D0 Meeting September 6, 2002 Rick Field - Florida/CDF Page 1 The “Underlying Event” in The “Underlying Event” in Hard Scattering Processes Hard Scattering Processes What happens when a proton and an antiproton collide with a center-of-mass energy of 2 TeV? Proton A ntiProton “Soft” C ollision (no hard scattering) Proton A ntiProton “H ard” Scattering PT(hard) O utgoing Parton O utgoing Parton U nderlying E vent U nderlying E vent Initial-State Radiation Final-State Radiation Proton A ntiProton 2 T eV Most of the time the proton and antiproton ooze through each other and fall apart (i.e. no hard scattering). The outgoing particles continue in roughly the same direction as initial proton and antiproton. Occasionally there will be a “hard” parton-parton collision resulting in large transverse momentum outgoing partons. Proton A ntiProton “U nderlying Event” Beam -Beam R em nants Beam -B eam R em nants Initial-State Radiation The “underlying event” is everything except the two outgoing hard scattered “jets”. It is an unavoidable background to many collider observables.
What happens when a proton and an antiproton collide with a center-of-mass energy of 2 TeV?
Proton AntiProton
“Soft” Collision (no hard scattering)
Proton AntiProton
“Hard” Scattering
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying Event Underlying Event
Initial-State Radiation
Final-State Radiation
Proton AntiProton 2 TeV
Most of the time the proton and antiproton ooze through each other and fall apart (i.e. no hard scattering). The outgoing particles continue in roughly the same direction as initial proton and antiproton.
Occasionally there will be a “hard” parton-parton collision resulting in large transverse momentum outgoing partons.
Proton AntiProton
“Underlying Event”
Beam-Beam Remnants Beam-Beam Remnants
Initial-State Radiation
The “underlying event” is everything except the two outgoing hard scattered “jets”. It is an unavoidable background to many collider observables.
D0 Meeting September 6, 2002
Rick Field - Florida/CDF Page 2
Min-Bias?
Beam-Beam RemnantsBeam-Beam Remnants
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).
However the “soft” component is color connected to the “hard” component so this separation is (at best) an approximation.
Proton AntiProton
“Hard” Collision
initial-state radiation
final-state radiation outgoing parton
outgoing parton
color string
color string
+
“Soft” Component “Hard” Component
initial-state radiation
final-state radiation outgoing jet
Beam-Beam Remnants
For ISAJET (no color flow) the “soft” and “hard” components are completely independent and the model for the beam-beam remnant component is the same as for min-bias (“cut pomeron”) but with a larger <PT>.
HERWIG breaks the color connection with a soft q-qbar pair and then models the beam-beam remnant component the same as HERWIG min-bias (cluster decay).
D0 Meeting September 6, 2002
Rick Field - Florida/CDF Page 3
Studying the “Underlying Event”Studying the “Underlying Event”
at CDFat CDF
The underlying event in a hard scattering process is a complicated and not very well understood object. It is an interesting region since it probes the interface between perturbative and non-perturbative physics.
There are two CDF analyses which quantitatively study the underlying event and compare with the QCD Monte-Carlo models.
It is important to model this region well since it is an unavoidable background to all collider observables. Also, we need a good model of min-bias (zero-bias) collisions.
The Underlying Event:beam-beam remnantsinitial-state radiation
multiple-parton interactions
Proton AntiProton
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying Event Underlying Event
Initial-State Radiation
Final-State Radiation
CDFCone AnalysisValeria TanoEve KovacsJoey Huston
Anwar Bhatti
CDFEvolution of Charged Jets
Rick FieldDavid Stuart
Rich Haas
Ph.D. Thesis PRD65:092002, 2002
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Rick Field - Florida/CDF Page 4
Evolution of Charged JetsEvolution of Charged Jets“Underlying Event”“Underlying Event”
Charged Jet #1Direction
“Transverse” “Transverse”
“Toward”
“Away”
“Toward-Side” Jet
“Away-Side” Jet
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 - space, x = 2x120o = 4/3.
Perpendicular to the plane of the 2-to-2 hard scattering
D0 Meeting September 6, 2002
Rick Field - Florida/CDF Page 5
Charged Multiplicity Charged Multiplicity versus Pversus PTT(chgjet#1)(chgjet#1)
Data on the average number of “toward” (||<60o), “transverse” (60<||<120o), and “away” (||>120o) charged particles (PT > 0.5 GeV, || < 1, including jet#1) as a function of the transverse momentum of the leading charged particle jet. Each point corresponds to the <Nchg> 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.
Charged Jet #1Direction
“Toward”
“Transverse” “Transverse”
“Away”
Underlying Event“plateau”
Nchg versus PT(charged jet#1)
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<N
ch
g>
in
1 G
eV
/c b
in
1.8 TeV ||<1.0 PT>0.5 GeV
"Toward"
"Away"
"Transverse"
CDF Preliminarydata uncorrected
Factor of 2 more active than an average Min-Bias event!
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Rick Field - Florida/CDF Page 6
““Transverse” PTransverse” PTT Distribution Distribution
Comparison of the “transverse” <Nchg> versus PT(charged jet#1) with the PT distribution of the “transverse” <Nchg>, dNchg/dPT. The integral of dNchg/dPT is the “transverse” <Nchg>. Shows how the “transverse” <Nchg> is distributed in PT.
““Max/Min Transverse” Nchg Max/Min Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)
Define “TransMAX” and “TransMIN” to be the maximum and minimum of the region 60o<<120o (60o<-<120o) on an event by event basis. The overall “transverse” region is the sum of “TransMAX” and “TransMIN”.
The plot shows the average “TransMAX” Nchg and “TransMIN” Nchg 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.
Charged Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
“Away”
"Max/Min Transverse" Nchg
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<N
ch
g>
in
1 G
eV
/c b
in
CDF Preliminarydata uncorrected
1.8 TeV ||<1.0 PT>0.5 GeV
"Max Transverse"
"Min Transverse"
“TransMAX”
“TransMIN”
Area 2x60o = 2/3
More sensitive to the “beam-beam remnants”
More sensitive to the “hard scattering” componentBryan Webber idea!
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Rick Field - Florida/CDF Page 8
ISAJET: ISAJET: “Transverse” Nchg “Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)
Plot shows the “transverse” <Nchg> vs PT(chgjet#1) compared to the QCD hard scattering predictions of ISAJET 7.32 (default parameters with PT(hard)>3 GeV/c) .
The predictions of ISAJET 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).
Beam-BeamRemnants
ISAJETCharged Jet #1
Direction
“Toward”
“Transverse” “Transverse”
“Away”
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
ns
ve
rse
" <
Nc
hg
> i
n 1
Ge
V/c
bin
1.8 TeV ||<1.0 PT>0.5 GeV
CDF Preliminarydata uncorrectedtheory corrected
Beam-Beam Remnants
Isajet Total
Hard Component
Outgoing Jetsplus
Initial & Final-StateRadiation
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Rick Field - Florida/CDF Page 9
HERWIG: “Transverse” Nchg HERWIG: “Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)
Plot shows the “transverse” <Nchg> vs PT(chgjet#1) compared to the QCD hard scattering predictions of HERWIG 5.9 (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).
Beam-BeamRemnants
Outgoing Jetsplus
Initial & Final-StateRadiation
HERWIG
Charged Jet #1Direction
“Toward”
“Transverse” “Transverse”
“Away”
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT (charged jet#1) (GeV/c)
"Tra
ns
ve
rse
" <
Nc
hg
> i
n 1
Ge
V/c
bin
1.8 TeV ||<1.0 PT>0.5 GeV Beam-Beam Remnants
Hard Component
CDF Preliminarydata uncorrectedtheory corrected
Herwig Total
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Rick Field - Florida/CDF Page 10
HERWIG: “Transverse”HERWIG: “Transverse”PPTT Distribution Distribution
Data on the “transverse” <Nchg> versus PT(charged jet#1) and the PT distribution of the “transverse” <Nchg>, dNchg/dPT, compared with the QCD Monte-Carlo predictions of HERWIG 5.9 (default parameters with with PT(hard) > 3 GeV/c). The integral of dNchg/dPT is the “transverse” <Nchg>.
PYTHIA models the “soft” component of the underlying event with color string fragmentation, but in addition includes a contribution arising from multiple parton interactions (MPI) in which one interaction is hard and the other is “semi-hard”.
Proton AntiProton
Multiple Parton Interaction
initial-state radiation
final-state radiation outgoing parton
outgoing parton
color string
color string
The probability that a hard scattering events also contains a semi-hard multiple parton interaction can be varied but adjusting the cut-off for the MPI.
One can also adjust whether the probability of a MPI depends on the PT of the hard scattering, PT(hard) (constant cross section or varying with impact parameter).
One can adjust the color connections and flavor of the MPI (singlet or nearest neighbor, q-qbar or glue-glue).
Also, one can adjust how the probability of a MPI depends on PT(hard) (single or double Gaussian matter distribution).
Pythia uses multiple partoninteractions to enhacethe underlying event.
Proton AntiProton
Multiple Parton Interactions
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying EventUnderlying Event
Parameter Value
Description
MSTP(81) 0 Multiple-Parton Scattering off
1 Multiple-Parton Scattering on
MSTP(82) 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 turn-off 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)
Hard Core
Multiple parton interaction more likely in a hard
(central) collision!
and now HERWIG
!
Herwig MPIJ. M. Butterworth
J. R. ForshawM. H. Seymour
D0 Meeting September 6, 2002
Rick Field - Florida/CDF Page 13
PYTHIAPYTHIAMultiple Parton InteractionsMultiple Parton Interactions
Plot shows “transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA with PT(hard) > 0 GeV/c.
Plot shows “Transverse” <Nchg> 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.
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Rick Field - Florida/CDF Page 16
Azimuthal CorrelationsAzimuthal Correlations
Predictions of PYTHIA 6.158 (CTEQ4L, PARP(67)=1) for the azimuthal angle, , between a b-quark with PT1 > 5 GeV/c and |y1| < 1 and a bbar-quark with PT2 > 0 GeV/c and |y2|<1 in proton-antiproton collisions at 1.8 TeV. The curves correspond to d/d (b/o) for flavor creation, flavor excitation, shower/fragmentation, and the resulting total.
b-quark direction
“Toward”
“Away”
bbar-quark
b-quark Correlations: Azimuthal Distribution
0.001
0.010
0.100
0 30 60 90 120 150 180
(degrees)
d /
d
(b
/deg
)
Pythia Total Flavor Creation Flavor Excitation Shower/Fragmentation
1.8 TeVPT1 > 5 GeV/cPT2 > 0 GeV/c
|y1| < 1 |y2| < 1
"Away""Toward"
Pythia CTEQ4L
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Rick Field - Florida/CDF Page 17
Azimuthal CorrelationsAzimuthal Correlations
Predictions of PYTHIA 6.206 (CTEQ5L) with PARP(67)=1 (new default) and PARP(67)=4 (old default) for the azimuthal angle, , between a b-quark with PT1 > 15 GeV/c, |y1| < 1 and bbar-quark with PT2 > 10 GeV/c, |y2|<1 in proton-antiproton collisions at 1.8 TeV. The curves correspond to d/d (b/o) for flavor creation, flavor excitation, shower/fragmentation, and the resulting total.
b-quark Correlations: Azimuthal Distribution
0.00001
0.00010
0.00100
0.01000
0 30 60 90 120 150 180
(degrees)
d /
d
(b
/deg
)
PY62 (67=1) Total Flavor Creation Flavor Excitation Shower/Fragmentation
Predictions of HERWIG 6.4 (CTEQ5L) for the azimuthal angle, , between a b-quark with PT1 > 15 GeV/c, |y1| < 1 and bbar-quark with PT2 > 10 GeV/c, |y2|<1 in proton-antiproton collisions at 1.8 TeV. The curves correspond to d/d (b/o) for flavor creation, flavor excitation, shower/fragmentation, and the resulting total.
b-quark Correlations: Azimuthal Distribution
0.00001
0.00010
0.00100
0.01000
0 30 60 90 120 150 180
(degrees)
d /
d
(b
/de
g)
HW64 Total Flavor Creation Flavor Excitation Shower/Fragmentation
Predictions of PYTHIA 6.158 (CTEQ5L) with PARP(67)=1 (new default) and PARP(67)=4 (old default) for diphoton system PT and the azimuthal angle, , between a photon with PT1 > 12 GeV/c, |y1| < 0.9 and photon with PT2 > 12 GeV/c, |y2|< 0.9 in proton-antiproton collisions at 1.8 TeV compared with CDF data.
DiPhoton Correlations: Azimuthal Distribution
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
90 105 120 135 150 165 180
(degrees)
1/
d /
d
(1/d
eg
)
PYC5 DiPhoton PARP(67)=4
PYC5 DiPhoton PARP(67)=1
CDF DiPhoton Data
1.8 TeV PT > 12 GeV || < 0.9
New Pythia
Old Pythia
Diphoton System Transverse Momentum
0.00
0.05
0.10
0.15
0.20
0.25
0 2 4 6 8 10 12 14 16 18
Diphoton System PT (GeV/c)
1/
d /
dP
T (
1/G
eV
/c)
PYC5 DiPhoton PARP(67)=1
PYC5 DiPhoton PARP(67)=4
CDF DiPhoton Data
1.8 TeV PT > 12 GeV || < 0.9New Pythia
Old Pythia
Photon direction
“Toward”
“Away”
Photon
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Rick Field - Florida/CDF Page 20
Parameter Tune 1 Tune 2
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) 0.16 0.16
PARP(67) 1.0 4.0
Old PYTHIA default(less initial-state radiation)
New PYTHIA default(less initial-state radiation)
Tuned PYTHIA 6.206Tuned PYTHIA 6.206
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
nsv
erse
" <
Nch
g>
in
1 G
eV/c
bin CDF
data uncorrectedtheory corrected
1.8 TeV ||<1.0 PT>0.5 GeV CTEQ5L
Tuned PYTHIA 6.206PARP(67)=1
Tuned PYTHIA 6.206PARP(67)=4
Plot shows “Transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard scattering predictions of two tuned versions of PYTHIA 6.206 (CTEQ5L, PARP(67)=1 and PARP(67)=4).
PYTHIA 6.206 CTEQ5L
Old PYTHIA default(less initial-state radiation)
New PYTHIA default(less initial-state radiation)
Bulk of Min-Bias events!
Can describe transition between “soft” and “hard” regime!
D0 Meeting September 6, 2002
Rick Field - Florida/CDF Page 21
Tuned PYTHIA 6.206Tuned PYTHIA 6.206“Transverse” P“Transverse” PTT Distribution Distribution
Data on the “transverse” <Nchg> versus PT(charged jet#1) and the PT distribution of the “transverse” <Nchg>, dNchg/dPT, compared with the QCD Monte-Carlo predictions of two tuned versions of PYTHIA 6.206 (PT(hard) > 0, CTEQ5L, PARP(67)=1 and PARP(67)=4).
PARP(67)=4.0 (old default) is favored over PARP(67)=1.0 (new default)!
PT(charged jet#1) > 30 GeV/c
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
nsv
erse
" <
Nch
g>
in
1 G
eV/c
bin CDF
data uncorrectedtheory corrected
1.8 TeV ||<1.0 PT>0.5 GeV CTEQ5L
Tuned PYTHIA 6.206PARP(67)=1
Tuned PYTHIA 6.206PARP(67)=4
"Transverse" PT Distribution (charged)
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
0 2 4 6 8 10 12 14
PT(charged) (GeV/c)
dN
chg
/dP
T (
1/G
eV/c
)
CDFdata uncorrectedtheory corrected
1.8 TeV ||<1
PYTHIA 6.206 CTEQ5L (3)
PARP(67)=1
PARP(67)=4
PT(chgjet#1) > 30 GeV/c
PT(chgjet#1) > 5 GeV/c
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Rick Field - Florida/CDF Page 22
Tuned PYTHIA 6.206 vs HERWIG 6.4Tuned PYTHIA 6.206 vs HERWIG 6.4 “TransMAX/MIN” vs P“TransMAX/MIN” vs PTT(chgjet#1)(chgjet#1)
Plots shows data on the “transMAX/MIN” <Nchg> and “transMAX/MIN” <PTsum> vs PT(chgjet#1). The solid (open) points are the Min-Bias (JET20) data.
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, CTEQ5L, PARP(67)=1 and PARP(67)=4).
<Nchg>
Charged Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
“Away” <PTsum>
"Max/Min Transverse" Nchg
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
nsv
erse
" <
Nch
g>
in
1 G
eV/c
bin
"Max Transverse"
"Min Transverse"
CDF Preliminarydata uncorrectedtheory corrected
1.8 TeV ||<1.0 PT>0.5 GeV
CTEQ5L
Tuned PYTHIA 6.206PARP(67)=1
Tuned PYTHIA 6.206PARP(67)=4
HERWIG 6.4
"Max/Min Transverse" PTsum
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<P
Tsu
m>
(G
eV/c
) in
1 G
eV/c
bin
"Max Transverse"
"Min Transverse"
CDF Preliminarydata uncorrectedtheory corrected
1.8 TeV ||<1.0 PT>0.5 GeV
CTEQ5L
Tuned PYTHIA 6.206PARP(67)=1
Tuned PYTHIA 6.206PARP(67)=4
HERWIG 6.4
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Rick Field - Florida/CDF Page 23
Tuned PYTHIA 6.206 vs HERWIG 6.4Tuned PYTHIA 6.206 vs HERWIG 6.4 “TransSUM/DIF” vs P“TransSUM/DIF” vs PTT(chgjet#1)(chgjet#1)
Plots shows data on the “transSUM/DIF” <Nchg> and “transSUM/DIF” <PTsum> vs PT(chgjet#1). The solid (open) points are the Min-Bias (JET20) data.
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, CTEQ5L, PARP(67)=1 and PARP(67)=4).
<Nchg>
Charged Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
“Away” <PTsum>
SUM/DIF "Transverse" Nchg
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<N
chg
> i
n 1
GeV
/c b
in
"Max+Min Transverse"
"Max-Min Transverse"
CDF Preliminarydata uncorrectedtheory corrected
1.8 TeV ||<1.0 PT>0.5 GeV
CTEQ5L
Tuned PYTHIA 6.206PARP(67)=1
Tuned PYTHIA 6.206PARP(67)=4
HERWIG 6.4
SUM/DIF "Transverse" PTsum
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<P
Tsu
m>
(G
eV/c
) in
1 G
eV/c
bin
CDF Preliminarydata uncorrectedtheory corrected
CTEQ5L
"Max+Min Transverse"
"Max-Min Transverse"
1.8 TeV ||<1.0 PT>0.5 GeV HERWIG 6.4
Tuned PYTHIA 6.206PARP(67)=1
Tuned PYTHIA 6.206PARP(67)=4
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Rick Field - Florida/CDF Page 24
Tuned PYTHIA 6.206 vs HERWIG 6.4Tuned PYTHIA 6.206 vs HERWIG 6.4 “Transverse” P“Transverse” PTT Distribution Distribution
Data on the PT distribution of the “transverse” <Nchg>, dNchg/dPT, 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, CTEQ5L, PARP(67)=1 and PARP(67)=4).
"Transverse" PT Distribution (charged)
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
0 1 2 3 4 5 6 7
PT(charged) GeV/c
dN
chg
/dP
T (
1/G
eV/c
)
PT(charged jet#1) > 5 GeV/c
CDFdata uncorrectedtheory corrected
1.8 TeV ||<1
CTEQ5L
Tuned PYTHIA 6.206PARP(67)=4
Tuned PYTHIA 6.206PARP(67)=1
HERWIG 6.4
"Transverse" PT Distribution (charged)
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
0 2 4 6 8 10 12 14
PT(charged) (GeV/c)
dN
chg
/dP
T (
1/G
eV/c
)
CDFdata uncorrectedtheory corrected
1.8 TeV ||<1
CTEQ5L
PT(chgjet1) > 30 GeV/c
Tuned PYTHIA 6.206PARP(67)=1
HERWIG 6.4
Tuned PYTHIA 6.206PARP(67)=4
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Rick Field - Florida/CDF Page 25
The Underlying Event:The Underlying Event:Summary & ConclusionsSummary & Conclusions
The “Underlying Event”
Combining the two CDF analyses gives a quantitative study of the underlying event from very soft collisions to very hard collisions.
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 beam-beam remnants with PT > 0.5 GeV/c.
PYTHIA (with multiple parton interactions) does the best job in describing the underlying event.
Perhaps the multiple parton interaction approach is correct or maybe we simply need to improve the way the Monte-Carlo models handle the beam-beam remnants (or both!).
Proton AntiProton
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying Event Underlying Event
Initial-State Radiation
Final-State Radiation
D0 Meeting September 6, 2002
Rick Field - Florida/CDF Page 26
Multiple Parton Interactions:Multiple Parton Interactions:Summary & ConclusionsSummary & Conclusions
Multiple Parton Interactions
The increased activity in the underlying event in a hard scattering over a soft collision cannot be explained by initial-state radiation.
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) describes the underlying event data fairly well and will also fit the min-bias data (must use MSTP(82)=4 “double Gaussian” and tune the parameters). More work is needed on the energy dependence.
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.
