top

DØ

top

b

WZ

tracks

Higgs

LeptonsJe

ts

Top Physics

At DØ

Top Physics

At DØFreya Blekman

NIKHEF / Universiteit van Amsterdam

NIKHEF Annual Scientific MeetingAmsterdam, December 19th, 2002

Freya BlekmanNIKHEF/ Universiteit van Amsterdam 2

Outline

1. Introduction

2. Triggers

3. Analyses:tt all hadrons

tt electron + jets

4. Conclusion, Prospects



Physics motivation• X-sec:

QCD HQ production

• Mass:Mtop provides

restriction on Mhiggs

• Top physics• spin correlations

• Vtb

DØ Run 1 Mtop

DØ Run 2 Mtop



Tevatron Top Physics (s)=1.96 TeV• Top pair

production (this talk)

• Single top

• t W + b BR > 99%

• tt analyses classified by W decay channels

• σ(pptt)≈ 7 pb

21%

15%

15%1%3%1%

44%

tau+Xmu+jetse+jetse+ee+mumu+muall hadronic



• Background reduction most important factor

• Lepton + jets: Background mainly QCD and W + jets

• Hadronic channel: Background mainly QCD

• All tt channels: 2 b-jets! B-tagging of vital importance

Event topology

eb-jet

jet

jet

b-jet

b-jet

jet

jetb-jet

jet

jet

Hadronic: 6 jets

e+jets:4 jets + e + νe



Triggers• Hadronic channel:

focus on jet multiplicity, high total transverse energy (= HT)

• Electron + jets channel: focus on electron identification + jet multiplicity

tt e+jets tt hadronic

SignalEfficiency

Background Trigger Rate

Signal Efficiency

Background Trigger Rate

Simulated BG Trigger Rate

L1 99 % 88 Hz 100 % 36 Hz 47.8 Hz(only pT > 20 QCD MC)

L2 95 % 52 Hz 99.0 % 8 Hz 4.1 Hz

L3 75 % 6 Hz 97.8 % 1.4 Hz 1.4 Hz



tt e + jets• Finding

electrons:EM ID efficiency

• High pT electrons:Count Z e+e- with and without ID cuts

ID cuts: isolation, EM fraction, pT > 20 GeV, shower shape.

(L. Phaf, I. Iashvili et al.)



tt e + jets candidate• passes Run I cuts • Isolated electron• 4 reconstructed jets above

15 GeV • HT = 191 GeV

• Aplanarity = 0.11

η

φ



Offline event selection• event-shape variables



tt e + jets candidate• passes Run I cuts • Isolated electron• 4 reconstructed jets above

15 GeV • HT = 191 GeV

• Aplanarity = 0.11

• Mtop (χ2 fit) = 165 GeVη

φ



tt e + jets candidate



tt MC Pythia,hadronic tt

Mtop = 174 GeV

QCD MC Pythia, pT > 80

tt hadrons• N jets > 5

• Centrality > 0.6

• HT > 225 GeV

• Aplanarity > 0.05

Data Lum: 2 pb-1

After selection signal/bg ~1/30 (MC signal eff ~ 44 %)

Centrality = HT/H Aplanarity

Jet multiplicity HT = total hadronic sum

(F.Blekman,M.Vreeswijk,G.Massaro)



eta

3.7

0.0

-3.7

-1.2

1.2

phi

0.0

2PI

ET GeV

40

+z

E scale: 51 GeV

0180

y

x

ET scale: 59 GeV

-2 2

• 6 jets• Passes selection cuts• Secondary vertices

tagged• Mtop (χ2 fit) = 168 GeV

tt hadrons candidate event

ηφ



Results Hadronic Channel

• Efficiencies and Purities

Results for preliminary study on 2 pb-1

Further study still to be done, but

promising for larger dataset….

Cuts

tt hadronsefficiency

S/B ratio

Selection (this study)

44 % 1/30

2 vertex tags + non-leading jet cut

25 % 1/2

Secondary vertex probability DØ Run 2 Preliminary

DØ Run 2 Data

tt hadronic MC

QCD MC

ANN output



Prospects

• Lepton + jet , di-lepton:• For now: Use Analysis

similar to Run 1 (based on event topology)

tt decay channel

Signal events DØ Run 1

‘Expected’ signal by spring 2003

Lepton+lepton 6 1 - 5Lepton+jet 20 5 - 27

Hadronic 13 7 - 40110 pb-1 @ 1.8 TeV 50 pb-1 @ 1.96 TeV, σ = 6

pb (expected eff 5 - 30 %)

• All hadronic:• Make use of improved

b-tagging

Expect to have 50 pb-1 reconstructed data by February 2003



Conclusion

• We see top events being produced at Tevatron

• Improved b-tagging starting to make a difference

• Potential measurement for the winter conferences

• More interesting high pT physics to come:

Stay Tuned…



Event Selection• event-shape variables ( hadronic channel)

ttbarQCDdata

Sphericity Aplanarity

Thrust

Sphericity

Oblateness

Documents

top