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Measurement of the Top Mass from the Leptons P T in the Dilepton Channel at CDF. Victoria Giakoumopoulou 30 March 2007. XXV Workshop on Recent Developments in High Energy Physics & Cosmology National Technical University, Athens, Greece. Outline. Why measure the top quark Motivation

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Measurement of the top mass from the leptons p t in the dilepton channel at cdf

Measurement of the Top Mass from the Leptons PT in the Dilepton Channel at CDF

Victoria Giakoumopoulou

30 March 2007

XXV Workshop on

Recent Developments in

High Energy Physics & Cosmology

National Technical University, Athens, Greece


  • Why measure the top quark

    • Motivation

    • Present Status

  • Methodology

    • Dilepton Event Selection

    • PT spectra

    • Top Mass with the Maximum Likelihood Method

    • Systematic errors

  • Conclusions & prospects

Why study the top quark
Why study the top quark

  • Currently the heaviest particle of the SM

  • Accurate measurement of the top quark mass

    better localization of SM Higgs mass

ΔΜW α Mt2

ΔΜW α lnMH


Main Injector


Top pair production decay

QCD Production (~6pb)dominates at Tevatron:




mu-mu (1/81)

tau-tau (1/81)

e -mu (2/81)

e -tau(2/81)

mu-tau (2/81)

e+jets (12/81)



jets (36/81)

Top Pair Production & Decay

  • W decays define channel:

    • Dilepton: 11.12%

    • Lepton+jets: 44.44%

    • All-hadronic: 44.44%

Measurements of the top mass
Measurements of the Top Mass

CDF +D0 March 07

(170.9±1.1(stat) ±1.5(syst))GeV/c2

CDF: (170.5±1.3(stat) ±1.8(syst))GeV/c2

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

    • The statistical error will decrease

    • The systematic error willdominate

Sensitivity to the top mass
Sensitivity to the Top Mass

The main contribution in the systematic uncertainty is associated with jets

Proposal (University of Athens CDF/ATLAS Group)  Seek a variable that:

  • doesnot depend on the jet energy scale

  • doesnot depend on b-tagging

  • it is the same for both the Dilepton and the Lepton+jets channels

    Such a variable could be the leptons (μ/e) PT/ET

    This can be calibrated against Z→ll to within ~50 MeV

Dilepton signal
Dilepton Signal

  • Expect to observe:

  • two high PT leptons of opposite sign

  • large missing ET from the two v’s

  • two or more jets


  • WW 16.13%

  • WZ  4.1%

  • ZZ  1%

  • Zττ  11.48%

  • DY  28.04 %

  • Fakes  38.26%




Drell–Yan: Zμμ, Ζee


Diboson WW/WZ/ZZ


We expect @ 1200 pb-1

23.11 background events

55.95 signal events


CDF note 8741

Methodology outline
Methodology Outline

  • Estimation of the Top Mass using the Lepton PT information

  • Create lepton PT templates for the signal for several top masses

    • PT is linearly dependent to the Top Mass

  • Create lepton PT templates for backgrounds

  • Fit the PT templates with Gamma x Fermi function

  • Implement Maximum Likelihood method to estimate the Top Mass

P t sensitivity to top mass
PT sensitivity to Top Mass


sensitiveto the

top quark mass

Linear dependency

<PT> = κMT+λ

SLOPE λ=13.52%

Shape of leptons p t
Shape of Leptons PT

  • Parameterization of PT

  • Fit 30 PT histograms for MT=155-192 GeV/c2

    • Find the optimum mass dependent parameters p=a1MT+a2 and q=a3MT+a4

  • Fit PT of combined background

    • Find the optimum p and q parameters


  • N: number of data events

  • nbexp: expected number of background events

  • σnb: expected background uncertainty

  • ns: number of estimated signal events

  • nb: number of estimated background events

Example of mass estimation mc@1200pb 1
Example of mass estimation[email protected]

Input Mass = 175 GeV

79 signal & bg events

Sanity tests only signal
Sanity tests - only Signal

[email protected] 200PE










Sources of systematics

  • Gluon radiation in the initial state (ISR)

  • Estimated by using the official CDF Pythia samples with “more” and

  • “less” ISR

  • Choice of Parton Distribution Functions (PDF)

  • Estimated using the sets CTEQ5L and MRST75

  • Choice of Monte Carlo generator

  • Estimated by comparing the results of Pythia and Herwig with the

  • same input top mass

  • Uncertainty associated with the measurement of leptons’ PT

  • Uncertainty in the expected background shape

Measurement of lepton p t
Measurement of lepton PT

  • Possible pT scale uncertainty and non-linearity

  • Tune using Z → dilepton data (CDF note 8632)

  • Bremsstrahlung from the detected electrons

  • Found to be negligible

Preliminary estimate of the PT scale uncertainty on the Mtop is < 1 GeV

Adapted from CDF Joint Physics Group


Blessed analysis by CDF @ 340pb-1


  • Expect to observe

  • high PT lepton

  • large MET

  • ≥ 4 jets

tWb lvb



  • W+jets: qq’ → W → lν (~80%)

  • Jets faking leptons (~20%)

Composition @ 340pb-1

  • Signal:70 expected events

  • Background:125 expected events

  • (100 W + 15 fake e + 10 fake μ)

  • Data: 197 events

Lj blessed result @ 340pb 1
LJ – Blessed result @ 340pb-1

Mtop=(220 ± 47.4(stat) ± 9.3 (syst) ) GeV/c2

Summary prospects
Summary - Prospects

  • New method to measure the Mtop from the lepton(s) PT has been proposed and developed by the UoA group

  • Method blessed (approved) by CDF

  • Present systematic error ~4 GeV

  • Projected combined L+jets &Dilepton uncertainties on MT for ∫Ldt~10fb-1 (~Tevatron end of RUN II)

    ±5 GeV statistical

    ±1-2 GeV systematic

  • Bright prospect for Mtop at LHC

  • CDF Notes






    • JINR/E1-2005-104 “Particles and Nuclei, Letters”



UoA CDF group

  • Victoria Giakoumopoulou

  • Athanasios Staveris-Polykalas

  • Constantinos Vellidis

  • Arkadios Manousakis-Katsikakis

  • Nikolaos Giokaris

  • Work partly supported by:

  • EPEAEK II program in the framework of the projects

    • Heraklitos

    • Pythagoras

  • GSRT

  • ELKE - UoA

Consistency of fit
Consistency of Fit

  • <PT> vsMtop from <PT> = κMT+λ, where κ=0.1327 and λ=32.56

  • <PT> from Γ x Fermi model

Perfect agreement

!! Model works fine

Statistical error @ tevatron lhc lj
Statistical Error @Tevatron/LHC (LJ)