Northwest Terascale Workshop
Parton Showers and Event Structure at the LHC
Modeling Min-Bias and Pile-Up
Rick Field
University of Florida
Outline of Talk
 Modeling “min-bias” collisions at CDF
University of Oregon February 24, 2009
(Pythia Tune A).
Outgoing Parton
PT(hard)
 Extrapolating “min-bias” to the LHC.
Initial-State Radiation
Proton
AntiProton
Underlying Event
Underlying Event
 Studying pile-up at CDF. (I cannot make
this available outside of CDF and hence it
is not included in this version of the talk.)
Outgoing Parton
Final-State
Radiation
 Studying pile-up at the LHC. You can not
talk about “event structure” at the LHC
without talking about pile-up!
CMS at the LHC
CDF Run 2
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
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 +sND
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 “inelastic non-diffractive”
component contains both
“hard” and “soft” collisions.
“Hard” Hard Core (hard scattering)
Outgoing Parton
“Soft” Hard Core (no hard scattering)
Proton
AntiProton
PT(hard)
Beam-Beam Counters
3.2 < |h| < 5.9
Proton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Outgoing Parton
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 2
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
1
0
-10
-8
-6
-4
-2
0
What you see for “Min-Bias”
Charged Particle Density: dN/dh
2.0
depends on your triggger!
Min-Bias 1.96 TeV
(HC+SD+DD+EL)
By comparing different
1.5
“Min-Bias” triggers one can
2
4
6
88
1.0
learn
about
the1010components!
Charged Particle Density
Charged
Charged Particle Density
4
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.
PYTHIA Tune DW
Charged Particles (PT > 0.5 GeV/c)
No Trigger
CDF Min-Bias
Trigger
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.
Oregon Terascale Workshop
February 24, 2009
rapidity distribution, dNchg/dhdf, at 1.96
TeV with the CDF Min-Bias trigger from
PYTHIA Tune DW.
Rick Field – Florida/CDF/CMS
Page 3
Min-Bias Particle Types
With Stable Particles
With ct = 10mm
This is a bigger effect
than I expected!
No-Bias at 14 TeV
charged particle density: 10mm vs Stable
Charged Particle Density: dN/dh
Charged Particle Density
6
Min-Bias 14 TeV
PY Tune DWT
4
2
pyDWT HC+SD+DD NoTrig
pyDWT HC+SD+DD NoTrig 10mm
Charged Particles (all PT)
0
-8
-6
-4
-2
0
2
4
6
8
PseudoRapidity h
Stable if ct ≥ 10 mm!
 Using ct = 10 mm reduces the
charged particle density by
almost 10%! Mostly from
Ks→p+p- (68.6%) and L →pp(64.2%).
Oregon Terascale Workshop
February 24, 2009
CDF Run 2
Proton
Rick Field – Florida/CDF/CMS
AntiProton
Primary
60 cm
Page 4
Particle Densities
DhDf = 4p = 12.6
2p
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 = 1/4p
3/4p = 0.08
0.24
13 GeV/c PTsum
0
-1
h
+1
Divide by 4p
dPTsum/dhdf = 1/4p
3/4p 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.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 5
CDF Run 1 “Min-Bias” Data
Charged Particle Density
Charged Particle Density: dN/dhdf
Charged Particle Pseudo-Rapidity Distribution: dN/dh
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
all PT
0.0
0
-4
-3
-2
-1
0
1
2
3
4
-4
-3
-1
0
1
2
3
4
Pseudo-Rapidity h
Pseudo-Rapidity h
<dNchg/dh> = 4.2
-2
<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).
DhxDf = 1
 Convert to charged particle density, dNchg/dhdf, by dividing by 2p.
Df = 1
There are about 0.67 charged particles per unit h-f in “Min-Bias”
0.25
0.67
collisions at 1.8 TeV (|h| < 1, all pT).
 There are about 0.25 charged particles per unit h-f in “Min-Bias”
Dh = 1
collisions at 1.96 TeV (|h| < 1, pT > 0.5 GeV/c).
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
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?
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 7
Herwig “Soft” Min-Bias
Can
Can we
we believe
believe HERWIG
HERWIG
“soft”
Min-Bias?
“soft”
Min-Bias?
No!
Charged Particle Density: dN/dhdf
Charged Particle Density: dN/dhdf
1.4
1.4
14 TeV
Herwig "Soft" Min-Bias
1.2
CDF Data
1.2
Charged density dN/dhdf
UA5 Data
dN/dhdf
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/dhdf, 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/dhdf, in “Min-Bias” collisions.
 HERWIG “Soft” Min-Bias predicts a 45% rise in dNchg/dhdf at h = 0 in going from the
Tevatron (1.8 TeV) to the LHC (14 TeV). 4 charged particles per unit h becomes 6.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 8
CDF Run 1 “Min-Bias” Data
pT Distribution
Lots of “hard” scattering
in “Min-Bias”!
Charged Particle Density: dN/dhdf
Charged Particle Density
1.4
1.0E+01
14 TeV
Herwig "Soft" Min-Bias
1.2
HERWIG “Soft” Min-Bias
|h|<1
1.0E+00
0.8
0.6
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
charged particle density, dNchg/dhdf, for
“Min-Bias” collisions compared with
HERWIG “Soft” Min-Bias.
Charged Density dN/dhdfdPT (1/GeV/c)
1.0
dN/dhdf
CDF Preliminary
CDF Min-Bias Data at 1.8 TeV
1.0E-01
1.0E-02
HW "Soft" Min-Bias
at 630 GeV, 1.8 TeV, and 14 TeV
1.0E-03
1.0E-04
1.0E-05
1.0E-06
0
2
4
6
8
10
12
14
PT (GeV/c)
 Shows the pT dependence of the charged particle density, dNchg/dhdfdPT, 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.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 9
CDF Run 1 “Min-Bias” Data
Combining “Soft” + “Hard”
No easy way to
“mix” HERWIG “hard”
with HERWIG “soft”.
sHC
Hard-Scattering Cross-Section
Charged
Particle Density
100.00
Charged Particle Density: dN/dhdf
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/dhdfdPT (1/GeV/c)
dN/dhdf
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?
Oregon Terascale Workshop
February 24, 2009
20
Hard-Scattering Cut-Off PTmin
Rick Field – Florida/CDF/CMS
Page 10
PYTHIA Tune A Min-Bias
“Soft” + ”Hard”
Tuned to fit the CDF Run 1
“underlying event”!
PYTHIA Tune A
CDF Run 2 Charged
DefaultParticle Density
Charged Particle Density: dN/dhdf
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 in
PT“Min-Bias”
(hard) > 0.
One
can simulate both “hard”
at the
Tevatron!
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)!
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 11
CDF Run 2 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 look
It is more probable
to find
a particle
at the the “associated”
density, dN
chg/dhdf,
in “min-bias” collisions (pT > 0.5 GeV/c, |h| <
accompanying
PTmax
than
it
is
to
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.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 12
CDF Run 2 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!).
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 13
CDF Run 2 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”).
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 14
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.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 15
PYTHIA Tune A
LHC Min-Bias Predictions
Hard-Scattering in Min-Bias Events
Charged Particle Density
50%
12% of “Min-Bias”
events
have|h|<1
PT(hard) > 10 GeV/c!
1.0E+00
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
LHC?
1.0E-05
 Shows the center-of-mass energy dependence
CDF Data
1.0E-06
0
2
4
6
8
PT(charged) (GeV/c)
1% of “Min-Bias” events
have PT(hard) > 10 GeV/c!
10
12
14
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!
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 16
Charged <PT> versus Nchg
The charged <PT>
rises with Nchg!
Average PT versus Nchg
1.6
CDF Preliminary
Average PT (GeV/c)
PYTHIA Tune A 1.96 TeV
data uncorrected
theory + CDFSIM
1.4
Min-Bias
1.2
1.0
0.8
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.6
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Number of Charged Particles
Shows the average transverse momentum of charged particles (|h|<1, pT>0.5 GeV)
versus the number of charged particles, Nchg, for the CDF Run 2 Min-Bias events.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 17
Min-Bias Correlations
Average PT versus Nchg
Average PT (GeV/c)
1.4
CDF Run 2 Preliminary
pyDW
data corrected
generator level theory
“Minumum Bias” Collisions
1.2
Min-Bias
1.96 TeV
pyA
Proton
1.0
AntiProton
ATLAS
0.8
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
0.6
0
10
20
30
40
50
Number of Charged Particles
 Data at 1.96 TeV on the average pT of charged particles versus the number of charged particles (pT >
0.4 GeV/c, |h| < 1) for “min-bias” collisions at CDF Run 2. The data are corrected to the particle level
and are compared with PYTHIA Tune A at the particle level (i.e. generator level).
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 18
Min-Bias: Average PT versus Nchg
 Beam-beam remnants (i.e. soft hard core) produces
Average PT versus Nchg
Average PT (GeV/c)
1.4
CDF Run 2 Preliminary
Min-Bias
1.96 TeV
data corrected
generator level theory
1.2
low multiplicity and small <pT> with <pT>
independent of the multiplicity.
 Hard scattering (with no MPI) produces large
pyA
multiplicity and large <pT>.
pyAnoMPI
1.0
 Hard scattering (with MPI) produces large
0.8
multiplicity and medium <pT>.
ATLAS
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
0.6
0
5
10
15
20
25
30
35
40
This observable is sensitive
to the MPI tuning!
Number of Charged Particles
“Hard” Hard Core (hard scattering)
Outgoing Parton
“Soft” Hard Core (no hard scattering)
PT(hard)
CDF “Min-Bias”
=
Proton
+
AntiProton
Proton
AntiProton
Underlying Event
Underlying Event
Initial-State
Radiation
Final-State
Radiation
Multiple-Parton Interactions
+
Proton
AntiProton
Underlying Event
Outgoing Parton
Oregon Terascale Workshop
February 24, 2009
Outgoing Parton
PT(hard)
Initial-State
Radiation
The CDF “min-bias” trigger
picks up most of the “hard
core” component!
Outgoing Parton
Underlying Event
Final-State
Radiation
Rick Field – Florida/CDF/CMS
Page 19
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
PseudoRapidity h
-4
-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.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 20
PYTHIA 6.2 Tunes
LHC Min-Bias Predictions
Average Number of Charged Particles vs PTmin
Charged PT Distribution
20
100.0
Generator Level
Min-Bias 14 TeV
pyA <PT> = 641 MeV/c
Generator Level
Min-Bias 14 TeV
pyDW <PT> = 665 MeV/c
15
pyA
pyDW
pyDWT
ATLAS
<Nchg>
1/Nev dN/dPT (1/GeV/c)
pyDWT <PT> = 693 MeV/c
ATLAS <PT> = 548 MeV/c
10.0
10
5
Charged Particles (|h|<1.0, PT>PTmin)
1.0
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Minimum PT (GeV/c)
Charged Particles (|h|<1.0)
0.1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Charged Particle PT (GeV/c)
 Shows the predictions of PYTHIA Tune A, Tune DW, Tune DWT, and the ATLAS tune for the
charged particle pT distribution at 14 TeV (|h| < 1) and the average number of charged
particles with pT > pTmin (|h| < 1).
 The ATLAS tune has many more “soft” particles than does any of the CDF Tunes. The
ATLAS tune has <pT> = 548 MeV/c while Tune A has <pT> = 641 MeV/c (100 MeV/c more
per particle)!
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 21
Studying Pile-Up at CDF
High PT Jet
CDF Run 2
Proton
AntiProton
Primary
Pile-Up
60 cm
396 ns
MB
 The primary vertex is the highest PTsum of charged particles pointing towards it.
 Normally one only includes those charged particles which point back to the primary
vertex.
 The primary vertex is presumably the collision that satisfied the trigger. Maybe not
for “min-bias” events?
 How well do we understand the pile-up at CDF?
Is the pile-up biased?
Is the pile-up the same for all triggers?
Does pile-up conspire to help satisfy your trigger?
How well do we model pile-up?
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 22
Pile-Up at the LHC
Tune DWT “Hard-Core” No
Trigger (ct =10mm)
Charged Particle Multiplicity Distribution
Charged Particle PseuoRapidity Distribution
4
Generator Level
HCMB 14 TeV
0.04
Charged Particles (|h|<2.0, all pT)
Mean = 24.39
0.03
Generator Level
HCMB 14 TeV
pyDWT <Nchg> = 81.7
<Nchg> in 0.4 bin
Probability per 1 Particle
0.05
pyDWT <Nchg> = 24.39
0.02
Npile = 1
3
2
1
0.01
Charged Particles (all pT)
0
0.00
0
10
20
30
40
50
60
70
80
90
100
-8
-6
-2
0
2
4
6
8
PseudoRapidity h
Number of Charged Particles: Nchg
 Shows the charged multiplicity distribution (|h|
< 2, all pT) for Npile = 1 (i.e. shows, on the
average, what one event looks like). The plot
shows the probability of finding 0, 1, 2, … etc.
charged particles. The sum of the points is
equal to one. The mean is 24.39 charged
particles and s = 19.7.
Oregon Terascale Workshop
February 24, 2009
-4
 Shows the charged particle pseudo-rapidity
distribution (all pT) for Npile = 1 (i.e. shows,
on the average, what one event looks like). The
plot shows the <Nchg> in a 0.4 bin (i.e. not
divided by bin size). The sum of the points
with |h| < 2 is 24.39.
Rick Field – Florida/CDF/CMS
Page 23
Pile-Up at the LHC
Charged Particle Multiplicity Distribution
“Central Limit Theorem”:
<Nchg> ~ Npile, s ~ sqrt(Npile)!
Charged Particle Multiplicity Distribution
0.16
Generator Level
HCMB 14 TeV
Duh!
0.06
Charged Particles (|h|<2.0, all pT)
pyDWT <Nchg> = 243.9
Mean = 243.9
0.04
Npile = 1 with Nchg->10*Nchg
Npile = 10
0.02
0.00
Probability per 50 Particles
Probability per 10 Particles
0.08
Generator Level
HCMB 14 TeV
0.12
True
for any
P(|h|<2.0,
cut!
Charged
Particles
all pT)
T(min)
pyDWT <Nchg> = 1219.5
0.08
Npile = 10 with Nchg->5*Nchg
Npile = 1 with Nchg->50*Nchg
0.04
Npile = 50
0.00
0
100
200
300
400
500
600
700
800
900
1000
0
500
Number of Charged Particles: Nchg
1000
1500
2000
2500
3000
3500
4000
4500
5000
Number of Charged Particles: Nchg
 Shows the charged multiplicity distribution (|h|
 Shows the charged multiplicity distribution
< 2, all pT) for Npile = 50 (i.e. shows, on the
(|h| < 2, all pT) for Npile = 10 (i.e. shows, on
average, what 50 events looks like). The plot
the average, what 10 events looks like). The
shows the probability of finding 0, 50, 100, …
plot shows the probability of finding 0, 10, 20,
etc. charged particles. The sum of the points is
… etc. charged particles. The sum of the
equal to one. The mean is 1219.5 charged
points is equal to one. The mean is 243.9
particles and s = 138.9. Also shown is the
charged particles and s = 62.3. Also shown
Npile = 1 distribution scaled by a factor of 50
is the Npile = 1 distribution scaled by a
(i.e. Nchg → 50×Nchg) and the Npile = 10
factor of 10 (i.e. Nchg → 10×Nchg).
distribution scaled by a factor of 5 (i.e. Nchg →
5×Nchg).
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 24
Pile-Up at the LHC
Log-Scale!
Charged Particle Multiplicity Distribution
Charged Particle Multiplicity Distribution
Charged Particle PseuoRapidity Distribution
1.0E+00
1.0E+01
0.06
All PT <Nchg> = 24.39
1.0E+00
0.04
0.02
PT > 1 <Nchg> = 11.6
PT > 1 GeV/c <Nchg> = 4.87
PT > 2.5 GeV/c <Nchg> = 0.636
1.0E-01
PT > 5 GeV/cPT
<Nchg>
= 0.0466= 1.3
> 2.5 <Nchg>
PT > 10 GeV/c <Nchg> = 0.0025
1.0E-02
PT > 5 <Nchg> = 0.089
1.0E-03
Probability per 1 Particle
Charged Particles
(|h|<2.0)= 81.7
All PT <Nchg>
Generator Level
HCMB 14 TeV
<Nchg> in 0.4 bin
Probability per 1 Particle
0.08
Charged Particles (|h|<2.0)
Generator Level
HCMB 14 TeV
1.0E-01
1.0E-02
1.0E-03
1.0E-04
PT > 10 <Nchg> =0.005
0.00
0
5
10
15
20
25
1.0E-04
30 -8 35
Number of Charged Particles: Nchg
1.0E-05
-6 40
-445
50
-2
0
0
2
PseudoRapidity h
54
10 6 15
8 20
25
30
35
40
45
Number of Charged Particles: Nchg
 Charged multiplicity distribution (|h| < 2) for Npile = 1 (i.e. shows, on the average, what one
event looks like). The plot shows the probability of finding 0, 1, 2, … etc. charged particles.
The five curves correspond to pT(min) = 0, 1.0 , 2.5, 5.0, and 10.0 GeV/c.
 Shows the charged particle pseudo-rapidity distribution for Npile = 1 (i.e. shows, on the
average, what one event looks like). The plot shows the <Nchg> in a 0.4 bin (i.e. not divided
by bin size). The five curves correspond to pT(min) = 0, 1.0 , 2.5, 5.0, and 10.0 GeV/c.
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 25
50
Pile-Up at the LHC
Charged Particle PT Distribution
Average
Average Number
Number of
of Charged
Charged Particles
Particles vs
vs PTmin
PTmin
1.0E+02
100.00
1.0E+02
Generator Level
HCMB 14 TeV
pyDWT <PT> = 693 MeV/c
1.0E-01
Charged Particles (|h|<2.0)
1.0E-02
1.0E+00
1.0E-02
0.10
1.0E-04
1.0E-03
0
2
4
6
8
10
12
14
16
18
20
pyDWT
pyDWT
fit
fit
1.00
1.0E-01
1.0E-03
1.0E-05
0.01
1.0E-04
0.00
Charged Particles (|h|<2.0)
Charged Particles (|h|<2.0)
1
2 1.03
4
5 2.06
7
8 3.09
10
11 4.0
12
13
14 5.0
15
Minimum
T T(GeV/c)
MinimumPP
(GeV/c)
PT (GeV/c)
 Shows the charged particle pT distribution
(|h| < 2) for Npile = 1 (i.e. shows, on the
average, what one event looks like). The plot
shows the <Nchg> in a 1.0 GeV/c bin (i.e. not
divided by bin size). The sum of the points
gives 24.39.
Oregon Terascale Workshop
February 24, 2009
Generator
Level
Generator
Level
HCMB
14
TeV
HCMB 14
TeV
Hard-Scattering
Tail!
1.0E+01
10.00
1.0E+00
<Nchg>
<Nchg>
<Nchg> in 1 GeV/c bin
1.0E+01
 Shows the average number of charged particle
the PT-cut (|h| < 2) for Npile = 1 (i.e. shows, on
the average, what one event looks like). The
first point corresponds to <Nchg> = 24.39.
The fit corresponds to
<Nchg>=24.39exp(-1.4pT(min)).
Rick Field – Florida/CDF/CMS
Page 26
Pile-Up at the LHC
<AssocNchg>+1 ≈ <Nchg>!
Charged Multiplicity & Associated Multiplicity
<AssocNchg>+1 >> <Nchg>!
Npile = 1 <Nchg> = 0.0466
Generator Level
HCMB 14 TeV
Npile = 1 <AssocNchg> = 0.277
0.8
5.0
<Nchg>
<AssocNchg>
<AssocNchg>+1
4.0
Average Number
1.0
Probability per 1 Particle
Charged Multiplicity & Associated Multiplicity
0.6
Charged Particles (pT > 5 GeV/c, |h|<2.0)
0.4
Npile = 1
Generator Level
HCMB 14 TeV
3.0
2.0
1.0
0.2
Charged Particles (pT > 5 GeV/c, |h|<2.0)
0.0
0.0
0
1
2
3
4
5
1
11
21
31
41
51
61
71
81
91
101
Number of Pile-Up Events
Nchg or AssocNchg
 Shows the charged multiplicity distribution (pT > 5 GeV/c, |h| < 2) for Npile = 1 (i.e. shows, on
the average, what one event looks like). The plot shows the probability of finding 0, 1, 2, … etc.
charged particles. The plot also shows the “associated multiplicity” distribution (open squares),
<AssocNchg> = <Nchg> -1, for events with at least one charged particle with pT > 5 GeV/c (i.e.
the over average multiplicity is <AssocNchg> +1 ) . Note that <AssocNchg> +1 = 1.277 and
<Nchg> = 0.0466. There are many more particles in events with at least one charged particle
with pT > 5 GeV/c, then in an average “min-bias” event. Also, note that the probability of
getting an additional particle in an event with at least one charged particle with pT > 5 GeV/c
(i.e. AssocNchg = 1 is greater than the probability of getting one particle in a typical “min-bias”
event, Nchg = 1).
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 27
Pile-Up at the LHC
Charged Particle Multiplicity Probability
Charged Particle Multiplicity Correlation
1.0
1.0
Prob(Nchg ≥ 2)
Prob(Nchg ≥ 1)
0.8
Prob(AssocNchg ≥ 1)
0.6
Correlation
Probability
0.8
Generator Level
HCMB 14 TeV
Generator Level
HCMB 14 TeV
0.4
0.2
0.6
Corr = 1 - Prob(AssocNchg ≥ 1)/Prob(Nchg ≥ 2)
0.4
0.2
Charged Particles (pT > 5 GeV/c, |h|<2.0)
Charged Particles (pT > 5 GeV/c, |h|<2.0)
0.0
0.0
1
11
21
31
41
51
61
71
81
91
101
1
11
Number of Pile-Up Events
21
31
41
51
61
71
81
91
101
Number of Pile-Up Events
 Shows the probability of finding Nchg ≥ 1 and  Shows the “correlation” (Corr = 1 Prob(AssocNchg≥ 1)/Prob(Nchg≥ 2)) versus
Nchg ≥ 2 (pT > 5 GeV/c, |h| < 2) versus Npile,
Npile, where Npile = 1 means one event, Npile
where Npile = 1 means one event, Npile = 10
= 10 means 10 events, etc.. . This correlation
means 10 events, etc.. . The plot also shows the
is very large for one event (i.e. Npile = 1) and
probability of finding AssocNchg ≥ 1 (overall
diminishes as Npile becomes large.
multiplicity ≥ 2) for events with at least one
charged particle with pT > 5 GeV/c. For Npile =
1 (i.e. one event) there is a strong correlation
“Central Limit Theorem”strikes again??
since Prob(AssocNchg≥ 1) is much greater than
Prob(Nchg≥ 2). However, this correlation
diminishes as Npile becomes large
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 28
Min-Bias & Pile-Up Summary
 “Min-Bias” is not well defined. What you
see depends on what you trigger on! Every
trigger produces some biases. We learn
about “min-bias” by comparing different
“low bias” triggers.
“Minumum Bias” Collisions
Proton
AntiProton
Charged Particle Density: dN/dh
 We are doing a fairly good job in modeling
“min-bias” and pile-up at CDF (i.e. Pythia
Tune A, Tune AW, Tune DW), but
extrapolations to the LHC are still uncertain!
Charged Particle Density
10
pyA
pyDW
pyDWT
ATLAS
Generator Level
14 TeV
8
6
4
2
Charged Particles (all pT)
0
 Studying pile-up at CDF is more difficult
than I thought due to primary vertex
splitting (and merging).
-10
-8
-6
-4
-2
0
2
4
6
8
10
PseudoRapidity h
I do not know to what extent pile-up is process
(i.e. trigger) dependent and to what extent it
helps satisfy the trigger requirements. This
would have big implications for the LHC!
Oregon Terascale Workshop
February 24, 2009
Rick Field – Florida/CDF/CMS
Page 29