Predicting “Min-Bias” and the
“Underlying Event” at the LHC
Important 900 GeV Measurements
Rick Field
University of Florida
Outline of Talk
 The inelastic non-diffractive cross section.
 New CDF charged multiplicity
CERN November 10, 2009
distribution and comparisons with
the QCD Monte-Carlo tunes.
Outgoing Parton
PT(hard)
 Relationship between the “underlying
event” in a hard scattering process and
“min-bias” collisions.
Initial-State Radiation
CDF Run 2
 “Min-Bias at 900 GeV and 2.2 TeV.
Proton
Proton
Underlying Event
Outgoing Parton
Underlying Event
Final-State
Radiation
 The “underlying event” at 900 GeV
and 7 TeV.
 Summary.
CMS-QCD Group Meeting
November 10, 2009
CMS at the LHC
Rick Field – Florida/CDF/CMS
UE&MB@CMS
Page 1
Inelastic Non-Diffractive Cross-Section
Inelastic Non-Diffractive Cross-Section: sHC
Inelastic Non-Diffractive Cross-Section: sHC
70
70
RDF Preliminary
60
py Tune DW generator level
Cross-Section (mb)
Cross-Section (mb)
60
50
40
My guess!
30
20
10
RDF Preliminary
py Tune DW generator level
50
40
30
20
10
K-Factor = 1.2
K-Factor = 1.2
0
0
0
2
4
6
8
10
12
14
0.1
1.0
100.0
Center-of-Mass Energy (TeV)
Center-of-Mass Energy (TeV)
Linear scale!
10.0
Log scale!
stot = sEL + sSD +sDD +sHC
 The inelastic non-diffractive cross section versus center-of-mass energy from PYTHIA (×1.2).
sHC varies slowly.
Only a 13% increase between 7 TeV (≈ 58 mb) and 14 teV (≈ 66 mb). Linear
on a log scale!
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 2
Charged Particle Multiplicity
Charged Multiplicity Distribution
Charged Multiplicity Distribution
1.0E+00
1.0E+00
CDF Run 2 Preliminary
CDF Run 2 Preliminary
1.0E-01
1.0E-01
CDF Run 2 <Nchg>=4.5
CDF Run 2 <Nchg>=4.5
1.0E-02
1.0E-02
1.0E-03
1.0E-03
py64 Tune A <Nchg> = 4.1
Probability
Probability
pyAnoMPI <Nchg> = 2.6
1.0E-04
1.0E-05
1.0E-04
1.0E-05
1.0E-06
1.0E-06
HC 1.96 TeV
Min-Bias 1.96 TeV
1.0E-07
1.0E-07
Normalized to 1
Normalized to 1 Charged Particles (|h|<1.0, PT>0.4 GeV/c)
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
1.0E-08
1.0E-08
0
5
10
15
20
25
30
35
40
45
50
55
0
5
20
25
30
35
40
No MPI!
“Minumum Bias” Collisions
Proton
15
45
50
55
Number of Charged Particles
Number of Charged Particles
7 decades!
10
Tune A!
AntiProton
 Data at 1.96 TeV on the charged particle multiplicity (pT > 0.4 GeV/c, |h| < 1) for “min-bias”
collisions at CDF Run 2.
 The data are compared with PYTHIA Tune A and Tune A without multiple parton
interactions (pyAnoMPI).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 3
Charged Particle Multiplicity
Charged Multiplicity DistributionCharged
0.20
1.0E+00
0.20
CDF Run 2 Preliminary
CDF
1.0E-01
CDF Run 2 <Nchg>=4.5
0.15
py64 Tune A <Nchg> = 4.1
0.15
HC 1.96 TeV
0.05
Normalized to 1
0
2
4
CDF Run 2 Preliminary
CDF Run 2 <Nchg>=4.5
CDF Run 2 <Nchg>=4.5
pyS320 <Nchg> = 4.2
py64 Tune A <Nchg> = 4.1
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
0.10
HC 1.96 TeV
0.05
0.00
Run 2 Preliminary
1.0E-02
pyAnoMPI <Nchg> = 2.6
Probability
0.10
Probability
Probability
Charged Multiplicity Distribution
Multiplicity Distribution
1.0E-03
py64DW <Nchg> = 4.2
1.0E-04
pyS320 <Nchg> = 4.2
pyP329 <Nchg> = 4.4
1.0E-05
1.0E-06
Normalized
to 1 14
12
16
Charged
Particles (|h|<1.0, PT>0.4 GeV/c)
Number of Charged
Particles
1.0E-07
6
8
10
HC 1.96 TeV
Normalized to 1
0.00
0
2
4
6
8 1.0E-08 10
0
12
5
Tune A!Number of Charged Particles
No MPI!
10
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
14
15
20
16
25
30
35
40
45
50
55
Number of Charged Particles
“Minumum Bias” Collisions
Proton
Tune S320!
AntiProton
 Data at 1.96 TeV on the charged particle multiplicity (pT > 0.4 GeV/c, |h| < 1) for “min-bias”
collisions at CDF Run 2.
 The data are compared with PYTHIA Tune A and Tune A without multiple parton
interactions (pyAnoMPI).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 4
The “Underlying Event”
Select inelastic non-diffractive events
that contain a hard scattering
Proton
Hard parton-parton
collisions is hard
(pT > ≈2 GeV/c)
Proton
The “underlying-event” (UE)!
Proton
Given that you have one hard
scattering it is more probable to
have MPI! Hence, the UE has
more activity than “min-bias”.
CMS-QCD Group Meeting
November 10, 2009
“Semi-hard” partonparton collision
(pT < ≈2 GeV/c)
Proton
+
+
Proton
Proton
Rick Field – Florida/CDF/CMS
Proton
Proton
+…
Multiple-parton
interactions (MPI)!
Page 5
The Inelastic Non-Diffractive
Cross-Section
Occasionally one of
the parton-parton
collisions is hard
(pT > ≈2 GeV/c)
Proton
Proton
Majority of “minbias” events!
Proton
“Semi-hard” partonparton collision
(pT < ≈2 GeV/c)
Proton
+
Proton
+
Proton
Proton
Proton
+
Proton
Proton
+…
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Multiple-parton
interactions (MPI)!
Page 6
The “Underlying Event”
Select inelastic non-diffractive events
that contain a hard scattering
Proton
Hard parton-parton
collisions is hard
(pT > ≈2 GeV/c)
Proton
The “underlying-event” (UE)!
Proton
Given that you have one hard
scattering it is more probable to
have MPI! Hence, the UE has
more activity than “min-bias”.
CMS-QCD Group Meeting
November 10, 2009
“Semi-hard” partonparton collision
(pT < ≈2 GeV/c)
Proton
+
+
Proton
Proton
Rick Field – Florida/CDF/CMS
Proton
Proton
+…
Multiple-parton
interactions (MPI)!
Page 7
Charged Particle Multiplicity
Charged Multiplicity Distribution
Charged Multiplicity Distribution
1.0E+00
1.0E+00
CDF Run 2 Preliminary
CDF Run 2 Preliminary
1.0E-01
1.0E-01
CDF Run 2 <Nchg>=4.5
1.0E-02
py64 Tune A <Nchg> = 4.1
pyA 900 GeV <Nchg> = 3.3
CDF Run 2 <Nchg>=4.5
1.0E-02
py64 Tune A <Nchg> = 4.1
py64A LHC22 <Nchg> = 4.1
Probability
Probability
pyAnoMPI <Nchg> = 2.6
1.0E-03
1.0E-04
1.0E-05
1.0E-03
The charged multiplicity distribution
does not change between
1.96 and 2.2 TeV
and proton-proton is the same as
Tune A prediction at
proton-antiproton!
900 GeV!
1.0E-04
1.0E-05
1.0E-06
1.0E-06
HC 1.96 TeV
Min-Bias
1.0E-07
1.0E-07
Normalized to 1
Normalized to 1
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
Charged Particles (|h|<1.0, PT>0.4 GeV/c)
1.0E-08
1.0E-08
0
5
10
15
20
25
30
35
40
45
50
55
0
“Minumum Bias” Collisions
Proton
10
15
20
25
30
35
40
45
50
55
Number of Charged Particles
Number of Charged Particles
No MPI!
5
TuneCollisions
A prediction
“Minumum Bias”
Tune A!
at
2.2 TeV!
AntiProton
Proton
Proton
 Data at 1.96 TeV on the charged particle multiplicity (pT > 0.4 GeV/c, |h| < 1) for “min-bias”
collisions at CDF Run 2. The data are compared with PYTHIA Tune A and Tune A without
multiple parton interactions (pyAnoMPI).
 Prediction from PYTHIA Tune A for proton-proton collisions at 900 GeV and 2.2 TeV.
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 8
LHC Predictions: 900 GeV
Charged Multiplicity Distribution
Charged Multiplicity Distribution
Charged Multiplicity Distribution
Distribution
Momentum
Transverse Momentum
Charged Transverse
Charged
Distribution
0.08
0.16
1.0E+00
RDF Preliminary
30
1.0E+02
py64DW <Nchg> = 14.4 <pT> = 787 MeV/c
RDF
0.06
0.12
1.0E+01
MeV/c
787MeV/c
<pT>==795
14.4<pT>
<Nchg>==14.7
py64DW
pyP329 <Nchg>
20
1.0E+00
HC 900 GeV
1.0E-05
Normalized to 1
1.0E-06
0
10
20
0.02
0.04
HC HC
900900
GeVGeV
Charged Particles (|h|<2.0)
Normalized
to 1 to 1
Normalized
RDF Preliminary
00
HC 900
HC GeV
900 GeV
0
1.0E-03
0.0 0.0 0.2
40
50
0.4
1.0
60
Number of Charged Particles
70
0.6
25
10
4
15
6
20
8
25
10
30
12
35
14
40
16
Number of Charged Particles
Charged Particles (|h|<2.0) Number of Charged Particles
802.0
0.8
1.0
3.0
1.2 4.0 1.4
(GeV/c)
Momentum
Transverse
Transverse
Momentum
(GeV/c)
“Minumum Bias” Collisions
Charged Particles (|h|<2.0, all pT)
0.00
0.00
1.0E-02
Proton
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
pyP329 <Nchg> = 14.7 <pT> = 795 MeV/c
10
30
0.04
pyDW
900
GeV<Nchg>
<Nchg>== 14.2
5.3
pyA
900
GeV
pyDW
14.4
pyS320900
900GeV
GeV<Nchg>
<Nchg> == 5.2
pyS320
900 GeV
GeV<Nchg>
<Nchg>
= 14.2
pyP329 900
= 5.3
pyP329 900 GeV <Nchg> = 14.7
0.08
<Nchg> = 14.2 <pT> = 778 MeV/c
pyS320
All pT
<Nchg> ~ 14
pT > 0.5 geV/c
<Nchg> ~ 5 1.0E-01
1.0E-04
Preliminary
pyS320 <Nchg> = 14.2 <pT> = 778 MeV/c
Probability
Probability
Probability
1.0E-03
d<Nchg>/dpT
d<Nchg>/dpT1/(GeV/c)
1/(GeV/c)
1.0E-01
1.0E-02
RDF
Preliminary
pyA <Nchg>
= 5.0 STdev = 4.5
RDF Preliminary
Charged Particles (|h|<2.0)
1.6
5.0
“Minumum Bias” Collisions
Proton
Proton
Proton
 Charged multiplicity distributions for proton-proton collisions at 900 GeV
(|h| < 2) from PYTHIA Tune A, Tune DW, Tune S320, and Tune P329.
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 9
“Transverse” Charged Density
"Transverse" Charged Particle Multiplicity
"Transverse" Charged Particle Density: dN/dhd
0.18
RDF Preliminary
generator level
py64DW <Nchg> = 3.5 <NchgDen> = 0.42
pyS320 <Nchg> = 3.4 <NchgDen> = 0.41
pyDW
0.4
Probability
"Transverse" Charged Density
0.6
pyS320
0.2
0.12
RDF Preliminary
generator level
0.06
HC 900 GeV PTmax > 5 GeV/c
HC 900 GeV
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
0.0
0.00
0
2
4
6
8
10
12
14
16
18
20
0
1
PTmax (GeV/c)
2
3
4
5
6
PTmax Direction
PTmax Direction


“Toward”
“Transverse”
7
8
9
10
11
12
13
14
Number of Charged Particles
“Toward”
“Transverse”
“Transverse”
“Away”
“Transverse”
PTmax > 5 GeV/c!
“Away”
 Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5
GeV/c, |h| < 2) at 900 GeV as defined by PTmax from PYTHIA Tune DW and Tune S320 at the
particle level (i.e. generator level).
 Shows the charged particle multiplicity distribution in the “transverse” region (pT > 0.5 GeV/c,
|h| < 2) at 900 GeV as defined by PTmax from PYTHIA Tune DW and Tune S320 at the particle
level (i.e. generator level).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 10
Charged Particle Multiplicity
NchgDen = 0.21
The “underlying event” is twice as
active as an average HC collision!
Charged Multiplicity Distribution
"Transverse" Charged Particle Multiplicity
0.18
0.16
RDF Preliminary
py64DW <Nchg> = 3.5 <NchgDen> = 0.42
pyA <Nchg> = 5.0 STdev = 4.5
pyDW 900 GeV <Nchg> = 5.3
pyS320 <Nchg> = 3.4 <NchgDen> = 0.41
0.12
Probability
Probability
0.12
RDF Preliminary
generator level
0.06
pyS320 900 GeV <Nchg> = 5.2
pyP329 900 GeV <Nchg> = 5.3
0.08
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
0.04
HC 900 GeV PTmax > 5 GeV/c
HC 900 GeV
Normalized to 1
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
0.00
0.00
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0
2
4
6
8
10
12
14
Number of Charged Particles
Number of Charged Particles
PTmax Direction
“Minumum Bias” Collisions

“Toward”
“Transverse”
Proton
Proton
“Transverse”
“Away”
 Shows the charged particle multiplicity distribution in the “transverse” region (pT > 0.5 GeV/c,
|h| < 2) at 900 GeV as defined by PTmax from PYTHIA Tune DW and Tune S320 at the particle
level (i.e. generator level).
 Shows the charged particle multiplicity distribution in HC collisions (pT > 0.5 GeV/c, |h| < 2) at
900 GeV PYTHIA Tune A, Tune DW, Tune S320, and Tune P329 at the particle level (i.e.
generator level).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 11
16
sHC: PTmax > 5 GeV/c
Inelastic HC Cross-Section: PTmax > 5 GeV/c
Inelastic HC Cross-Section: PTmax > 5 GeV/c
Number
of Events Fraction
in 1/nb: PTmax
>>55GeV/c
HC
Cross-Section:
PTmax
GeV/c
1.6
1.5
1,500,000
2.0%
py Tune DW generator level
py Tune
generator
level level
py DW
Tune
DW generator
K-Factor = 1.2
1.0
0.5
0.0
0
2
Numberof
ofEvents
Events
Percent
Cross-Section (mb)
RDF Preliminary
RDF RDF
Preliminary
Preliminary
1.5%
1,000,000
K-Factor = 1.2
Still lots of events!
1.0%
Cross-Section (mb)
RDF Preliminary
py Tune DW generator level
1.2
K-Factor = 1.2
Charged Particles (PTmax > 5 GeV/c |h| < 1)
0.8
0.4
500,000
Charged Particles (PTmax > 5 GeV/c |h| < 1)
0.5%
4
0.0
6
8
10
12
0.1
1.0
10.0
100.0
Charged Particles (PTmax > 5 GeV/c
|h| < 1)
Center-of-Mass
Energy (TeV)
Center-of-Mass Energy (TeV)
0
0.0%
0
Linear scale!
14
0
2
2
4 4
6 6
88
1010
Log scale!
12
12
14
14
Center-of-Mass
Energy
(TeV)
Center-of-Mass
Energy
(TeV)
stot = sEL + sSD +sDD +sHC
In 1,000,000 HC collisions at
900 GeV you get 940 with
PTmax > 5 GeV/c!
 The inelastic non-diffractive PTmax > 5 GeV/c cross section (|h| < 1) versus center-of-mass
energy from PYTHIA (×1.2).
sHC(PTmax > 5 GeV/c) varies more rapidly. Factor of 2.3 increase between 7 TeV (≈ 0.56 mb)
and 14 teV (≈ 1.3 mb). Linear on a linear scale!
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 12
PTmax Cross-Section 900 GeV
Very important to do BOTH
“soft” and “hard” physics!
Number of Events with PTmax > PT0
Differential Cross-Section: ds/dPTmax
1.0E+02
1,000,000
RDF Preliminary
pyDW 1,885 Events with PTmax > 5 GeV/c
generator level
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
1.0E+00
1.0E-01
pyS320
1.0E-02
1.0E-03
HC 900 GeV
1.0E-04
RDF Preliminary
generator level
10,000
1,000
HC 900 GeV
100
pyDW
If we get 3,400,000 HC collisions
at 900 GeV we could do both
“min-bias” AND
the “underlying event”
In 1,000,000 HC collisions at
900 GeV you get ~3,000 with
them!
stot = sELand+ compare
sSD
PTmax+s
> 5 GeV/c!
DD +sHC
K-Factor = 1.2
1.0E-05
pyS320 3,160 Events with PTmax > 5 GeV/c
100,000
Number of Events
ds/dPT (mb/GeV/c)
1.0E+01
1,000,000 HC Collisions
Charged Particles (|h|<2.0)
10
0
2
4
6
8
10
12
14
16
18
20
0
1
PTmax (GeV/c)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
PT0 (GeV/c)
 The inelastic non-diffractive PTmax > 5 GeV/c cross section (|h| < 2) at 900 GeV from PYTHIA
Tune DW and Tune S320.
10,000 HC
eventsGeV
with from
 Number of events with PTmax > PT0 (|h| < 2) for 1,000,000Need
HCabout
collisions
at 900
PTmax > 5 GeV/c to do a nice analysis!
PYTHIA Tune DW and Tune S320.
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 13
Min-Bias “Associated”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Particle Density: dN/dhd
1.2
RDF Preliminary
14 TeV
Min-Bias
"Transverse" Charged Density
"Transverse" Charged Density
1.2
py Tune DW generator level
10 TeV
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
0.8
LHC10
LHC7
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
2

“Toward”
RHIC
“Transverse”
“Transverse”
0.2 TeV → 1.96 TeV
(UE increase ~2.7 times)
Tevatron
“Away”
6
8
10
12
14
Center-of-Mass Energy (TeV)
PTmax (GeV/c)
PTmax Direction
4
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, 7 TeV, 10 TeV, 14 TeV predicted by PYTHIA Tune DW at the particle level
Linear scale!
(i.e. generator level).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 14
Min-Bias “Associated”
Charged Particle Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Particle Density: dN/dhd
1.2
RDF Preliminary
14 TeV
Min-Bias
"Transverse" Charged Density
"Transverse" Charged Density
1.2
py Tune DW generator level
10 TeV
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
LHC10
LHC7
0.8
Tevatron
0.4
900 GeV
RHIC
PTmax = 5.25 GeV/c
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0.0
0
5
10
15
20
25
0.1

“Toward”
LHC7
“Transverse”
100.0
PTmax Direction
7 TeV → 14 TeV
(UE increase ~20%)

“Toward”
LHC14
“Transverse”
“Away”
10.0
Center-of-Mass Energy (TeV)
PTmax (GeV/c)
PTmax Direction
1.0
Linear on a log plot!
“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, 7 TeV, 10 TeV, 14 TeV predicted by PYTHIA Tune DW at the particle level
Log scale!
(i.e. generator level).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 15
“Transverse” Charge Density
"Transverse" Charged Particle Density: dN/dhd
"Transverse" Charged Particle Density: dN/dhd
1.2
"Transverse" Charged Density
"Transverse" Charged Density
0.6
RDF Preliminary
generator level
pyDW
0.4
pyS320
0.2
HC 900 GeV
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
RDF Preliminary
py Tune DW generator level
7 TeV
0.8
factor of 2!
900 GeV
0.4
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
0.0
0.0
0
2
4
6
8
10
12
14
16
18
20
0
2
4

LHC
900 GeV
“Toward”
“Transverse”
8
10
12
14
16
18
20
PTmax (GeV/c)
PTmax (GeV/c)
PTmax Direction
6
PTmax Direction
900 GeV → 7 TeV
(UE increase ~ factor of 2.1)
“Transverse”
“Away”

“Toward”
LHC
7 TeV
“Transverse”
“Transverse”
“Away”
 Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5
GeV/c, |h| < 2) at 900 GeV as defined by PTmax from PYTHIA Tune DW and Tune S320 at the
particle level (i.e. generator level).
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 16
Important 900 GeV Measurements
“Minumum Bias” Collisions
Proton
Charged Multiplicity Distribution
0.08
0.16
Proton
pyA <Nchg>
= 5.0 STdev = 4.5
RDF
Preliminary
RDF Preliminary
pyDW
900
GeV
<Nchg>==14.2
5.3
pyA
900
GeV
<Nchg>
pyS320
900GeV
GeV<Nchg>
<Nchg> = 5.2
pyDW
900
14.4
pyS320
GeV<Nchg>
<Nchg>= =5.3
14.2
pyP329 900
900 GeV
pyP329 900 GeV <Nchg> = 14.7
 The amount of activity in “min-bias” collisions (multiplicity distribution,
Probability
0.06
0.12
pT distribution, PTsum distribution, dNchg/dh).
0.04
0.08
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
0.02
0.04
Normalized to 1
Normalized to 1
PTmax Direction
0

25
4
10
“Transverse”
12
30
14
35
16
40
Final-State
Radiation
0.6
0.18
“Transverse”
“Away”
T
"Transverse"
Charged Density
Probability
Outgoing Parton
10
25
"Transverse"
Charged
Particle
Density:
dN/dhd
"Transverse"
Charged
Particle
Multiplicity
Proton
Underlying Event
8
20
“Toward”
Initial-State Radiation
Underlying Event
6
15
Number of Charged Particles
PT(hard)
Proton
Charged Particles (|h|<2.0, all pT)
0.00
It is very important to measure
BOTH “min-bias” and the and the
“underlying event” at 900 GeV!
To do this
we need to collect
 The amount of activity in the “underlying
event” in hard scattering
events (“transverse” Nchg distribution,
“transverse”
p distribution,
about
5,000,000
CMS
“transverse” PTsum distribution for events with PTmax > 5 GeV/c).
min-bias triggers!
Outgoing Parton
For every 1,000
events here
HC HC
900900
GeVGeV
RDF Preliminary
generator level
py64DW <Nchg> = 3.5 <NchgDen> = 0.42
pyS320 <Nchg> = 3.4 <NchgDen> = 0.41
pyDW
0.4
0.12
RDF Preliminary
generator level
We get 3 events
here!
pyS320
0.2
0.06
HC 900 GeV PTmax > 5 GeV/c
HC 900 GeV
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
Charged Particles (|h|<2.0, PT>0.5 GeV/c)
0.0
0.00
00
12
2 4 3
PTmax Direction
64
58
6
107
812
9 1410
16
11
1218 13
20
14
PTmax
(GeV/c)Particles
Number
of Charged

Outgoing Parton
PT(hard)
Initial-State Radiation
Proton
"Transverse" Charged Particle Density: dN/dhd
“Toward”
1.2
Underlying Event
Underlying Event
“Transverse”
Outgoing Parton
Final-State
Radiation
“Transverse”
“Away”
 We should map out the energy dependence of the “underlying
event” in a hard scattering process from 900 GeV to 14 TeV!
"Transverse" Charged Density
Proton
RDF Preliminary
py Tune DW generator level
0.8
0.4
PTmax = 5.25 GeV/c
Charged Particles (|h|<1.0, PT>0.5 GeV/c)
0.0
0.1
1.0
10.0
100.0
Center-of-Mass Energy (TeV)
CMS-QCD Group Meeting
November 10, 2009
Rick Field – Florida/CDF/CMS
Page 17