Underlying Event Studies at CDF

Stefano LamiDIS 2003

Underlying Event Studies at CDFUnderlying Event Studies at CDF Stefano LamiStefano Lami

The Rockefeller University


The “Underlying Event” inHard Scattering Processes

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

“Underlying Event”

Beam-Beam Remnants Beam-Beam Remnants


UE = unavoidable background to be removed from the jets before comparing to NLO QCD predictions

Tevatron Collider: most of collisions are ``Soft’’, outgoing particles roughly in the same direction as initial proton and antiproton.

Occasional ``Hard’’ interaction results in large transverse momentum outgoing partons.

The ``Underlying Event’’ is everything but the two outgoing Jets, including :

initial/final gluon radiation

beam-beam remnants

secondary semi-hard interactions

Min-BiasMin-BiasMin-Bias


The Underlying Event

Precise Jet measurements requires accurate modeling of the UE.

The physics of the UE is complicated and involves both pQCD and non-pQCD.

None of the QCD Monte-Carlo models correctly describes the properties of the underlying event

Proton AntiProton

Multiple Parton Interactions

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying EventUnderlying Event

Pythia uses Multiple Parton Interactions to enhance the underlying event.

(MPI more likely in a hard (central) collision!)

Can we tune the QCD MonteCarlo models to fit collider data?

Are Minimum-Bias events a good approximation to the Underlying Event? (The beam-beam remnants in the underlying event are color connected to the hard component)


Run I Results

MAX/MIN cones

Sensitive to UE and NLOperturbative corrections

Sensitive to UE only

First study: Cone analysis for 50-300 GeV Jets

-1 +1

2

0

Leading Jet

Cone 1

Cone 2

• Sum the PT of charged particles 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.

Charged particle tracks in Central Tracking to study low momentaCharged particle tracks in Central Tracking to study low momenta


0.4 GeV/c

50 < ET(jet1) < 300 GeV/c

The MAX cone increases with increasing ET of the leading jetThe MIN cone stays flat

The MAX cone increases with increasing ET of the leading jetThe MIN cone stays flat

The MIN cone constant at a level similar to that found in Min-Bias events.

HERWIG agrees well with Jet data, but does not reproduce well Min-bias events (lack of semi-hard physics)

PYTHIA parameters can be tuned to fit the data

(PT0 =2 GeV cut-off for

multiple parton scattering)


Run I Results

Second study: 0.5 - 50 GeV Charged particle Jets (PRD65:09202, 2002)

• Study only the charged particle components of jets: charged particle jets• Min-bias and Jet20 data• Compare to Monte-Carlo models: HERWIG, ISAJET, and PYTHIA

6 particles 5 ‘jets’

Use simple, non-standard, jet definition with R=0.7:

Assign all charged particles (PT> 0.5 GeV/c, ||<1) to a jet

Jets contain particles from the UE as well as from outgoing partons

Even one charged particle can be jet

Look at 3 regions (towards, away and transverse) in phi


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 same size in - space, x=2x120o=4/3.


“Toward”


“Away”

-1 +1

2

0

Leading Jet

Toward Region

Transverse Region

Transverse Region

Away Region

Away Region

PT > 0.5 GeV/c || < 1

Charged Particle Correlations

Very sensitive to the “underlying event”


Charged Multiplicity versus PT(chgjet1)

• 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)

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!

Jet Data

Min-Bias


“Transverse” PT 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.

"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)d

Nc

hg

/dP

T (

1/G

eV

/c)


1.8 TeV ||<1 PT>0.5 GeV/c

PT(chgjet1) > 2 GeV/c



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

0

1

2

3

4

5

0 5 10 15 20 25 30 35 40 45 50


"Tra

ns

ve

rse

" <

Nc

hg

> in

1 G

eV

/c b

in CDF Min-Bias

CDF JET20

1.8 TeV ||<1.0 PT>0.5 GeV


PT(charged jet#1) > 5 GeV/c“Transverse” <Nchg> = 2.2

PT(charged jet#1) > 30 GeV/c“Transverse” <Nchg> = 2.3


“Max/Min Transverse” Nchg versus PT(chgjet1)

"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


<Nch

g>

in 1

GeV

/c b

in


1.8 TeV ||<1.0 PT>0.5 GeV

"Max Transverse"

"Min Transverse"

“TransMAX”

“TransMIN” More sensitive to the “beam-beam remnants”

"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


<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

HERWIG 6.4 CTEQ4L

Data PTsum for

Max/Min transverse regions in agreement with first study, once

normalized to different area.

The charged particle jets in the Min-Bias data are a smooth continuation of the high PT

charged jets observed in the Jet20 data.

More sensitive to the “hard scattering” component

Herwig does not have enough activity in the

Transverse region


Beam-BeamRemnants


“Toward”


“Away”


0

1

2

3

4

0 5 10 15 20 25 30 35 40 45 50


"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

Isajet Total

Hard Component

Outgoing Jetsplus

Initial & Final-StateRadiation

Run I “transverse” data compared to Models


0

1

2

3

4

5

0 5 10 15 20 25 30 35 40 45 50


"Tra

nsvers

e"

<N

ch

g>

CTEQ3L CTEQ4L CTEQ5L CDF Min-Bias CDF JET20

CDFdata uncorrectedtheory corrected

1.8 TeV ||<1.0 PT>0.5 GeV/c

Pythia 6.206 (default)MSTP(82)=1

PARP(81) = 1.9 GeV/c PYTHIA 6.206PYTHIA default parameters give very poor description of the Underlying Event

ISAJET 7.32 has a lot of

activity in the transverse region,

but with the wrong dependence

on PT(chgj1)



0

1

2

3

4

0 5 10 15 20 25 30 35 40 45 50


"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 6.206

Average number of Charged tracks in the “Transverse” region vs PT(leading jet) compared to QCD hard scattering predictions of two tuned versions of PYTHIA 6.206 (CTEQ5L). Multiple Parton Interactions with varying impact parameter, double Gaussian matter distribution and smooth turn-off PT

0

PYTHIA 6.206 CTEQ5L

Bulk of Min-Bias events!

Can describe transition between “soft” and “hard” regime!


Proton AntiProton

“Hard” Scattering

PT(hard)

Outgoing Parton

Outgoing Parton

Underlying Event Underlying Event


Final-State Radiation

Proton AntiProton 2 TeV

The “Underlying Event” in Run II

Center of mass Energy from 1.8 to 1.96 TeV

New Central Tracking, Plug Calorimeter, Electronics

• Repeat Run I analysis on Charged particle jets

• Same Run I track selection (and <2 vertices)

• Same PYTHIA version tuned on Run I data

• Min-Bias and Jet data (~85 pb-1 for jet triggers so far)



Published CDF Run I data on the average density of charged particles in the ``transverse region’’

dN/ ddvs PT(leading jet)

Excellent agreement between

Run I and Run II

PYTHIA tuned to fit Run I data



Average ``transverse’’ charged PTsum density (PT > 0.5 GeV, || < 1)

as a function of the transverse momentum of the leading charged particle jet.


Conclusions

• Combining Minimum Bias and Jet CDF data gives a quantitative study of the underlying event from very soft collisions to very hard collisions.

• Studies of the underlying event at CDF have revealed inadequacies of some MonteCarlo generators and have led to improved tuning.

• Tuned PYTHIA (with multiple parton interactions) does a good job in describing the underlying event in CDF data.

• Run I and Run II data show an excellent agreement for charged particles. The underlying event is the same in Run II as in Run I but now we can study the evolution out to much higher energies.


Tuned PYTHIA 6.206 vs HERWIG 6.4 “TransMAX/MIN” vs PT(chgjet1)

• 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


"Tra

nsv

erse

" <

Nch

g>

in

1 G

eV/c

bin

"Max Transverse"

"Min Transverse"


1.8 TeV ||<1.0 PT>0.5 GeV

CTEQ5L



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


<P

Tsu

m>

(G

eV/c

) in

1 G

eV/c

bin

"Max Transverse"

"Min Transverse"


1.8 TeV ||<1.0 PT>0.5 GeV

CTEQ5L



HERWIG 6.4


“Transverse” PT Distribution

• Run I average number of charged particles per unit PT –dd, dNchg/dPTdd. The open squares correspond to Min-Bias collisions, the solid circles (squares) correspond to events with PT(chgjet#1)>5 GeV (> 30 GeV).

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Underlying Event Studies at CDF

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