the underlying event in hard scattering processes

Run 2 Monte-Carlo Workshop April 20, 2001

Rick Field - Florida/CDF

The Underlying Event inThe Underlying Event inHard Scattering ProcessesHard Scattering Processes

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.

It is important to model this region well since it is an unavoidable background to all collider observables.

I will report on two CDF analyses which quantitatively study the underlying event and compare with the QCD Monte-Carlo models.

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

CDFQFL+ConesValeria TanoEve KovacsJoey Huston

Anwar Bhatti

CDFWYSIWYG+

Rick FieldDavid Stuart

Rich Haas

Ph.D. ThesisPh.D. Thesis



Require PT > 0.5 GeV, || < 1

Make an 8% correction for the track finding efficiency

Errors (statistical plus systematic) of around 5%

Zero or one vertex

|zc-zv| < 2 cm, |CTC d0| < 1 cm


Assume a uniform track finding efficiency of 92%

Errors include both statistical and correlated systematic uncertainties

WYSIWYG: Comparing DataWYSIWYG: Comparing Datawith QCD Monte-Carlo Modelswith QCD Monte-Carlo Models

WYSIWYGWhat you see is what you get.

Almost!

Select“clean”region

Charged ParticleData

Makeefficiency

corrections

QCDMonte-Carlo

Uncorrected data

compare

Corrected theory

Look only at the charged particles measured by

the CTC.

SmallCorrections!



Charged Particle Charged Particle CorrelationsCorrelations

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.


“Toward”


“Away”

-1 +1

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0

Leading Jet

Toward Region

Transverse Region

Transverse Region

Away Region

Away Region



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.


“Toward”


“Away”

Underlying Event“plateau”

Nchg versus PT(charged jet#1)

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PT(charged jet#1) (GeV/c)

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"Toward"

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"Transverse"

CDF Preliminarydata uncorrected



Shape of an AverageShape of an AverageEvent with Event with PPTT(chgjet#1) = 20 GeV/c(chgjet#1) = 20 GeV/c

<Nchg> = 8.2

<Nchg> = 1.2

<Nchg> = 4.5

<Nchg> = 1.2

PT(charged jet#1) = 20 GeV

Includes Jet#1

Underlying event“plateau”

Remember|| < 1 PT > 0.5 GeV

Shape in Nchg

Nchg versus PT(charged jet#1)

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"Toward"

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““Height” of the UnderlyingHeight” of the UnderlyingEvent “Plateau”Event “Plateau”

<Nchg> = 8.2

<Nchg> = 1.2

<Nchg> = 4.5

<Nchg> = 1.2

PT(jet#1) = 20 GeV

Implies 1.09*3(2.4)/2 = 3.9 charged particles per unit

with PT > 0.5 GeV.

Implies 2.3*3.9 = 9 charged particles per unit

with PT > 0 GeV which isa factor of 2 larger

than “soft” collisions.

Charged Particle Pseudo-Rapidity Distribution

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-10 -8 -6 -4 -2 0 2 4 6 8 10

Pseudo-Rapidity

Herwig "Soft" MB Isajet "Soft" MB MBR CDF DATA

dNchg/d

1.8 TeV Proton-Antiproton Collisions

Hard

Soft4 per unit



““Transverse” Nchg Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)

Plot shows the “Transverse” <Nchg> versus PT(chgjet#1) compared to the the QCD hard scattering predictions of Herwig 5.9, Isajet 7.32, and Pythia 6.115 (default parameters with PT(hard)>3 GeV/c).

Only charged particles with || < 1 and PT > 0.5 GeV are included and the QCD Monte-Carlo predictions have been corrected for efficiency.

Pythia 6.115

Herwig 5.9

Isajet 7.32Charged Jet #1

Direction

“Toward”


“Away”

"Transverse" Nchg versus PT(charged jet#1)

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Herwig Isajet Pythia 6.115 CDF Min-Bias CDF JET20

1.8 TeV ||<1.0 PT>0.5 GeV

CDF Preliminarydata uncorrectedtheory corrected



““Transverse” PTsum Transverse” PTsum versus Pversus PTT(chgjet#1)(chgjet#1)

Isajet 7.32

Pythia 6.115

Herwig 5.9


“Toward”


“Away”

"Transverse" PTsum versus PT(charged jet#1)

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Plot shows the “Transverse” <PTsum> versus PT(chgjet#1) compared to the the QCD hard scattering predictions of Herwig 5.9, Isajet 7.32, and Pythia 6.115 (default parameters with PT(hard)>3 GeV/c).

Only charged particles with || < 1 and PT > 0.5 GeV are included and the QCD Monte-Carlo predictions have been corrected for efficiency.



The Underlying Event:The Underlying Event:DiJet vs Z-JetDiJet vs Z-Jet



“Toward”

“Away”

“Toward-Side” Jet

“Away-Side” Jet

Z-BosonDirection


“Toward”

“Away”

“Away-Side” Jet

Charged Jet #1or

Z-Boson Direction

“Toward”


“Away”

PT > 0.5 GeV || < 1

Look at charged particle correlations in the azimuthal angle relative to the leading charged particle jet or the Z-boson.

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.



Z-boson: Charged Multiplicity Z-boson: Charged Multiplicity versus Pversus PTT(Z)(Z)

Z-boson data on the average number of “toward” (||<60o), “transverse” (60<||<120o), and “away” (||>120o) charged particles (PT > 0.5 GeV, || < 1, excluding decay products of the Z-boson) as a function of the transverse momentum of the Z-boson. The errors on the (uncorrected) data include both statistical and correlated systematic uncertainties.

Nchg versus PT(Z-boson)

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PT(Z-boson) (GeV)


1.8 TeV |eta|<1.0 PT>0.5 GeV

<Nchg>

"Toward" "Transverse"

"Away"

Z-bosonDirection

“Toward”


“Away”



DiJet vs Z-JetDiJet vs Z-Jet“Transverse” Nchg“Transverse” Nchg

Comparison of the dijet and the Z-boson data on the average number of charged particles (PT > 0.5 GeV, || <1) for the “transverse” region.

The plot shows the QCD Monte-Carlo predictions of PYTHIA 6.115 (default parameters with PT(hard)>3 GeV/c) for dijet (dashed) and “Z-jet” (solid) production.

DiJet

Z-boson

PYTHIA"Transverse" Nchg vs PT(charged jet#1 or Z-boson)

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PT(charged jet#1 or Z-boson) (GeV)

Pythia Z-jet Pythia DiJet CDF Z-boson CDF Min-Bias CDF JET20

<Nchg>

1.8 TeV |eta|<1.0 PT>0.5 GeV


data corrected

Charged Jet #1or

Z-bosonDirection

“Toward”


“Away”




Correct for track finding efficiency

Calorimeter: tower threshold = 50 MeV, Etot < 1800 GeV, |lj| < 0.7, |zvtx| < 60 cm, 1 and only 1 class 10, 11, or 12 vertex

Tracks: |zc-zv| < 5 cm, |CTC d0| < 0.5 cm, PT > 0.4 GeV, || < 1

QFL: Comparing DataQFL: Comparing Datawith QCD Monte-Carlo Modelswith QCD Monte-Carlo Models

Selectregion

Charged ParticleAnd Calorimeter

Data

QFLdetector

simulation

QCDMonte-Carlo

Uncorrected data

compare

Corrected theory

Look only at both the charged particles

measured by the CTC and the calorimeter data.

Tano-Kovacs-Huston-Bhatti



““Transverse” ConesTransverse” Cones

Tano-Kovacs-Huston-Bhatti

-1 +1

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Leading Jet

Toward Region

Transverse Region

Transverse Region

Away Region

Away Region

Sum the PT of charged particles (or the energy) in two cones of radius 0.7 at the same as the leading jet but with || = 90o.

Plot the cone with the maximum and minimum PTsum versus the ET of the leading (calorimeter) jet..

-1 +1

2

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Leading Jet

Cone 1

Cone 2

Transverse Region:

2(/3)=0.66

Transverse Cone:

(0.7)2=0.49



Transverse RegionTransverse Regionvs Transverse Conesvs Transverse Cones

Add max and min cone: 2.1 GeV/c + 0.4 GeV/c = 2.5 GeV/c.

Multiply by ratio of the areas: (2.5 GeV/c)(1.36) = 3.4 GeV/c.

The two analyses are consistent! Tano-Kovacs-Huston-Bhatti

2.1 GeV/c

0.4 GeV/c

3.4 GeV/c

0 < PT(chgjet#1) < 50 GeV/c

0 < ET(jet#1) < 50 GeV/c


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Field-Stuart-Haas



Max/Min ConesMax/Min Conesat 630 GeV/cat 630 GeV/c

Tano-Kovacs-Huston-Bhatti HERWIG+QFL slightly lower at 1,800 GeV/c

agrees at 630 GeV/c.



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 three categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants), charged particles that arise from initial-state radiation, and charged particles that result from the outgoing jets plus final-state radiation.

Beam-BeamRemnants

Initial-StateRadiation

ISAJETCharged Jet #1

Direction

“Toward”


“Away”

Outgoing Jets


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Beam-Beam Remnants

Outgoing Jets +Final-State Radiation

Isajet Total




PYTHIA: “Transverse” Nchg PYTHIA: “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 PYTHIA 6.115 (default parameters with PT(hard)>3 GeV/c).

The predictions of PYTHIA 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

PYTHIACharged Jet #1

Direction

“Toward”


“Away”


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Beam-Beam Remnants

Pythia 6.115 Total

Outgoing Jets +Initial & Final-State Radiation



Hard Scattering Component:Hard Scattering Component: “Transverse” Nchg vs P “Transverse” Nchg vs PTT(chgjet#1)(chgjet#1)

QCD hard scattering predictions of HERWIG 5.9, ISAJET 7.32, and PYTHIA 6.115.

Plot shows the dijet “transverse” <Nchg> vs PT(chgjet#1) arising from the outgoing jets plus initial and finial-state radiation (hard scattering component).

HERWIG and PYTHIA modify the leading-log picture to include “color coherence effects” which leads to “angle ordering” within the parton shower. Angle ordering produces less high PT radiation within a parton shower.

HERWIG

PYTHIA

ISAJETCharged Jet #1Direction

“Toward”


“Away”


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theory corrected

Outgoing Jets +Initial & Final-State Radiation Isajet

Pythia 6.115

Herwig



PYTHIA: Multiple PartonPYTHIA: Multiple PartonInteractionsInteractions

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 new HERWIG

!



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) > 3 GeV.

PYTHIA 6.115: GRV94L, MSTP(82)=1, PTmin=PARP(81)=1.4 GeV/c.


PYTHIA 6.115: GRV94L, MSTP(81)=0, no multiple parton interactions.

6.115

No multiplescattering


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Pythia 6.115 Pythia 6.125 Pythia No MS CDF Min-Bias CDF JET20

1.8 TeV ||<1.0 PT>0.5 GeV


Parameter 6.115 6.125

MSTP(81) 1 1

MSTP(82) 1 1

PARP(81) 1.4 GeV/c

1.9 GeV/c

PARP(82) 1.55 GeV/c

2.1 GeV/c 6.125

PYTHIA default parameters

ConstantProbabilityScattering



Note: Multiple parton interactions depend

sensitively on the PDF’s!




PYTHIA 6.115: CTEQ3L, MSTP(82)=1, PTmin =PARP(81)=1.4 GeV/c.

PYTHIA 6.115: CTEQ3L, MSTP(82)=1, PTmin =PARP(81)=0.9 GeV/c.


“Toward”


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GRV94L MSTP(82)=1PARP(81) = 1.4 GeV/c CTEQ3L MSTP(82)=1

PARP(81) = 0.9 GeV/c

CTEQ3L MSTP(82)=1PARP(81) = 1.4 GeV/c

ConstantProbabilityScattering





PYTHIA 6.115: GRV94L, MSTP(82)=3, PT0=PARP(82)=1.55 GeV/c.

PYTHIA 6.115: CTEQ3L, MSTP(82)=3, PT0=PARP(82)=1.55 GeV/c.



“Toward”


“Away”

Varying Impact

Parameter




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CDF Preliminarydata uncorrectedtheory corrected CTEQ3L MSTP(82)=3

PARP(82) = 1.35 GeV/c


GRV94L MSTP(82)=3PARP(82) = 1.55 GeV/c









“Toward”


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Varying Impact

ParameterHard Core




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CTEQ3L MSTP(82)=4PARP(82) = 1.55 GeV/cCTEQ4L MSTP(82)=4

PARP(82) = 1.55 GeV/c








“Toward”


“Away”

Describes correctly the rise from soft-collisions

to hard-collisions!


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1.8 TeV ||<1.0 PT>0.5 GeV

Varying Impact

Parameter




Plot shows “Transverse” <PTsum> versus PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA with PT(hard) > 0 GeV.




“Toward”


“Away”

Describes correctly the rise from soft-collisions

to hard-collisions!

Varying Impact

Parameter


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The Underlying Event:The Underlying Event:Summary & ConclusionsSummary & Conclusions

The “Underlying Event”

The underlying event is very similar in dijet and the Z-boson production as predicted by the QCD Monte-Carlo models.

The number of charged particles per unit rapidity (height of the “plateau”) is at least twice that observed in “soft” collisions at the same corresponding energy.

ISAJET (with independent fragmentation) produces too many (soft) particles in the underlying event with the wrong dependence on PT(jet#1) or PT(Z). 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. HERWIG 5.9 does not have enough activity in the underlying event.

PYTHIA (with multiple parton interactions) does the best job in describing the underlying event.

Combining the two CDF analyses gives a quantitative study of the underlying event from very soft collisions to very hard collisions.

Proton AntiProton

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event Underlying Event


Final-State Radiation



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 data very nicely! I need to check out the new version of HERWIG.

Multiple parton interactions are very sensitive to the parton structure functions. You must first decide on a particular PDF and then tune the multiple parton interactions to fit the data.

Proton AntiProton

Hard CoreHard Core

Slow!

the underlying event in hard scattering processes

Documents

charged particles

qcd montecarlo predictions

unit hwith pt

cmrequire pt

leading charged particle

nchgrick field floridacdf

unit hrick field floridacdf

transverse nchg