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

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### 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

Outline

- 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

- 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

QCD Production (~6pb)dominates at Tevatron:

85%:

15%:

e-e(1/81)

mu-mu (1/81)

tau-tau (1/81)

e -mu (2/81)

e -tau(2/81)

mu-tau (2/81)

e+jets (12/81)

mu+jets(12/81)

tau+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

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

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

- Expect to observe:
- two high PT leptons of opposite sign
- large missing ET from the two v’s
- two or more jets

Signal

- WW 16.13%
- WZ 4.1%
- ZZ 1%
- Zττ 11.48%
- DY 28.04 %
- Fakes 38.26%

tW+bl+v

tW-bl-v

Background

Drell–Yan: Zμμ, Ζee

Fakes

Diboson WW/WZ/ZZ

Zττ

We expect @ 1200 pb-1

23.11 background events

55.95 signal events

S/B=2.42

CDF note 8741

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

PT sensitivity to Top Mass

PT

sensitiveto the

top quark mass

Linear dependency

<PT> = κMT+λ

SLOPE λ=13.52%

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

Likelihood

- 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

Sanity tests - only Signal

[email protected] 200PE

<Pull>=p1Min+p0

<Pull_RMS>=p1Min+p0

<MF>=p1Min+p0

p0=-0.19±0.25

p1=0.001±0.001

p0=-0.96±0.22

p1=0.000±0.001

p0=-1.9±4.5

p1=1.02±0.03

Fitted Mass – Signal + Background

p0=0.017±0.255

p1=-0.0004±0.001

p0=0.95±5.7

p1=0.998±0.033

p0=0.91±0.23

p1=0.0003±0.001

- 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 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

Signal

- Expect to observe
- high PT lepton
- large MET
- ≥ 4 jets

tWb lvb

tWbqq’b

Background

- 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

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

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/DOC/TOP/PUBLIC/8741
- CDF/PHYS/TOP/PUBLIC/8632
- CDF/PHYS/TOP/PUBLIC/8617
- CDF/PHYS/TOP/PUBLIC/8616
- CDF/PUB/TOP/PUBLIC/7888
- JINR/E1-2005-104 “Particles and Nuclei, Letters”
- CDF/PHYS/TOP/CDFR/7685
- CDF/PHYS/TOP/CDFR/7684

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

- <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

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