Study of the to Dilepton Channel with the Total

Transverse Energy Kinematic Variable

Athens, April 17th 2003 Victoria Giakoumopoulou

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University of Athens, Physics Department

Section of Nuclear and Particle Physics

Workshop on RECENT DEVELOPMENTS IN HIGH ENERGY PHYSICS AND COSMOLOGY

OUTLINE

• Introduction• Top Quark Production and Decay• Physics Motivation• H Analysis for the Dilepton Channel• Results from HERWIG simulation

study• Conclusions and Future Work

Introduction on top quark

• Top quark was predicted by the SM as the I3=+1/2 member of a weak SU(2) isodoublet that also contains the b quark

• It was discovered both by CDF end D0 at the Femilab Tevatron in 1995

b

t

Top Production and Decay

At high energy collisions and for Mtop > 100 GeV/c2

fusion to a gluon is the main production mechanism.

pp qq

PROTON

ANTIPROTON

q

q

b

b

+w

-wGLUON

2qv,

3q,l

4q,v

1,ql

t

t

Decay Modes

vl,w

v,lw

2

1

Dilepton BR=11.2%

two leptons of opposite sign

q,q'w

q,q'w

2

1

All Hadronic BR=44.4%

6 or more jets

qq'w

vl,w

2

1

Lepton + jets BR=44.4%

or W+jets

What about the Dilepton Signal?

tW+bjet

l+vl

tW-

jet

l-

b

lv

Expect to observe:

• two leptons with high PT

• large missing ET from the two v’s

• two or more jets

Why study the to Dilepton channel

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• another measurement of the top quark mass with smallest systematic error (?)

• i t can provide many checks on the SM

better ‘localization’ of SM Higgs mass

Top quark mass from Fermilab Tevatron RUN I

Channel CDF

All-hadronic 186±10 ±5.7

Lepton+MET+jets

176±5.1 ±5.3

Dilepton 167±10.3 ±4.8

All above numbers are in GeV/c2

The first uncertainty is statistical and the second is systematic.

Direct checks on the Standard Model ...

From Standard Model :

N(e-e+ και μ-μ+) = Ν (e-μ+ και e+μ-)

From CDF RUN I data :

N(e-e+ και μ-μ+) = 2

Ν (e-μ+ και e+μ-) =7

CDF Detector at Fermilab Tevatron

New

Old

Partially new

Forward muonEndplugcalorimeter Silicon and drift

chamber trackers

Central muonCentral calorimeters

Solenoid

Front endTriggerDAQOffline

TOF

H Analysis

• H variable for the study of top in the Dilepton Channel.

• Motivation for the use of H variable the decay products have higher ET’s than

the decay products in the background processes.

(jets)EE(leptons)PH TTT

backgroundtt HH

tt

Main background in the W+jets channel

QCD W + jets productionThe W boson recoils against a significant jet activity

PROTON

ANTIPROTON

JET

JET

JET

q

q

w

q

q

g

g

g

H distribution for the Signal and the Backgrounds in the W+jets

channel

• Solid line : CDF RUN I data

• Double dotted line: top events from HERWIG 180

• Dotted line : top events from HERWIG and background from VECBOS

• Yellow histogram : events selected with b-tagging

F. Abe et al, ‘Study of Production in Collisions Using Total Transverse Energy.’ Phys. Rev.Lett. 75 (1995) 3997

Dilepton Channel

Signal : electrons and muons

not tau leptons

Two main backgrounds from Run II

channel Dilepton→tt

Total number of background events: 0.103±0.056 events

Observe 5 events

p

p

gluon

jet

p

p

ql

γ*, Ζ*

l

q

p

p

τ-

p

pτ+

q

q

0Z

gluon

jet

Drell-Yan Zτ+τ-

Data analysis

vl,w

v,lw

2

1

For event selection we impose

• ‘CDF cuts’ και

• ‘Reduced CDF cuts’

Minv<75 or

Minv> 105 GeVInvariant Mass

>200Δφ( ,jets)

>25 GeV>25 GeV

>2>2Number of jets

2.02.0|η|

>10 GeV>10 GeVΕΤ

Jets

1.01.0|η|

>20 GeV>20 GeVΡTLeptons

Reduced CDF cutsCDF cutsSelection criteria

TE

TE

H distribution for the Signal and the backgrounds in the Dilepton

Channel in RUN I

J. Cassada, M. Kruse, P. Tipton, ‘Top Dilepton Events with the Top Quark Hypothesis. CDF –note 4278.

9 events from CDF Run I

e+e-+ ή μ+μ-

eμ

HERWIG Μt=175 GeV

background events

top + background

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data

H distribution for the Signal and the backgrounds in the Dilepton

Channel in RUN II

5 events from CDF Run II

e+e-+

eμ

μ+μ-

MC x 10tt

Motivated by these last plots we decided to examine the possibility to :

• relax or eliminate completely the Δφ and MZ cuts

• impose a cut on H

TO :

a. get higher efficiency

b. better signal/background ratio (S/B)

Our analysis, so far, has been performed by the HERWIG MC at generation level

Simulation of Signal and Background Events

• HERWIG59 is used for the generation production of and background events tt

H distribution for events with HERWIG

tt

Production of 20000 events

995 events in Dilepton

5% of events decay in the Dilepton Channel

tt

tt

H distribution of with HERWIG, CDF cuts

Mtop=175 GeV

CDF cuts306 dilepton events

31% of all dilepton events

187 eμ

119 ee ή μμ

tt

H for background events with HERWIG, CDF cuts

CDF cuts Drell-

Yan

49 events

ττZ

5 events

H signal and background with HERWIG, CDF cuts

CDF cuts signal

Drell-Yan ττZ

306 signal events

49 Drell-Yan events

5 events

S/B=5.7±1.1

ττZ

H distribution for with HERWIG, reduced CDF cuts

Reduced CDF cuts350 dilepton events

35% of all dilepton events

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H distribution for signal and background with HERWIG, reduced

CDF cutsDrell-Yan

ττZ

471 events 6 events

Reduced CDF cuts

H distribution for signal and background with HERWIG, reduced

CDF cuts

signal

Drell-Yan

350 signal events

471 Drell-Yan events

6 events

ττZ

Reduced CDF cuts

Introduction of H cut in events selected with reduced CDF cuts

H cut

H cut= 275 GeV

308 events

37 background events

S/B=8.3±1.4

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H cut=275GeV

Reduced CDF cuts

• Application of H variable in the analysis of Dilepton channel we have a good separation of signal and background

• Selecting events with H variable we have better efficiency for the signal and much better signal/background ratio relative to the analysis used in CDF RUN I.

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Conclusions and Future Work

Present Work• Use the CDF full simulation package to study the to dilepton signal and all of the backgrounds• Analysis of CDF RUN II data.

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Decay Channels

Beyond ‘Standard’ Cuts

• Cut on the Invariant Mass of the two leptons

Mll<75 GeV/c2 or Mll>105 GeV/c2 , if leptons are of the same type

to reduce events from Ζee(or μμ) decays

• For events with Total Transverse Energy < 50 GeV we require

Δφ(ΜΕΤ, jet) > 200 and Δφ(ΜΕΤ,lepton) > 200

This reduces the Drell-Yan background, because in this process there are not real neutrinos and comes from energy mismeasurement in the hadronic calorimeter. Therefore we expect strong correlation between and the jets

E/

E/

9 events from CDF Run I