Kinematics of Top Decays in the Dilepton and the Lepton + Jets

channels: Probing the Top Mass

University of Athens - Physics Department

Section of Nuclear and Particle Physics

Nikos Giokaris

Introduction Top Quark Production and Decay Physics Motivation Sensitivity to the Top Mass at TEVATRON

and LHC Results from PYTHIA/HERWIG simulation

study Conclusions and Future Work

OUTLINE

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 by both CDF end DØ at the Fermilab Tevatron in 1995

Top Production and Decay

At high energy collisions at Tevatron and for Mtop>100 GeV/c2 fusion to a gluon is the main production mechanism.

ppqq

p p

t

t

bW

q

l

'q

v

b

ql ,

',qv

W

Decay Modes

vl,w

v,lw

2

1

q,q'w

q,q'w

2

1

qq'w

vl,w

2

1

Dilepton

BR=11.2%

All Hadronic

BR=44.4%

Lepton + jets

BR=44.4%

What about the Dilepton Signal?

Expect to observe:

• two leptons with high PT

• large missing ET from the two v’s

• two or more jets

tW+bjet

l+vl

W-

jet

l-

b

v

t

Why study the to Dilepton channeltt

CDF/D02 fb-1goal

another measurement of the top quark mass with smallest systematic error

better ‘localization’ of SM Higgs mass

it can provide many checks on the SM

The Top Mass

New World Average (2004) (including RunI results) Mt = 178.0 ± 4.3 (±2.7st±3.3sys) GeV/c2

hep-ex/0404010

Moderate (CDF RUN II) or very high (LHC) statistics of top production are expected soon

The statistical error will decreaseThe systematic error will dominate

23.73.6t GeV/c 1.7(syst)jets)(stat173.5M

CDF Run II :

The Top Mass

Mt = 178.0 ± 4.3 GeV (±2.7st±3.3sys)

Stat: 2.7 GeV/c2

Syst: 3.3 GeV/c2

•2.6 GeV/c2 JES

•1.6 GeV/c2 signal

•0.88 GeV/c2 background

•0.83 GeV/c2 UN/MI

•0.35 GeV/c2 fit

•0.12 MC

Sensitivity to the Top Mass

PROBLEM The main source of the systematic error is the jet energy scale.

PROPOSAL Use a variable(s) that does not depend on the jet energy.

EXAMINE LEPTON sensitivity to the top mass in the decay channelswhere leptons are electrons or muons

)(PETT

jetslepton & Dilepton tttt

Analysis Outline

Generation of events for several top masses (130 -230 GeV/c2) for: CDF energy, 2 TeV (HERWIG) LHC energy, 14 TeV (Pythia)

Selection of dilepton events, in parton level, where leptons are electrons or muonsRequirements on leptons: PT > 20 GeV/c |η| <1.1

Study of the mean value of the leptons’ PT for the samples generated with the above top masses

tt

Leptons’ PT events generated with

HERWIG

Top Quark Mass (GeV/c2)

Lepton <PT> (GeV/c)

<PT> standard deviation (GeV/c)

130 50.59 26.15

150 53.33 28.43

160 55.08 30.39

170 56.78 31.55

180 58.17 32.10

190 58.91 31.73

200 60.83 34.21

210 62.72 34.73

230 67.11 37.64

2TeVs

Mtop = 170 GeV/c2

Distributions of Leptons’ PT

events generated with HERWIG 2TeVs

Mtop = 180 GeV/c2

Leptons’ PT vs top mass events

generated with HERWIG 2TeVs

PT

sensitive to the

top quark mass

Fit to a straight line

gives

slope: 0.16

Other kinematic variables vs top mass events generated with

HERWIG

Kinematic Variable

Slope

PT 0.1574±0.0036

P 0.1739±0.0042

Leading PT 0.2278±0.0054

Leading P 0.2523±0.0064

Sum of PT’s 0.316±0.008

Sum of P’s 0.3523±0.0092

2TeVs

Top Quark Mass (GeV/c2)

Lepton <PT> (GeV/c)

<PT> standard deviation (GeV/c)

130 54.99 33.13

140 55.5 33.47

150 57.37 35.75

160 59.37 37.1

170 61.69 38.97

180 63.89 40.83

190 66.28 43.91

200 67.63 44.52

210 69.81 45.11

220 72.67 47.32

230 75.65 51.03

Leptons’ PT events generated with Pythia

14TeVs

Distributions of Leptons’ PT

events generated with Pythia 14TeVs

Mtop = 170 GeV/c2

Mtop = 180 GeV/c2

PT

sensitive to the

top quark mass

Fit to a straight line

gives

slope: 0.21

Leptons’ PT vs top mass events

generated with Pythia 14TeVs

Kinematic Variable

Slope

PT 0.2068±0.0039

P 0.242±0.005

Leading PT 0.2294±0.0062

Leading P 0.3526±0.0075

Sum of PT’s 0.381±0.008

Sum of P’s 0.451±0.0010

Other kinematic variables vs top mass events generated with

Pythia 14TeVs

Kinematic Variable

Slope

PT0.1574±0.0036

P0.1739±0.0042

Leading PT0.2278±0.0054

Leading P0.2523±0.0064

Sum of PT’s 0.316±0.008

Sum of P’s0.3523±0.0092

Again, the numbers for 2TeVs

Leptons’ PT events generated with HERWIG,

after simulation

Top Quark Mass (GeV/c2)

Lepton <PT> (GeV/c)

<PT> standard deviation (GeV/c)

130 54.43 30.00

150 57.24 31.94

160 59.54 33.55

170 60.99 34.12

180 63.43 35.47

190 62.85 34.82

200 65.86 37.86

210 66.54 47.58

230 70.17 38.79

2TeVs

Leptons’ PT vs top mass events

generated with HERWIG, after simulation 2TeVs

PT

sensitive to the

top quark mass

Fit to a straight line

gives

slope: 0.15

Other kinematic variables vs top mass events generated with

HERWIG, after simulation

Kinematic Variable

Slope

PT 0.1546±0.0106

P 0.1667±0.0122

Leading PT 0.2172±0.0157

Leading P 0.2309±0.0182

Sum of PT’s 0.3091±0.0237

Sum of P’s 0.3317±0.0273

2TeVs

Kinematic Variable

Slope

PT0.1574±0.0036

P0.1739±0.0042

Leading PT0.2278±0.0054

Leading P0.2523±0.0064

Sum of PT’s 0.316±0.008

Sum of P’s0.3523±0.0092

Again, the numbers in parton level

Estimation of the statistical error of the top mass (TEVATRON)

NPP

mrms

TstatT

stattop

1)(

)(

Expected statistical error in the top mass, as extracted from the Pt spectrum of the two leptons in the dilepton channel, as a function of Luminosity L

Integrated Luminosity

Expected by

Expected number of dilepton events

(δMtop)stat

(pb-1) (GeV/c2) (%)

193 Feb. 2003 10 51 28

400 Sep. 2004 21 35 20

1200 Dec. 2005 63 20 11

3000 Dec. 2006 158 13 7

8000 Dec. 2008 420 8 4

For

TEV

ATR

ON

Top mass is linear dependent to the <PT> <PT>=λmtop+κ

Estimation of the statistical error of the top mass (LHC)

For

LH

C

NPP

mrms

TstatT

stattop

1)(

)(

Integrated Luminosity

Expected by

Expected number of dilepton events

(δMtop)stat

(pb-1) (GeV/c2) (%)

1000End of 1st year of

operation8000 1.6 0.8

10,000End of 2nd

year of operation

80,000 0.5 0.3

Top mass is linear dependent to the <PT> <PT>=λmtop+κ

Estimation of the systematic error of the top mass

Top mass is linear dependent to the <PT> <PT>=λmtop+κ

Contributions to systematic error: uncertainties in

• the fit parameters due to finite MC statistics & omission of non linear terms

• the measurement of leptons’ PT

• the measurement of jets’ Pt (MET also)

• the MC event generator

• the knowledge of background

Uncertainty

(δ<PT>)sy

st

δκ δλ (δMtop)stat

(Gev/c) (Gev/c)(GeV/

c2)(%)

Leptons’s PT

0.05

Linear fit

Jet energy

Monte Carlo

Background

Total

2222 )()()(1

)(

topsystTsysttop

mPm

Summary

Top mass analysis, using only lepton PT information in the ttbarDilepton channel, looks promising

The method is applicable both in Tevatron and LHC experiments

The systematic error of the top mass is expected to decrease considerably by this method

The statistical error will also be very small at LHC