D Findings on the Top Quark

1
DÆ Findings on the Top Quark

• Boaz Klima
• Fermilab
• for
• The DÆ Collaboration
• Les Rencontres de Physique de la Valee DAosteLa
  Thuile, Aosta Valley
• Mar. 4, 1998

2
Outline

• Introduction
• Production Cross Section
• Dilepton and Lepton jets channels
• All jets channel
• Measurement of the Top Quark Mass
• Lepton jets events
• Dilepton events
• Other Studies
• Kinematic Properties of Top pairs
• Search for charged Higgs in Top decays
• Future Outlook
• Summary

3
Top Production

• At the Tevatron, top quarks are mainly
  pair-produced via
• Assuming standard model couplings,
• BR(t Wb) 100
• Categorize Channels by W Decay Mode
• W ln, lepton
• W qq, jets
• dilepton, lepton jets, all jets

4
A Brief History

• 1977-1992 A collection of null results
• 1992-1993 Interesting events show up
• e.g. DØs event 417 (e-m)
• 1994
• Mass limit mt gt 131 GeV/c2 _at_ 95 C.L.
• PRL 72, 2138 (1994) - DØ
• First Evidence
• Phys. Rev. D50, 2966(1994) - CDF
• - 2.8s excess

• Feb. 24, 1995 Discovery

PRL 74, 2626 (1995) - CDF
- 4.8s excess
-
PRL 74, 2632 (1995) - DØ
- 4.6s excess
-
5
Top Decay Channels

• Dilepton (2 isolated, high pT leptons, ³ 2 jets,
  ET)
• Branching Ratio 5 (emeemm)
• Small Background
• WW
• (for ee.mm Zl l-)
• QCD bb,cc l l-
• Fake leptons from mis-identification
• Leptonjets (1 isolated, high pT lepton, many
  jets (³ 3) , ET)
• Branching Ratio 15 per lepton
  flavor Dominant Backgrounds
• Wjets production
• Fake leptons from QCD multijet events
• Two Strategies
• Event Shape and kinematic cuts (topological)
• b m tagging (tag)
• All jets (Several jets (³ 6), 1 or 2 b-tags,
  event shape)
• Branching Ratio 44
• Backgrounds
• QCD Multijet Production

b-jet
W
W
b-jet
b-jet
jet
jet
W
W
b-jet
jet
b-jet
jet
W
jet
W
b-jet
jet
6
Dilepton Analysis
Other cuts for Z and Cosmic ray background
removal
7
en Analysis

• Selection

Results for òLdt 108 pb-1
l
4 events observed
Background 1.160.36 events

Expected top (events)
mt 170 GeV/c2 1.66 0.48

• Extends dilepton search for events which fail
  standard selection ( about 50 emee,
  33 ejets, 17 et)

8
Leptonjets Topological Analysis

• Event shape / kinematic variables
• Total leptonic ET ETL ETl ET
• Hadronic Activity HT S ETjet
• Large HT signifies the decay of massive objects
• Aplanarity (A)
• Mab Spapb/Sp2 momentum tensor for the
  W and jets
• A 3/2 x (smallest eigenvalue)
• A 1/2 for spherical events, 0 for planar events
• tt events are expected to be more spherical than
  those from radiative QCD processes

9
Leptonjets b-tag Analysis
e
jets/
m
m
E
(
e
p
(
m

e

h
(
m
) lt 1.7
E
gt 20
GeV
E
T
T
T

2
P
D
R(
m
,jet) lt 0.5
gt 4
GeV/c and

T
³ 3 jets with E
gt 20
GeV and
h
lt 2
T
T
10
Leptonjets Summary

• Results

Channel topological
b-tag ò L dt (pb-1)
110.0
106.6 Events observed 19
11 Background events 8.67
1.74 2.44 0.47
Expected top yields (events) mtop 150 GeV/c2
18.31 6.33 9.13
1.66 mtop 170 GeV/c2 14.14 3.08
5.83 0.98 mtop 190 GeV/c2
9.18 1.42 3.69 0.59
tt production cross sections from E. Laenen et.
al. Phys. Lett. 321B (1994) 254
11
All Jets Analysis

• Basic Selection 6 jets m -tag
• Compared to QCD multijets, top events are
• harder, more spherical (less planar), more
  central and stiffer non-leading jets
  i.e., jets 3,4,5

• Use two neural networks
• Kinematic variables input to 1st network
• HT Total scalar ET
• Ös Total invariant mass
• ET(1)/ HT ET fraction carried by leading jet
• A Aplanarity
• S Sphericity
• C Centrality HT / SE
• hRMS Weighted RMS in h
• h5 h6 áh2ñ of 5th 6th jets
• HT (3j) Total scalar ET of non-leading jets
• áNjñ Threshold weighted number of
  jets
• ÖET(5) ET(6) áETñ of 5th 6th jets

top
background
output of neural network 1
12
All Jet Analysis (Cont.)
NN 2
10 kinematic variables
NN 1

• Inputs to neural network 2
• Output of neural network 1
• pT of muon
• Variable sensitive to quality of constrained fit
  to any top mass
• Fisher discriminant sensitive to jet width
  (signalq, bkgndg)

13
Combined Fit to Data
(preliminary)

• stt 7.1 2.8 (stat) 1.6 (syst) pb
• at a top mass of 172 GeV/c2
• With a cut of 0.94 on network output
• 18 events remain in data
• 6.9 0.9 Expected Background
• 3s effect

14

Summary of DØ Cross Section Results
òL dt 125 pb-1
at mt 172 GeV/c2
(1) Published in Phys. Rev. Lett. 79, 1203 (1997)
(2) To be submitted for publication in PRD
(1998)
15
Top Cross Section Summary
Tevatron cross section average is unofficial
16
Measuring the Top Quark Mass

• mt is a Standard Model parameter
• Important measurement with mW to constrain mHiggs
  in SM
• Goal Determine mt
• as accurately as possible
  (optimal use of information)
• using as many decay modes as possible
• using several methods to cross check techniques,
  systematics, etc.
• Amazing amount of work done since the discovery
  in 1995
• Measurement significantly improved
• d mt(1997) / d mt(1995) 1/4
• mt is the best known quark mass today
• dmt /mt 4

17
Dilepton Mass Analysis

• Underconstrained Kinematics
• 2 unmeasured neutrinos in final state (0c fit)
• DÆ Strategy - Calculate a probability as a
  function of assumed top mass
• Matrix Element Weighting - Assume mt and solve.
  Assign a weight using PDFs and lepton Pts
• Neutrino Weighting - Assume mt and rapidity for
  each neutrino and solve. Assign a weight based on
  the agreement between the calculated neutrino Pt
  and the measured ET.
• Extended from previous work by
• K. Kondo, J. Phys. Soc. Jpn. 57, 4126 (1988)
  and 60, 836 (1991).
• R.H. Dalitz and G.R. Goldstein, Phys. Rev. D
  51, 4763 (1995).

18
Dilepton Top Mass

• Six events little background 1.4 0.5
• Four dimensional likelihood fit using four
  dimensional top and background templates

(GeV/c2)
mt 168.4 12.3 (stat.) 3.6 (syst.)
GeV/c2 Phys. Rev. Lett. 80, 2063 (1998),
hep-ex/9706014.
19
Lepton jets Mass Analysis in a Nutshell

• Selection - events with four or more jets
• 2C Kinematic fit to
• mfit , c2
• 12 (6) Permutations for untagged (tagged) events
• Compute signal probability for each
• event D
• 2D Likelihood fit in (D, mfit)-plane to signal
  background models
  L(mt)
• Quadratic fit to -ln L to extract mt and smt
• Signal and background models
• HERWIG MC (v 5.7) for tt
• VECBOS MC (v 3.0) for W jets background
• Data for fake lepton background

20
Event Selection

91 events selected 7 events b-tagged
Require fit c2 lt 10, select fit with smallest c2
Þ 77 events in data, 5 tagged
21
Multivariate Discriminants

• Variables
• x3 measures the centrality of the event
• x4 measures the extent to which jets are
  clustered together
• We combine the four variables into a multivariate
  discriminant (single variable) D 0
  - 1

22
NN Discriminant(DNN vs mfit )
Background
Signal (170 GeV/c2)
23
DNN vs mfit
Background
Signal 172 GeV/c2
Data
24
Leptonjets Top Mass
Background-rich

Signal-rich
LB Fit 174.0 5.6(stat.) GeV/c2 NN Fit 171.3
6.0(stat.) GeV/c2 Correlation between LB and
NN fits (88 4)
mt 173.3 5.6(stat.) 5.5 (syst.) GeV/c2
25
Systematic Errors
Source of uncertainty
Error in GeV/c2

• Jet Energy Scale 4.0
• Generator
• ISAJET/HERWIG 1.9
• VECBOS Models 2.5
• Multiple Interactions 1.3
• MC Statistics 0.8
• LB/NN Diff. 0.8
• Likelihood Fit 1.0
• Total 5.5

Combining leptonjets and dilepton top mass
results
26
Top Quark Mass Measurements Summary
Tevatron top mass average is unofficial
27
Top Quark Cross Section vs. Mass
DØ stt 5.6 1.8 pb (_at_mt 172 GeV/c2)
28

Mt vs. MW (DØ)
29

Mt vs. MW (world)
Direct vs. Indirect
30
Kinematic Properties of tt Pairs
mtt
2c
3c
Fix mt173 GeV
pTt
No evidence for new physics
31
Top Quark Rare Decays
Rare decays - SM and beyond Within Standard
Model t Wb g/g t Wb
Z Near threshold t Wb H0
Might be beyond threshold t W s/d
Measure CKM matrix element Beyond
SM t c/u g/g (FCNC) lt 1.4 / 0.3
t c/u Z (FCNC) lt 2 t c/u
H0 (FCNC) t H b (SUSY) lt
11 (SUSY)
32
Search for Charged Higgs in Top Decays
t Hb

• Motivated by SUSY models
• Assume BR(t Wb) BR(t Hb) 1.0
• If MH lt Mtop - Mb then
• t Hb competes with t Wb at low and high
  tanb, within the MSSM
• If one or both top quarks decay to Hb, then
  dilepton and leptonjets events can be used to
  set limits on BR(t Hb)
• At low tanb H cs at high tanb H tnt

33
Search for Charged Higgs in Top Decays
t Hb

• Acceptance for different channels

34
Exclusion region in M(H) vs. tan b plane
(Preliminary) Disappearance analysis in
leptonjets channels
35
Future Top Physics at the Tevatron (TeV-2)

• Tevatron Run 2 begins in the year 2000.
• Expect òL dt 2 fb-1 (20 times current data).
• Ös 2.0 TeV (35 increase in tt cross section
  relative to current Ös 1.8 TeV).
• Upgraded detector more efficient (SVX,t,)
• Expected top event yield about 40 times
  current sample

36
Future Top Physics at the Tevatron (TeV-2)

• B tagging efficiency (at least one b in top
  event)
• SMT 50-60
• SLT 30
• Total 80
• Improved measurements
• mt 1
• stt 8
• BR(t Wb)/BR(t Xb) 7
• ...
• New measurements
• Top quark width (using single top events)
• Vtb
• W polarization
• Top spin correlations

37
Summary

• Top quark has been observed by DÆ in many decay
  modes and its pair production cross section stt
  has been measured to be
• Top mass measurements make optimal use of
  information. Combining results from the lepton
  jets and dilepton channels mt has been measured
  to be
• No inconsistencies with Standard Model seen in
  top production and decays
• Search for Charged Higgs in top decays yields
  exclusion regions in M(H) vs. tan b plane
• The outlook for the future is bright and
  promising