“Min-Bias” and the
“Underlying Event”
Predictions at 7 TeV (and 900 GeV)
Rick Field
University of Florida
Outline of Talk
 The PYTHIA MPI energy scaling parameter
PARP(90).
 The “underlying event” at STAR.
CERN August 28, 2009
Extrapolations to RHIC.
Outgoing Parton
PT(hard)
 LHC predictions at 14 TeV for the
“underlying event” (hard scattering
QCD & Drell-Yan).
Initial-State Radiation
CDF Run 2
 LHC predictions at 7 TeV (and 900
Proton
Proton
Underlying Event
Outgoing Parton
Underlying Event
Final-State
Radiation
GeV).
 Summary & Conclusions.
CMS at the LHC
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
UE&MB@CMS
Page 1
“Transverse” Charged Density
PTmax Direction

"Transverse" Charged Particle Density: dN/dhd
0.8
“Transverse”
“Transverse”
“Away”
ChgJet#1 Direction

“Toward”
“Transverse”
“Transverse”
“Away”
"Transverse" Charged Density
“Toward”
RDF Preliminary
1.96 TeV
py Tune A generator level
0.6
0.6
0.4
Jet#1
ChgJet#1
0.2
PTmax
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
Jet#1 Direction

0
5
10
15
20
25
30
PT(jet#1) or PT(chgjet#1) or PTmax (GeV/c)
“Toward”
“Transverse”
“Transverse”
“Away”
 Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5
GeV/c, |h| < 1) at 1.96 TeV as defined by PTmax, PT(chgjet#1), and PT(jet#1) from PYTHIA
Tune A at the particle level (i.e. generator level).
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 2
Tuning PYTHIA:
Multiple Parton Interaction Parameters
Parameter
Default
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.
Determines the energy
Probability
that of
thethe
MPI
produces two gluons
dependence
MPI!
with color connections to the “nearest neighbors.
0.33
PARP(86)
0.66
PARP(89)
PARP(82)
PARP(90)
PARP(67)
1 TeV
1.9
GeV/c
0.16
1.0
CMS-LowPT Meeting
August 28, 2009
Multiple Parton Interaction
Color String
Color String
Multiple PartonDetermine
Interactionby comparing
Probability thatAffects
the MPI
theproduces
amount two
of gluons
either as described
by PARP(85)
or as a closed
initial-state
radiation!
gluon loop. The remaining fraction consists of
quark-antiquark pairs.
with 630 GeV data!
Color String
Hard-Scattering Cut-Off PT0
Determines the reference energy E0.
The cut-off PT0 that regulates the 2-to-2
scattering divergence 1/PT4→1/(PT2+PT02)2
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.
Rick Field – Florida/CDF/CMS
5
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
PARP(85)
Description
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 3
All use LO as
with L = 192 MeV!
UE Parameters
ISR Parameter
PYTHIA 6.2 Tunes
Parameter
Tune AW
Tune DW
Tune D6
PDF
CTEQ5L
CTEQ5L
CTEQ6L
MSTP(81)
1
1
1
MSTP(82)
4
4
4
PARP(82)
2.0 GeV
1.9 GeV
1.8 GeV
PARP(83)
0.5
0.5
0.5
PARP(84)
0.4
0.4
0.4
PARP(85)
0.9
1.0
1.0
PARP(86)
0.95
1.0
1.0
PARP(89)
1.8 TeV
1.8 TeV
1.8 TeV
PARP(90)
0.25
0.25
0.25
PARP(62)
1.25
1.25
1.25
PARP(64)
0.2
0.2
0.2
PARP(67)
4.0
2.5
2.5
MSTP(91)
1
1
1
PARP(91)
2.1
2.1
2.1
PARP(93)
15.0
15.0
15.0
Uses CTEQ6L
Tune A energy dependence!
(not the default)
Intrinsic KT
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 4
All use LO as
with L = 192 MeV!
UE Parameters
Tune A
ISR Parameter
PYTHIA 6.2 Tunes
Parameter
Tune DWT
Tune D6T
ATLAS
PDF
CTEQ5L
CTEQ6L
CTEQ5L
MSTP(81)
1
1
1
MSTP(82)
4
4
4
PARP(82)
1.9409 GeV
1.8387 GeV
1.8 GeV
PARP(83)
0.5
0.5
0.5
0.4
0.4
0.5
PARP(67)
2.5
2.5
1.0
MSTP(91)
1
1
1
PARP(91)
Tune D
PARP(93)
Tune 2.1
DW
15.0
2.1
15.0
1.0
Tune D6
5.0
Tune D6T
Intrinsic KT
CMS-LowPT Meeting
August 28, 2009
ATLAS energy dependence!
(PYTHIA default)
Tune B
Tune AW
PARP(85)
1.0
0.33 tunes!
These are 1.0
“old” PYTHIA
6.2
PARP(86)
1.0
0.66
There 1.0
are new 6.420
tunes
by
Tune BW
PARP(89)
1.96 TeV
1.96 TeV
1.0 TeV
Peter Skands (Tune S320, update of S0)
PARP(90)
0.16
0.16
0.16
Peter
Skands
(Tune
N324,
N0CR)
PARP(62)
1.25
1.25
1.0
Hendrik
Hoeth
(Tune0.2P329, “Professor”)
PARP(64)
0.2
1.0
PARP(84)
Rick Field – Florida/CDF/CMS
Page 5
Min-Bias “Associated”
Charged Particle Density
35% more at RHIC means
"Transverse" Charged Particle Density: dN/dhd
26% less at the LHC!
1.6
RDF Preliminary
"Transverse" Charged Density
0.3
"Transverse" Charged Density
"Transverse" Charged Particle Density: dN/dhd
PY Tune DW
generator level
0.2
~1.35
PY Tune DWT
0.1
Min-Bias
0.2 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
RDF Preliminary
PY Tune DWT
generator level
1.2
~1.35
0.8
PY Tune DW
0.4
Min-Bias
14 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0.0
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
PTmax Direction

“Toward”
“Transverse”
12
14
16
18
20
PTmax (GeV/c)
PTmax (GeV/c)
RHIC
10
PTmax Direction
0.2 TeV → 14 TeV
(~factor of 70 increase)
“Transverse”
“Away”

“Toward”
LHC
“Transverse”
“Transverse”
“Away”
 Shows the “associated” charged particle density in the “transverse” regions as a function of
PTmax for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events
at 0.2 TeV and 14 TeV from PYTHIA Tune DW and Tune DWT at the particle level (i.e. generator
level). The STAR data from RHIC favors Tune DW!
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 6
The “Underlying Event” at STAR
 At STAR they have measured the “underlying event at W = 200 GeV (|h| < 1, pT > 0.2 GeV)
and compared their uncorrected data with PYTHIA Tune A + STAR-SIM.
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 7
Peter’s Pythia Tunes WEBsite
 http://home.fnal.gov/~skands/leshouches-plots/
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 8
Min-Bias “Associated”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Density
1.2
RDF Preliminary
14 TeV
Min-Bias
py Tune DW generator level
0.8
~1.9
0.4
1.96 TeV
~2.7
0.2 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0
5
10
15
20
25
PTmax (GeV/c)
PTmax Direction

“Toward”
RHIC
“Transverse”
“Transverse”
“Away”
0.2 TeV → 1.96 TeV
(UE increase ~2.7 times)
Tevatron
PTmax Direction

“Toward”
“Transverse”
PTmax Direction
1.96 TeV → 14 TeV
(UE increase ~1.9 times)
LHC
“Transverse”
“Away”

“Toward”
“Transverse”
“Transverse”
“Away”
 Shows the “associated” charged particle density in the “transverse” region as a function of PTmax
for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events at 0.2
TeV, 1.96 TeV and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e. generator
level).
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 9
LHC Predictions
“Minumum Bias” Collisions
Proton
AntiProton
Charged Particle Density: dN/dh
8.0
Charged Particle Density
I believe because of the STAR analysis we are now
in a position to make some predictions at the LHC!
 The amount of activity in “min-bias” collisions.
Outgoing Parton
Underlying Event
Final-State
Radiation
PY Tune DW
PY Tune A
PY64 Tune S320
2.0
-8
“Away”
scattering events.
-6
-4
-2
0
2
4
6
8
PseudoRapidity h
"Transverse" Charged Particle Density: dN/dhd
“Toward”
“Transverse”
Min-Bias
14 TeV
Charged Particles (all PT)
1.6
 The amount of activity in the “underlying event” in hard
Drell-Yan Production
4.0
If the LHC data are not in
the range shown here then
we learn new (QCD) physics!
“Transverse”
Outgoing Parton
PY ATLAS
0.0
"Transverse" Charged Density
AntiProton
PY Tune DWT
6.0

Initial-State Radiation
Underlying Event
generator level
PTmax Direction
PT(hard)
Proton
PY64 Tune P329
RDF Preliminary
PY ATLAS
RDF Preliminary
PY Tune DWT
generator level
1.2
0.8
PY64 Tune P329
PY Tune A
0.4
PY Tune DW
PY64 Tune S320
Min-Bias
14 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0
5
10
Z-BosonDirection
15
20
25
PTmax (GeV/c)

Lepton
"Toward" Charged Particle Density: dN/dhd
“Toward”
Underlying Event
Underlying Event
PY ATLAS
RDF Preliminary
AntiProton
“Transverse”
“Transverse”
“Away”
Anti-Lepton
 The amount of activity in the “underlying event” in DrellYan events.
"Toward" Charged Density
Proton
1.6
PY Tune DWT
generator level
1.2
0.8
PY Tune DW
PY64 Tune P329
PY64 Tune S320
0.4
70 < M(pair) < 110 GeV
Drell-Yan
14 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0
25
50
75
100
125
150
Lepton-Pair PT (GeV/c)
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 10
Min-Bias “Associated”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Density
1.2
RDF Preliminary
14 TeV
Min-Bias
py Tune DW generator level
0.8
~1.9
0.4
~1.2
7 TeV
~1.5
1.96 TeV
~2.7
0.2 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0
5
10
15
20
PTmax (GeV/c)
PTmax Direction

“Toward”
RHIC
“Transverse”
“Transverse”
“Away”
0.2 TeV → 1.96 TeV
(UE increase ~2.7 times)
Tevatron
PTmax Direction

“Toward”
“Transverse”
25
For this observable 7
TeV is not 40% of the
way from the Tevatron
to 14 TeV it is ~80% of
the way!
PTmax Direction
1.96 TeV → 7 TeV
(UE increase ~1.5 times)
LHC
“Transverse”
“Away”

“Toward”
“Transverse”
“Transverse”
“Away”
 Shows the “associated” charged particle density in the “transverse” region as a function of PTmax
for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events at 0.2
TeV, 1.96 TeV, 7 TeV, and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e.
generator level).
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 11
Min-Bias “Associated”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Particle Density: dN/dhd
1.2
RDF Preliminary
"Transverse" Charged Density
"Transverse" Charged Density
1.2
14 TeV
Min-Bias
py Tune DW generator level
7 TeV
0.8
1.96 TeV
0.9 TeV
0.4
0.2 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
RDF Preliminary
LHC14
py Tune DW generator level
LHC7
0.8
Tevatron
900 GeV
0.4
PTmax = 5.25 GeV/c
RHIC
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0.0
0
5
10
For this observable
LHC at 7 TeV teaches us a lot!
15
20
0
25
2

“Toward”
RHIC
“Transverse”
“Transverse”
“Away”
6
8
10
12
14
Center-of-Mass Energy (TeV)
PTmax (GeV/c)
PTmax Direction
4
0.2 TeV → 1.96 TeV
(UE increase ~2.7 times)
Tevatron
PTmax Direction

“Toward”
“Transverse”
PTmax Direction
1.96 TeV → 14 TeV
(UE increase ~1.9 times)
LHC
“Transverse”
“Away”

“Toward”
“Transverse”
“Transverse”
“Away”
 Shows the “associated” charged particle density in the “transverse” region as a function of PTmax
for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events at 0.2
TeV, 0.9 TeV, 1.96 TeV and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e.
Linear scale!
generator level).
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 12
Min-Bias “Associated”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Particle Density: dN/dhd
1.2
RDF Preliminary
"Transverse" Charged Density
"Transverse" Charged Density
1.2
14 TeV
Min-Bias
py Tune DW generator level
7 TeV
0.8
1.96 TeV
0.9 TeV
0.4
0.2 TeV
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
RDF Preliminary
py Tune DW generator level
LHC14
LHC7
0.8
Tevatron
900 GeV
0.4
PTmax = 5.25 GeV/c
RHIC
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0.0
0
5
10
15
20
0
25
1
PTmax (GeV/c)
PTmax Direction

“Toward”
RHIC
“Transverse”
“Transverse”
“Away”
10
100
Center-of-Mass Energy (TeV)
0.2 TeV → 1.96 TeV
(UE increase ~2.7 times)
Tevatron
PTmax Direction

“Toward”
“Transverse”
PTmax Direction
1.96 TeV → 14 TeV
(UE increase ~1.9 times)
LHC
“Transverse”
“Away”

“Toward”
“Transverse”
“Transverse”
“Away”
 Shows the “associated” charged particle density in the “transverse” region as a function of PTmax
for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events at 0.2
TeV, 0.9 TeV, 1.96 TeV and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e.
Log scale!
generator level).
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 13
Min-Bias “Charged Particle Density
Charged Particle Density: dN/dh
Charged Particle Density (h = 0): dN/dh
3.0
3.0
14 TeV
7 TeV
2.0
1.96 TeV
0.9 TeV
1.0
LHC14
RDF Preliminary
Min-Bias
Charged Density (h = 0)
Charged Particle Density
RDF Preliminary
py Tune DW generator level
0.2 TeV
py Tune DW generator level
LHC7
2.0
Tevatron
900 GeV
RHIC
1.0
Min-Bias
Charged particles pT > 0.5 GeV/c
Charged Particles (PT>0.5 GeV/c)
0.0
0.0
-8
-6
-4
-2
0
For this observable
LHC at 7 TeV teaches us a lot!
2
4
6
0
8
2
PseudoRapidity h
8
Proton
10
12
14
1.96 TeV → 14 TeV
(dN/dh increase ~1.58 times)
“Minumum Bias” Collisions
“Minumum Bias” Collisions
“Minumum Bias” Collisions
RHIC
6
Center-of-Mass Energy (TeV)
0.2 TeV → 1.96 TeV
(dN/dh increase ~1.63 times)
Proton
4
Proton
AntiProton
Tevatron
Proton
Proton
LHC
 Shows the “min-bias” charged particle density, dN/dh, for charged particles (pT > 0.5 GeV/c) for at
0.2 TeV, 0.9 TeV, 1.96 TeV and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e.
generator level).
Linear scale!
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 14
Min-Bias “Charged Particle Density
Charged Particle Density: dN/dh
Charged Particle Density (h = 0): dN/dh
3.0
3.0
14 TeV
RDF Preliminary
Min-Bias
py Tune DW generator level
Charged Density (h = 0)
Charged Particle Density
RDF Preliminary
7 TeV
2.0
1.96 TeV
0.9 TeV
1.0
0.2 TeV
LHC14
py Tune DW generator level
LHC7
2.0
Tevatron
900 GeV
RHIC
1.0
Min-Bias
Charged particles pT > 0.5 GeV/c
Charged Particles (PT>0.5 GeV/c)
0.0
0.0
-8
-6
-4
-2
0
2
4
6
0
8
1
PseudoRapidity h
RHIC
1.96 TeV → 14 TeV
(dN/dh increase ~1.58 times)
“Minumum Bias” Collisions
“Minumum Bias” Collisions
“Minumum Bias” Collisions
Proton
100
Center-of-Mass Energy (TeV)
0.2 TeV → 1.96 TeV
(dN/dh increase ~1.63 times)
Proton
10
Proton
AntiProton
Tevatron
Proton
Proton
LHC
 Shows the “min-bias” charged particle density, dN/dh, for charged particles (pT > 0.5 GeV/c) for at
0.2 TeV, 0.9 TeV, 1.96 TeV and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e.
generator level).
Log scale!
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
Page 15
Summary & Conclusions
However, I believe that the
better fits to the LEP
fragmentation data at high z
will lead to small improvements
Outgoing Parton
of Tune A at the
Tevatron!
 We are making good progress in understanding and modeling the
“underlying event”. RHIC data at 200 GeV are very important!
PT(hard)
Initial-State Radiation
Proton
 The new Pythia pT ordered tunes (py64 S320 and py64 P329)
are very similar to Tune A, Tune AW, and Tune DW. At present
the new tunes do not fit the data better than Tune AW and Tune
DW. However, the new tune are theoretically preferred!
AntiProton
Underlying Event
Underlying Event
Outgoing Parton
Final-State
Radiation
Hard-Scattering Cut-Off PT0
PYTHIA 6.206
e = 0.25 (Set A))
4
PT0 (GeV/c)
 It is clear now that the default value PARP(90) = 0.16 is
not correct and the value should be closer to the Tune A
value of 0.25.
 The new and old PYTHIA tunes are beginning to
converge and I believe we are finally in a position to make
some legitimate predictions at the LHC!
5
3
2
e = 0.16 (default)
1
 All tunes with the default value PARP(90) = 0.16 are
wrong and are overestimating the activity of min-bias and
the underlying event at the LHC! This includes all my
“T” tunes and the ATLAS tunes!
 Need to measure “Min-Bias” and the “underlying
event” at the LHC (7 TeV) as soon as possible to see if
there is new QCD physics to be learned!
CMS-LowPT Meeting
August 28, 2009
Rick Field – Florida/CDF/CMS
100
1,000
10,000
100,000
CM Energy W (GeV)
UE&MB@CMS
Page 16