverse" Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
1.0E+00
CDF Preliminary
PYTHIA 6.206 (Set A)
PARP(67)=4
data uncorrected
theory corrected
0.75
CDF Data
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
PT(charged jet#1) (GeV/c)
PT(charged jet#1) > 30 GeV/c
50
Charged Density dN/dhddPT (1/GeV/c)
"Transverse" Charged Density
1.00
data uncorrected
theory corrected
PT(chgjet#1) > 30 GeV/c
1.0E-01
PYTHIA 6.206 Set A
PARP(67)=4
1.0E-02
1.0E-03
1.0E-04
PT(chgjet#1) > 5 GeV/c
1.0E-05
PYTHIA 6.206 Set B
PARP(67)=1
1.8 TeV |h|<1 PT>0.5 GeV/c
PARP(67)=4.0 (old default) is favored
over PARP(67)=1.0 (new default)!
1.0E-06
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Compares the average “transverse” charge particle density (|h|<1, PT>0.5 GeV) versus
PT(charged jet#1) and the PT distribution of the “transverse” density, dNchg/dhddPT with
the QCD Monte-Carlo predictions of two tuned versions of PYTHIA 6.206 (PT(hard) > 0,
CTEQ5L, Set B (PARP(67)=1) and Set A (PARP(67)=4)).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 36
Tuned PYTHIA 6.206
Set A
Describes the rise
from “Min-Bias” to
1.0E+00
“underlying event”!
"Transverse" Charged Particle Density: dN/dhd
1.00
PYTHIA 6.206 Set A
data uncorrected
theory corrected
PYTHIA 6.206 Set A
CDF Preliminary
0.75
0.50
0.25
CTEQ5L
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
15
20
25
30
35
40
45
PT(charged jet#1) (GeV/c)
Charged Particle Density: dN/dhd
1.0
CDF Published
50
Charged Density dN/dhddPT (1/GeV/c)
"Transverse" Charged Density
CDF Preliminary
Charged Particle Density
1.0E-01
"Transverse"
PT(chgjet#1) > 30 GeV/c
1.0E-02
1.0E-03
1.0E-04
CDF Min-Bias
dN/dhd
0.8
“Min-Bias”
data uncorrected
theory corrected
"Transverse"
PT(chgjet#1) > 5 GeV/c
CTEQ5L
0.6
1.8 TeV |h|<1 PT>0.5 GeV/c
0.4
1.0E-05
0
0.2
2
4
6
8
10
12
14
Pythia 6.206 Set A
1.8 TeV all PT
CDF Min-Bias 1.8 TeV
PT(charged) (GeV/c)
0.0
-4
-3
-2
-1
0
1
2
3
4
Pseudo-Rapidity h
 Compares the average “transverse” charge particle density (|h|<1, PT>0.5 GeV) versus
PT(charged jet#1) and the PT distribution of the “transverse” and “Min-Bias” densities with
the QCD Monte-Carlo predictions of a tuned version of PYTHIA 6.206 (PT(hard) > 0,
CTEQ5L, Set A). Describes “Min-Bias” collisions! Describes the “underlying event”!
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 37
Tuned PYTHIA 6.206
Set A
Describes the rise
from “Min-Bias” to
1.0E+00
“underlying event”!
"Transverse" Charged Particle Density: dN/dhd
1.00
PYTHIA 6.206 Set A
data uncorrected
theory corrected
PYTHIA 6.206 Set A
CDF Preliminary
0.75
0.50
0.25
CTEQ5L
1.8 TeV |h|<1.0 PT>0.5 GeV
0.00
0
5
10
15
20
25
30
35
PT(charged jet#1) (GeV/c)
40
45
50
Charged Density dN/dhddPT (1/GeV/c)
"Transverse" Charged Density
CDF Preliminary
Charged Particle Density
Set A PT(charged jet#1) > 30 GeV/c
“Transverse” <dNchg/dhd> = 0.60
data uncorrected
theory corrected
"Transverse"
PT(chgjet#1) > 5 GeV/c
1.0E-01
"Transverse"
PT(chgjet#1) > 30 GeV/c
1.0E-02
1.0E-03
1.0E-04
CDF Min-Bias
CTEQ5L
“Min-Bias”
1.8 TeV |h|<1 PT>0.5 GeV/c
1.0E-05
Set A Min-Bias
<dNchg/dhd> = 0.24
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 Compares the average “transverse” charge particle density (|h|<1, PT>0.5 GeV) versus
PT(charged jet#1) and the PT distribution of the “transverse” and “Min-Bias” densities with
the QCD Monte-Carlo predictions of a tuned version of PYTHIA 6.206 (PT(hard) > 0,
CTEQ5L, Set A). Describes “Min-Bias” collisions! Describes the “underlying event”!
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 38
True “Transverse”
Particle Density at 1.8 TeV
CDF “Min-Bias”
"Transverse" Charged Particle Density:
(|h|<1,dN/dhd
all PT)
"Transverse" Charged Particle Density: dN/dhd
<dNchg/dhd> = 0.67
CDF Preliminary
data uncorrected
theory corrected
2.0
"Transverse" Charged Density
"Transverse" Charged Density
1.00
PY6.206 (Set A)
0.75
0.50
0.25
CTEQ5L
PY6.206 (Set C)
HW 6.4
HW 6.4
PY6.206 (Set A)
PT > 0 (true)
1.5
PY6.206 (Set C)
1.0
PT > 0.5 (corrected)
0.5
CTEQ5L
1.8 TeV |h|<1.0 PT>0.5 GeV
1.8 TeV |h|<1.0
0.0
0.00
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
PT(charged jet#1) (GeV/c)
 Shows the average “transverse” charge particle density (|h|<1, PT>0.5 GeV, corrected) and
the true (|h|<1, PT>0) “transverse” charged particle density, dNchg/dhd predicted by
HERWIG 6.4 (PT(hard) > 3 GeV/c, CTEQ5L) and two tuned versions of PYTHIA 6.206
(PT(hard) > 0, CTEQ5L, Set A & Set C).
 There are roughly 1.4 charged particles per unit h- (PT > 0) in
the “transverse” region compared to 0.67 for a typical CDF
“Min-Bias” collision (9 charged particles per unit h compared
to 4).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
2 charged particles
in cone of
radius R=0.7
at 1.8 TeV
Page 39
True “Transverse”
PTsum Density at 1.8 TeV
"Transverse" Charged PTsum Density: dPTsum/dhd
"Transverse" Charged PTsum Density: dPTsum/dhd
1.25
CDF Preliminary
data uncorrected
theory corrected
"Transverse" PTsum Density (GeV)
"Transverse" PTsum Density
1.00
PY6.206 (Set A)
0.75
0.50
0.25
CTEQ5L
PY6.206 (Set C)
HW 6.4
1.8 TeV |h|<1.0 PT>0.5 GeV
PY6.206 (Set C)
HW 6.4
PY6.206 (Set A)
1.00
PT > 0 (true)
0.75
0.50
PT > 0.5 GeV (corrected)
0.25
1.8 TeV |h|<1.0
CTEQ5L
0.00
0.00
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
PT(charged jet#1) (GeV/c)
 Shows the average “transverse” charge PTsum density (|h|<1, PT>0.5 GeV, corrected) and the
true (|h|<1, PT>0) “transverse” charged PTsum density, dPTsum/dhd predicted by HERWIG
6.4 (PT(hard) > 3 GeV/c, CTEQ5L) and two tuned versions of PYTHIA 6.206 (PT(hard) > 0,
CTEQ5L, Set A & Set C).
 There is roughly 1 GeV/c per unit h- (PT > 0) from charged
particles in the “transverse” region for PT(chgjet#1) = 35 GeV/c.
Note, however, that the “transverse” charged PTsum density
increases rapidly as PT(chgjet#1) increases.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
1.5 GeV/c (charged)
in cone of
radius R=0.7
at 1.8 TeV
Page 40
“Transverse” Charged Densities
Energy Dependence
"Transverse" Charged PTsum Density: dPTsum/dhd
"Transverse" Charged Particle Density: dN/dhd
1.8 TeV
CDF Preliminary
"Transverse" Charged Density
1.00
PYTHIA 6.206 (Set A)
"Transverse" PTsum Density (GeV)
0.75
data uncorrected
theory corrected
0.50
0.25
630 GeV e = 0
630 GeV e = 0.16
HW 630 GeV
CTEQ5L
|h|<1.0 PT>0.5 GeV
PYTHIA 6.206 (Set A)
CDF Preliminary
1.8 TeV
data uncorrected
theory corrected
0.75
0.50
0.25
630 GeV e = 0.16
630 GeV e = 0
HW 630 GeV
|h|<1.0 PT>0.5 GeV
0.00
0.00
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
PT(charged jet#1) (GeV/c)
Hard-Scattering Cut-Off PT0
 Shows the “transverse” charge particle density
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
and charged PTsum density (|h|<1, PT>0.5 GeV)
versus PT(charged jet#1) at 1.8 TeV and 630
GeV predicted by HERWIG 6.4 (PT(hard) > 3
GeV/c, CTEQ5L) and a tuned version of
PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, Set A, e
= 0 and e = 0.16 (default)).
5
3
2
e = 0.16 (default)
1
100
1,000
10,000
100,000
CM Energy W (GeV)
Fermilab MC Workshop
October 4, 2002
Reference point
Rick Field - Florida/CDF E0 = 1.8 TeV
Page 41
“Transverse” Charged Densities
Energy Dependence
"Transverse" Charged PTsum Density: dPTsum/dhd
"Transverse" Charged Particle Density: dN/dhd
1.8 TeV
CDF Preliminary
"Transverse" Charged Density
1.00
PYTHIA 6.206 (Set A)
data uncorrected
theory corrected
0.50
0.25
630 GeV e = 0
630 GeV e = 0.16
HW 630 GeV
CTEQ5L
|h|<1.0 PT>0.5 GeV
"Transverse" PTsum Density (GeV)
0.75
PYTHIA 6.206 (Set A)
CDF Preliminary
0.75
1.8 TeV
data uncorrected
theory corrected
0.50
0.25
630 GeV e = 0.16
630 GeV e = 0
HW 630 GeV
|h|<1.0 PT>0.5 GeV
0.00
0.00
0
5
10
15
20
25
PT(charged jet#1)
0 a
35
40
50 e=0) gives
A constant
P45
T0 (i.e.
(GeV/c)
large cut-off at 630 GeV and less
activity in the UE resulting in a
large energy dependence.
30
 Shows the “transverse” charge particle density
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
Hard-Scattering Cut-Off PT0
5
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
and charged PTsum density (|h|<1, PT>0.5 GeV)
Lowering
PT0 at 630jet#1)
GeV (i.e.
versus
PT(charged
at 1.8 TeV and 630
increasing e) increases UE activity
GeV
predicted by HERWIG 6.4 (PT(hard) > 3
resulting in less energy dependence.
GeV/c, CTEQ5L) and a tuned version of
PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, Set A, e
= 0 and e = 0.16 (default)).
5
3
2
e = 0.16 (default)
1
100
1,000
10,000
100,000
CM Energy W (GeV)
Fermilab MC Workshop
October 4, 2002
Reference point
Rick Field - Florida/CDF E0 = 1.8 TeV
Page 42
“Transverse” Cones
vs “Transverse” Regions
2p
2p
Transverse
Region

“Cone Analysis”
Transverse
Cone:
p(0.7)2=0.49p
Away Region
(Tano, Kovacs, Huston, Bhatti)
Cone 1

Leading
Jet
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 in two cones of
radius 0.7 at the same h as the leading jet but with
|F| = 90o.
Plot the cone with the maximum and minimum PTsum
versus the ET of the leading (calorimeter) jet.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 43
Energy Dependence
of the “Underlying
Event”
“Cone Analysis”
(Tano, Kovacs, Huston, Bhatti)
630 GeV
1,800 GeV
PYTHIA 6.115
PT0 = 1.4 GeV
PYTHIA 6.115
PT0 = 2.0 GeV



Sum the PT of charged particles (PT > 0.4 GeV/c) in two cones of radius 0.7 at the same h as the
leading jet but with |F| = 90o. Plot the cone with the maximum and minimum PTsum versus the ET
of the leading (calorimeter) jet.
Note that PYTHIA 6.115 is tuned at 630 GeV with PT0 = 1.4 GeV and at 1,800 GeV with PT0 = 2.0
GeV. This implies that e = PARP(90) should be around 0.3 instead of the 0.16 (default).
For the MIN cone 0.25 GeV/c in radius R = 0.7 implies a PTsum density of dPTsum/dhd = 0.16 GeV/c
and 1.4 GeV/c in the MAX cone implies dPTsum/dhd = 0.91 GeV/c (average PTsum density of 0.54
GeV/c per unit h-).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 44
“Transverse” Charged Densities
Energy Dependence
"Transverse" Charged PTsum Density: dPTsum/dhd
"Min Transverse" PTsum Density: dPTsum/dhd
0.60
0.3
Charged PTsum Density (GeV)
Charged PTsum Density (GeV)
e = 0.25
HERWIG 6.4
0.40
e = 0.16
e=0
0.20
CTEQ5L
Pythia 6.206 (Set A)
630 GeV |h|<1.0 PT>0.4 GeV
Pythia 6.206 (Set A)
0.2
0.1
CTEQ5L
e = 0.16
e=0
630 GeV |h|<1.0 PT>0.4 GeV
0.0
0.00
0
5
10
15
20
25
30
35
40
45
50
0
5
10
 Shows the “transverse” charged PTsum density
20
25
30
35
40
45
50
Hard-Scattering Cut-Off PT0
5
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
(|h|<1, PT>0.4 GeV) versus PT(charged jet#1) at
630 GeV predicted by HERWIG 6.4 (PT(hard) > 3
GeV/c, CTEQ5L) and a tuned version of PYTHIA
6.206 (PT(hard) > 0, CTEQ5L, Set A, e = 0, e =
0.16 (default) and e = 0.25 (preferred)).
Also shown are the PTsum densities (0.16 GeV/c
and 0.54 GeV/c) determined from the Tano cone
analysis at 630 GeV
Fermilab MC Workshop
October 4, 2002
15
PT(charged jet#1) (GeV/c)
PT(charged jet#1) (GeV/c)

HERWIG 6.4
e = 0.25
Rick Field - Florida/CDF
3
2
e = 0.16 (default)
1
100
1,000
10,000
100,000
CM Energy W (GeV)
Reference point
E0 = 1.8 TeV
Page 45
“Transverse” Charged Densities
Energy Dependence
"Transverse" Charged PTsum Density: dPTsum/dhd
"Min Transverse" PTsum Density: dPTsum/dhd
0.60
0.3
Charged PTsum Density (GeV)
Charged PTsum Density (GeV)
e = 0.25
HERWIG 6.4
0.40
e = 0.16
e=0
0.20
630 GeV |h|<1.0 PT>0.4 GeV
0.2
0.1
CTEQ5L
630 GeV |h|<1.0 PT>0.4 GeV
0.0
0.00
0
5
10
15
20
25
30
35
40
45
50
0
5
10
Lowering PT0 at 630 GeV (i.e.
increasing e) increases UE activity
charged PTsum density
resulting in less energy dependence.
(|h|<1, PT>0.4 GeV) versus PT(charged jet#1) at
630 GeV predicted by HERWIG 6.4 (PT(hard) > 3
GeV/c, CTEQ5L) and a tuned version of PYTHIA
6.206 (PT(hard) > 0, CTEQ5L, Set A, e = 0, e =
0.16 (default) and e = 0.25 (preferred)).
Also shown are the PTsum densities (0.16 GeV/c
and 0.54 GeV/c) determined from the Tano cone
analysis at 630 GeV
Fermilab MC Workshop
October 4, 2002
20
25
30
35
40
45
50
Hard-Scattering Cut-Off PT0
5
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
 Shows the “transverse”
15
PT(charged jet#1) (GeV/c)
PT(charged jet#1) (GeV/c)

HERWIG 6.4
e = 0.25
Increasing e produces less energy
dependence for the
UE resulting
in
e = 0.16
e=0
less UE activity at the LHC!
CTEQ5L
Pythia 6.206 (Set A)
Pythia 6.206 (Set A)
Rick Field - Florida/CDF
3
2
e = 0.16 (default)
1
100
1,000
10,000
100,000
CM Energy W (GeV)
Reference point
E0 = 1.8 TeV
Page 46
Tuned PYTHIA (Set A)
LHC Predictions
"Transverse" Charged PTsum Density: dPTsum/dhd
"Transverse" Charged Particle Density: dN/dhd
2.5
3.0
"Transverse" PTsum Density (GeV)
"Transverse" Charged Density
HERWIG 6.4
14 TeV
2.5
2.0
1.5
1.8 TeV
1.0
PYTHIA 6.206 Set A
0.5
CTEQ5L
|h|<1.0 PT>0 GeV
PYTHIA 6.206 Set A
2.0
14 TeV
Factor of 2!
HERWIG 6.4
1.5
1.0
1.8 TeV
0.5
CTEQ5L
|h|<1.0 PT>0 GeV
0.0
0.0
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
PT(charged jet#1) (GeV/c)
 Shows the average “transverse” charge particle and PTsum density (|h|<1, PT>0) versus
PT(charged jet#1) predicted by HERWIG 6.4 (PT(hard) > 3 GeV/c, CTEQ5L). and a tuned
versions of PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, Set A) at 1.8 TeV and 14 TeV.
 At 14 TeV tuned PYTHIA (Set A) predicts roughly 2.3 charged particles per unit h- (PT > 0)

in the “transverse” region (14 charged particles per unit h) which is larger than the HERWIG
prediction.
At 14 TeV tuned PYTHIA (Set A) predicts roughly 2 GeV/c charged PTsum per unit h- (PT
> 0) in the “transverse” region at PT(chgjet#1) = 40 GeV/c which is a factor of 2 larger than
at 1.8 TeV and much larger than the HERWIG prediction.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 47
Tuned PYTHIA (Set A)
LHC Predictions
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged PTsum Density: dPTsum/dhd
3.0
PYTHIA 6.206 (default)
"Transverse" PTsum Density (GeV)
"Transverse" Charged Density
3.5
HERWIG 6.4
3.0
14 TeV
2.5
2.0
1.5
1.8 TeV
1.0
PYTHIA 6.206 Set A
0.5
CTEQ5L
|h|<1.0 PT>0 GeV
0.0
PYTHIA 6.206 Set A
2.5
PYTHIA 6.206 (default)
14 TeV
2.0
HERWIG 6.4
1.5
1.0
1.8 TeV
0.5
|h|<1.0 PT>0 GeV
CTEQ5L
0.0
0
5
10
15
20
25
30
35
40
45
50
0
5
PT(charged jet#1) (GeV/c)
10
15
20
25
30
35
40
45
50
PT(charged jet#1) (GeV/c)
 Shows the average “transverse” charge particle and PTsum density (|h|<1, PT>0) versus
PT(charged jet#1) predicted by HERWIG 6.4 (PT(hard) > 3 GeV/c, CTEQ5L). and a tuned
versions of PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, Set A) at 1.8 TeV and 14 TeV. Also
shown is the 14 TeV prediction of PYTHIA 6.206 with the default value e = 0.16.
 Tuned PYTHIA (Set A) predicts roughly 2.3 charged particles per unit h- (PT > 0) in
the “transverse” region (14 charged particles per unit h) which is larger than the
HERWIG prediction and much less than the PYTHIA default prediction.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 48
Tuned PYTHIA (Set A)
LHC Predictions
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged PTsum Density: dPTsum/dhd
2.5
"Transverse" PTsum Density (GeV)
"Transverse" Charged Density
3.0
PYTHIA 6.206 Set A
2.5
2.0
1.5
HERWIG 6.4
1.0
0.5
CTEQ5L
14 TeV |h|<1.0 PT>0 GeV
0.0
PYTHIA 6.206 Set A
2.0
Big difference!
1.5
1.0
HERWIG 6.4
0.5
CTEQ5L
14 TeV |h|<1.0 PT>0 GeV
0.0
0
10
20
30
40
50
60
70
80
90
100
0
10
PT(charged jet#1) (GeV/c)
20
30
40
50
60
70
80
90
100
PT(charged jet#1) (GeV/c)
 Shows the average “transverse” charge particle and PTsum density (|h|<1, PT>0) versus

PT(charged jet#1) predicted by HERWIG 6.4 (PT(hard) > 3 GeV/c, CTEQ5L). and a tuned
versions of PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, Set A) at 1.8 TeV and 14 TeV. Also
shown is the 14 TeV prediction of PYTHIA 6.206 with the default value e = 0.16.
Tuned PYTHIA (Set A) predicts roughly 2.5 GeV/c per unit h-
(PT > 0) from charged particles in the “transverse” region for
3.8 GeV/c (charged)
PT(chgjet#1) = 100 GeV/c. Note, however, that the “transverse”
in cone of
radius R=0.7
charged PTsum density increases rapidly as PT(chgjet#1)
at 14 TeV
increases.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 49
Tuned PYTHIA (Set 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 the a tuned version of PYTHIA 6.206 (Set 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 (Set A) predicts a 42% rise in dNchg/dhd at h = 0 in going from the Tevatron (1.8
TeV) to the LHC (14 TeV).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 50
Tuned PYTHIA (Set A)
LHC Predictions
Charged Particle Density
12% of “Min-Bias” events
50%
have PT(hard) > 10 GeV/c!
1.0E+00
|h|<1
Pythia 6.206 Set A
Hard-Scattering in Min-Bias Events
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 the a tuned version of
PYTHIA 6.206 (Set A) with PT(hard) > 0.
 This PYTHIA fit 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!
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 51
The “Underlying Event”
Summary & Conclusions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event
Outgoing Parton
Underlying Event
The “Underlying Event”
Final-State
Radiation
 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 beambeam remnants with PT > 0.5 GeV/c.
The CDF “Min-Bias” data describes the “beam-beam remnants” better than HERWIG
does. Adding HERWIG’s initial & final state radiation to the CDF “Min-Bias” data
comes close to describing the “underlying event”, but the CDF “Min-Bias” data contain
a lot of “hard” scatterings. Thus, maybe the “beam-beam remnants” also contain
“hard” scatterings (i.e. multiple parton collisions).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 52
Multiple Parton Interactions
Summary & Conclusions
Multiple Parton Interactions
Proton
AntiProton
Hard Core
JETWEB
Hard Core
 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.
PYTHIA (with varying impact parameter) does a good job fitting the “underlying event”
data and also describes fairly well the “Min-Bias” data with the same program (PT(hard)
> 0).
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.
 Also check out Jon Butterworth’s JETWEB at http://jetweb.hep.ucl.ac.uk/Results/MI/.
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 53
LHC Predictions
Summary & Conclusions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event
Outgoing Parton
Tevatron
LHC
Underlying Event
Final-State
Radiation
 Both HERWIG and the tuned PYTHIA (Set 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 (Set 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)!
For the “underlying event” in hard scattering processes the predictions of HERWIG and
the tuned PYTHIA (Set A) differ greatly (factor of 2!). HERWIG predicts a smaller
increase in the activity of the “underlying event” in going from the Tevatron to the LHC.
The tuned PYTHIA (Set A) predicts about a factor of two increase at the LHC in the
charged PTsum density of the “underlying event” at the same PT(jet#1) (the “transverse”
charged PTsum density increases rapidly as PT(jet#1) increases).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 54
LHC Predictions
Summary & Conclusions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event




Tevatron
LHC
Underlying Event
12 times more likely
to find a 10 GeV
Both HERWIG and the tuned PYTHIA (Set A) predict a 40-45% rise
inindN
“jet”
“Min-Bias”
chg/dhd at
at theparticles
LHC!
= 0 in going from the Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged
per
Outgoing Parton
Final-State
Radiation
h
unit h at the Tevatron becomes 6 per unit h at the LHC.
Twice as much
The tuned PYTHIA (Set A) predicts that 1% of all “Min-Bias” events at the Tevatron
activity in the
(1.8 TeV) are the result of a hard 2-to-2 parton-parton scattering with PT(hard)
> 10
“underlying event”
GeV/c which increases to 12% at LHC (14 TeV)!
at the LHC!
For the “underlying event” in hard scattering processes the predictions of HERWIG and
the tuned PYTHIA (Set A) differ greatly (factor of 2!). HERWIG predicts a smaller
increase in the activity of the “underlying event” in going from the Tevatron to the LHC.
The tuned PYTHIA (Set A) predicts about a factor of two increase at the LHC in the
charged PTsum density of the “underlying event” at the same PT(jet#1) (the “transverse”
charged PTsum density increases rapidly as PT(jet#1) increases).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 55
LHC Predictions
Summary & Conclusions
Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
AntiProton
Underlying Event


Tevatron
LHC
Underlying Event
12 times more likely
to find a 10 GeV
the LHC
contains a 40-45% rise in dN /dhd at h
Both HERWIG and the tuned “Min-Bias”
PYTHIAat(Set
A) predict
“jet” in “Min-Bias”
chg
much more hard collisions than at the
at
the
LHC!
= 0 in going from the Tevatron Tevatron!
(1.8 TeV)Atto
LHCthe(14 TeV). 4 charged particles per
thethe
Tevatron
“underlying
event”
is athe
factor
of 2
unit h at the Tevatron becomes
6 per unit
h at
LHC.
more active than “Tevaron Min-Bias”.
Twice as much
The tuned PYTHIA (Set A) At
predicts
1% of allevent”
“Min-Bias”
events at the Tevatron
the LHCthat
the “underlying
will
activity in the
at least aparton-parton
factor of 2 more scattering with PT(hard) > 10
(1.8 TeV) are the result of a hardbe2-to-2
“underlying event”
active
than(14
“LHC
Min-Bias”!
GeV/c which increases to 12% at
LHC
TeV)!
at the LHC!
Outgoing Parton
Final-State
Radiation
 For the “underlying event” in hard scattering processes the predictions of HERWIG and

the tuned PYTHIA (Set A) differ greatly (factor of 2!). HERWIG predicts a smaller
increase in the activity of the “underlying event” in going from the Tevatron to the LHC.
The tuned PYTHIA (Set A) predicts about a factor of two increase at the LHC in the
charged PTsum density of the “underlying event” at the same PT(jet#1) (the “transverse”
charged PTsum density increases rapidly as PT(jet#1) increases).
Fermilab MC Workshop
October 4, 2002
Rick Field - Florida/CDF
Page 56