C2CR07
Rick Field
University of Florida
(for the CDF Collaboration)
Min-Bias at the Tevatron
Outline of Talk
 What is “Min-Bias”? Is it unique?
 “Jets” in Min-Bias Collisions. Clear Jet
CDF Run 2
structure as low as 1 GeV/c!
 Extrapolating Tevatron “Min-Bias” to the
LHC.
Proton
“Minumum Bias” Collisions
AntiProton
 Using Pile-Up collisions to Study Min-Bias.
Is Pile-Up Unbiased?
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 1
Proton-AntiProton Collisions
at the Tevatron
Elastic Scattering
The CDF “Min-Bias” trigger
picks up most of the “hard
core” cross-section plus a
Double
Diffraction
small
amount of single &
double diffraction.
M2
M1
Single Diffraction
M
stot = sEL + sIN
SD +sDD +sHC
1.8 TeV: 78mb
= 18mb
+ 9mb
+ (4-7)mb + (47-44)mb
CDF “Min-Bias” trigger
1 charged particle in forward BBC
AND
1 charged particle in backward BBC
Hard Core
The “hard core” component
contains both “hard” and
“soft” collisions.
“Hard” Hard Core (hard scattering)
Outgoing Parton
“Soft” Hard Core (no hard scattering)
Proton
AntiProton
Beam-Beam Counters
3.2 < |h| < 5.9
PT(hard)
Proton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Outgoing Parton
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 2
No-Bias vs Min-Bias
Charged Particle Density: dN/dh
Charged Particle Density: dN/dh
5
5
No Trigger
900 GeV Generated
pyDWT DD NoTtrig
4
pyDW HC Mbtrig
Charged Particle Density
Charged Particle Density
pyDWT HC NoTrig
pyDWT SD NoTrig
3
2
1
CDF Min-Bias Trigger
900 GeV Generated
pyDW DD MBtrig
4
pyDW SD MBtrig
3
2
1
Charged Particles (all PT)
Charged Particles (all PT)
0
0
-8
-6
-4
-2
0
2
4
6
8
-8
-6
-4
-2
0
2
What you see for “Min-Bias” PseudoRapidity h
depends on your triggger!
Charged Particle Density: dN/dh
Charged Particle Density: dN/dh
4
6
8
PseudoRapidity h
5
5
No Trigger
900 GeV Weighted
pyDWT HC NoTrig
4
pyDWT Sum MBtrig
Charged Particle Density
Charged Particle Density
pyDWT Sum NoTrig
pyDWT DD NoTtrig
pyDWT SD NoTrig
3
2
1
Charged Particles (all PT)
0
CDF Min-Bias Trigger
900 GeV Weighted
pyDWT HC MBtrig
4
pyDWT DD MBtrig
pyDWT SD MBtrig
3
2
1
Charged Particles (all PT)
0
-8
-6
-4
-2
0
2
4
6
8
-8
-6
PseudoRapidity h
C2CR07-Lake Tahoe
February 28, 2007
-4
-2
0
2
4
6
8
PseudoRapidity h
Rick Field – Florida/CDF
Page 3
No-Bias vs Min-Bias
About 2.5 charged particles
per unit h at h = 0.
Charged Particle Density: dN/dh
PYTHIA Tune
Tune DW
DW
Charged Particles
Particles
No-Bias 1.96 TeV
(HC+SD+DD+EL)
3
About 0.9 charged particles (pT >
0.5 GeV/c) per unit h at h = 0.
All
All PT
PT
PT > 0.5 GeV/c
2
Charged Particle Density: dN/dh
1
0
-10
2.0
-8
-6
-4
-2
0
2
4
6
88
10
10
PseudoRapidity h
 Charged particle (all pT) pseudo-rapidity
distribution, dNchg/dhdf, at 1.96 TeV with
no trigger (i.e. no-bias) from PYTHIA
Tune DW.
Charged Particle Density
Charged
Charged Particle Density
4
PYTHIA Tune DW
Charged Particles (PT > 0.5 GeV/c)
Min-Bias 1.96 TeV
(HC+SD+DD+EL)
1.5
No Trigger
CDF Min-Bias
Trigger
1.0
0.5
0.0
-10
-8
-6
-4
-2
0
2
4
6
8
10
PseudoRapidity h
 Charged particle (pT > 0.5 GeV/c) pseudo-
About 1.5 charged particles (pT >
0.5 GeV/c) per unit h at h = 0
with CDF min-bias trigger.
C2CR07-Lake Tahoe
February 28, 2007
rapidity distribution, dNchg/dhdf, at 1.96
TeV with the CDF Min-Bias trigger from
PYTHIA Tune DW.
Rick Field – Florida/CDF
Page 4
Particle Densities
DhDf = 4 = 12.6
2
f
31 charged
charged particles
particle
Charged Particles
pT > 0.5 GeV/c |h| < 1
CDF Run 2 “Min-Bias”
CDF Run 2 “Min-Bias”
Observable
Average
Nchg
Number of Charged Particles
(pT > 0.5 GeV/c, |h| < 1)
3.17 +/- 0.31
0.252 +/- 0.025
PTsum
(GeV/c)
Scalar pT sum of Charged Particles
(pT > 0.5 GeV/c, |h| < 1)
2.97 +/- 0.23
0.236 +/- 0.018
Average Density
per unit h-f
dNchg
chg/dhdf = 1/4
3/4 = 0.08
0.24
13 GeV/c PTsum
0
-1
h
+1
Divide by 4
dPTsum/dhdf = 1/4
3/4 GeV/c = 0.08
0.24 GeV/c
 Study the charged particles (pT > 0.5 GeV/c, |h| < 1) and form the charged
particle density, dNchg/dhdf, and the charged scalar pT sum density,
dPTsum/dhdf.
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 5
CDF Run 1 “Min-Bias” Data
Charged Particle Density
Charged Particle Pseudo-Rapidity Distribution: dN/dh
Charged Particle Density: dN/dhdf
1.0
7
CDF Published
CDF Published
6
0.8
dN/dhdf
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
0
all PT
0.0
-4
-3
-2
-1
0
1
2
3
4
-4
-3
Pseudo-Rapidity h
<dNchg/dh> = 4.2
-2
-1
0
1
2
3
4
Pseudo-Rapidity h
<dNchg/dhdf> = 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/dhdf, by dividing by 2. There are about 0.67
charged particles per unit h-f in “Min-Bias” collisions at 1.8 TeV (|h| < 1, all PT).
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 6
CDF Run 1 “Min-Bias” Data
Energy Dependence
Charged Particle Density: dN/dhdf
Charged Particle Density: dN/dhdf
1.4
1.0
CDF Published
CDF Data
UA5 Data
Fit 2
Fit 1
1.2
Charged density dN/dhdf
dN/dhdf
0.8
0.6
0.4
0.2
CDF Min-Bias 630 GeV
CDF Min-Bias 1.8 TeV
1.0
0.8
0.6
0.4
0.2
all PT
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)
LHC?
<dNchg/dhdf> = 0.51
h = 0 630 GeV
24% increase
<dNchg/dhdf> = 0.63
h = 0 1.8 TeV
 Shows the center-of-mass energy dependence of the charged particle density, dNchg/dhdf,

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?
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 7
PYTHIA Tune A Min-Bias
“Soft” + ”Hard”
Tuned to fit the
“underlying event”!
PYTHIA Tune A
CDF Run 2 Default
Charged Particle Density: dN/dhdf
Charged Particle Density
1.0
CDF Published
1.0E+00
0.8
CDF Min-Bias Data
1.0E-01
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/dhdfdPT (1/GeV/c)
dN/dhdf
Pythia 6.206 Set A
1.8 TeV |h|<1
1.0E-02
12% of “Min-Bias” events
have PT(hard) > 5 GeV/c!
PT(hard) > 0 GeV/c
1.0E-03
1% of “Min-Bias” events
have PT(hard) > 10 GeV/c!
1.0E-04
1.0E-05
CDF Preliminary
1.0E-06
0
2
4
6
8
10
12
14
PT(charged) (GeV/c)
 The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned.
 This PYTHIA fit predicts that 12% of all “Min-Bias” events are a result of a hard 2-to-2
parton-parton scattering with PT(hard) > 5 GeV/c (1% with PT(hard) > 10 GeV/c)!
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 8
Min-Bias “Associated”
Charged Particle Density
“Associated” densities do
not include PTmax!
Highest pT
charged particle!
Charged Particle Density: dN/dhdf
PTmax Direction
PTmax Direction
0.5
Df
Correlations in f
Charged Particle Density
CDF Preliminary
Associated Density
PTmax not included
data uncorrected
0.4
Df
Charge Density
0.3
0.2
0.1
Min-Bias
Correlations
in f
Charged Particles
(|h|<1.0, PT>0.5 GeV/c)
PTmax
0.0
0
30
60
90
120
150
180
210
240
270
300
330
360
Df (degrees)
 Use the maximum pT charged particle in the event, PTmax, to define a direction and

is “associated”
more probable
to finddN
a chg
particle
look at the It
the
density,
/dhdf, in “min-bias” collisions (pT > 0.5
accompanying
PTmax
than
it
is to
GeV/c, |h| < 1).
find a particle in the central region!
Shows the data on the Df dependence of the “associated” charged particle density,
dNchg/dhdf, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative
to PTmax (rotated to 180o) for “min-bias” events. Also shown is the average charged
particle density, dNchg/dhdf, for “min-bias” events.
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 9
Min-Bias “Associated”
Charged Particle Density
Rapid rise in the particle
density in the “transverse”
region as PTmax increases!
Associated Particle Density: dN/dhdf
PTmaxDirection
Direction
PTmax
Df
“Toward”
“Transverse”
“Transverse”
Correlations in f
“Away”
Associated Particle Density
Jet #1
Df
PTmax > 2.0 GeV/c
1.0
PTmax > 2.0 GeV/c
PTmax > 1.0 GeV/c
0.8
Charged Particles
(|h|<1.0, PT>0.5 GeV/c)
CDF Preliminary
data uncorrected
PTmax > 0.5 GeV/c
Transverse
Region
0.6
Transverse
Region
0.4
0.2
Jet #2
PTmax
PTmax not included
Min-Bias
0.0
0
30
60
90
120
150
180
210
240
270
300
330
360
Df (degrees)
Ave Min-Bias
0.25 per unit h-f
PTmax > 0.5 GeV/c
 Shows the data on the Df dependence of the “associated” charged particle density,
dNchg/dhdf, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative
to PTmax (rotated to 180o) for “min-bias” events with PTmax > 0.5, 1.0, and 2.0 GeV/c.
 Shows “jet structure” in “min-bias” collisions (i.e. the “birth” of the leading two jets!).
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 10
Min-Bias “Associated”
Charged Particle Density
PY Tune A
PTmax > 2.0 GeV/c
PTmax Direction
Direction
PTmax
Df
“Toward”
“Transverse”
“Transverse”
Correlations in f
“Away”
PTmax > 2.0 GeV/c
Associated Particle Density
Df
Associated Particle Density: dN/dhdf
1.0
CDF Preliminary
PY Tune A
0.8
data uncorrected
theory + CDFSIM
PTmax > 0.5 GeV/c
PY Tune A
Transverse
Region
0.6
PY Tune A 1.96 TeV
Transverse
Region
0.4
0.2
PTmax
PTmax not included
(|h|<1.0, PT>0.5 GeV/c)
0.0
0
30
60
90
120
PTmax > 0.5 GeV/c
150
180
210
240
270
300
330
360
Df (degrees)
 Shows the data on the Df dependence of the “associated” charged particle density,
dNchg/dhdf, for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) relative
to PTmax (rotated to 180o) for “min-bias” events with PTmax > 0.5 GeV/c and PTmax >
2.0 GeV/c compared with PYTHIA Tune A (after CDFSIM).
 PYTHIA Tune A predicts a larger correlation than is seen in the “min-bias” data (i.e.
Tune A “min-bias” is a bit too “jetty”).
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 11
PYTHIA Tune A
LHC Min-Bias Predictions
Charged Particle Density: dN/dhdf
Charged Particle Density: dN/dhdf
1.4
1.4
Pythia 6.206 Set A
CDF Data
14 TeV
Charged density dN/dhdf
1.0
dN/dhdf
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/dhdf,
for “Min-Bias” collisions compared with PYTHIA Tune A with PT(hard) > 0.
 PYTHIA was tuned to fit the “underlying event” in hard-scattering processes at 1.8 TeV
and 630 GeV.
 PYTHIA Tune A predicts a 42% rise in dNchg/dhdf at h = 0 in going from the Tevatron (1.8
TeV) to the LHC (14 TeV). Similar to HERWIG “soft” min-bias, 4 charged particles per
unit h becomes 6.
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 12
PYTHIA Tune A
LHC Min-Bias Predictions
Hard-Scattering in Min-Bias Events
Charged Particle Density
12% of “Min-Bias”
50% events
have PT(hard) > 10 GeV/c!
1.0E+00
|h|<1
Pythia 6.206 Set A
Pythia 6.206 Set A
40%
% of Events
Charged Density dN/dhdfdPT (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
1,000
10,000
100,000
CM Energy W (GeV)
630 GeV
1.0E-05
 Shows the center-of-mass energy
CDF Data
1.0E-06
0
2
LHC?
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/dhdfdPT, for “Min-Bias” collisions
compared with PYTHIA Tune A with
PT(hard) > 0.
 PYTHIA Tune A predicts that 1% of all “Min-Bias” events at 1.8 TeV are a result of a hard
2-to-2 parton-parton scattering with PT(hard) > 10 GeV/c which increases to 12% at 14
TeV!
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 13
PYTHIA 6.2 Tunes
LHC Min-Bias Predictions
Charged Particle Density: dN/dY
Charged Particle Density: dN/dh
12
pyA
pyDW
pyDWT
ATLAS
Generator Level
14 TeV
8
Charged Particle Density
Charged Particle Density
10
6
4
2
pyA
pyDW
pyDWT
ATLAS
Generator Level
14 TeV
10
8
6
4
2
Charged Particles (all pT)
Charged Particles (all pT)
0
0
-10
-8
-6
-4
-2
0
2
4
6
8
10
-10
-8
-6
-4
PseudoRapidity h
-2
0
2
4
6
8
10
Rapidity Y
 Shows the predictions of PYTHIA Tune A, Tune DW, Tune DWT, and the ATLAS tune for
the charged particle density dN/dh and dN/dY at 14 TeV (all pT).
 PYTHIA Tune A and Tune DW predict about 6 charged particles per unit h at h = 0, while
the ATLAS tune predicts around 9.
 PYTHIA Tune DWT is identical to Tune DW at 1.96 TeV, but extrapolates to the LHC using
the ATLAS energy dependence.
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 14
Using Pile-Up to Study Min-Bias
AntiProton
Proton
Primary
Pile-Up
60 cm
MB
High PT Jet
Warning! This analysis is very
 The primary vertex is the highest PTsum of charged particles pointing towards it.
preliminary and not “blessed”.
You
cannot
believe
it until
it ispoint back to the primary
 Normally one only includes
those
charged
particles
which
“blessed”!
vertex.
 However, the primary vertex is presumably the collision that satisfied the trigger
and is hence biased.
 Perhaps the pile-up is not biases and can serve as a new type of “Min-Bias”
collisions.
 This assumes that the pile-up is not affected by the trigger (i.e. it is the same for all
primary processes).
C2CR07-Lake Tahoe
February 28, 2007
Rick Field – Florida/CDF
Page 16
Summary and Conclusions
“Minumum Bias” Collisions
 “Min-Bias” is not well defined. What you
see depends on what you trigger on! Every
trigger produces some biases.
Proton
 Very preliminary results seem to show that
pile-up conspires to help give you what you
ask for (i.e. satisfy your “trigger” or your
event selection)!
If true this means the pile-up is not the
same for all processes. It is process (i.e.
trigger) dependent!
C2CR07-Lake Tahoe
February 28, 2007
Charged Particle Density: dN/dh
10
Charged Particle Density
 We have learned a lot about “Min-Bias” at the
Tevatron, but we do not know what to expect at
the LHC. This will depend on the Min-Bias Trigger!
AntiProton
pyA
pyDW
pyDWT
ATLAS
Generator Level
14 TeV
8
6
4
2
Charged Particles (all pT)
0
-10
-8
-6
-4
-2
0
2
4
6
8
10
PseudoRapidity h
I must double check my analysis and
get it “blessed” before you can trust
what I have shown!
Rick Field – Florida/CDF
Page 17