Top pair production cross section and branching ratio measurements

1
Top pair production cross section and branching
ratio measurements

• For the DØ Collaboration
• Elizaveta Shabalina
• University of Illinois at Chicago
• Joint Theoretical and Experimental Seminar
• Fermilab, 09/16/05

2
Outline

• Introduction
• Top pair production
• One lepton and jets
• Event kinematics method
• b-jet tagging method
• Br (t?Wb)/Br (t?Wq)
• Top pair production
• all hadronic
• two leptons and jets
• Conclusions

3
Top quark physics today

• The biggest accomplishment of Run I of Tevatron
  was the top quark discovery in 1995
• Run I cross sections with gt100 pb-1
• CDF D0
• Precision (25) was severely limited by
  statistics

• At present
• ?s 1.96 TeV - 30 higher production rate
• much higher luminosity
• Current goal deliver precision measurements
• Theoretical prediction of cross section 6.5
  accuracy
• Tev2000 study projected precision of ttbar cross
  section measurement
• Five WC seminars since June 1st were dedicated
  to top physics

1 fb-1 11
10 fb-1 6
4
Top cross section - motivation

• Important test of perturbative QCD
• Higher production rate e. g. ttbar resonances
  
• Measure in different decay channels
• Exotic top decays (to charged Higgs or light
  stop) different cross sections in different
  channels
• Dilepton to ljets cross sections ratio probes
  non-W boson top decays

• Measure with different methods
• b-jet tagging method normally assumes Br
  (t ? Wb) 1
• an implicit use of the SM prediction
  Vtb0.9990 ? 0.9992 (at 90C.L.)
• Topological method is free from this assumption
• Using both test of top decays without b
  quark in the final state

5
Top production

• Standard model pair production via the strong
  interaction
• Standard model electroweak production (single top)

Discovered in Run I
? 6.77 0.42 pb for mtop 175 GeV
To be observed in Run II
Top pair production is one of the main
backgrounds to single top
6
and decay

• Very short lifetime ? decays as a free quark
• Br (t ? Wb) ? 100

• W decay modes determine top quark final state
• Dilepton (ee, µµ, eµ)
• Both Ws decay leptonically
• BR 6
• Lepton (e or µ) jets
• One W decays leptonically, another one
  hadronically
• BR 34
• All-hadronic
• Both Ws decay hadronically
• BR 46
• thad X
• BR 14

Leptonjets
Leptonjets
7
DØ detector
All detector subsystems are important for high
quality top quark measurements

• Electrons - energy clusters in EM section of the
  calorimeter and track in the central tracking
  system
• Muons - track segments in muon chambers and
  track in the central tracking system
• Jets - clusters of energy in EM and hadronic
  parts of calorimeter
• Jet tagging requires tracks

8
Electron and muon identification

• Electron
• High fraction of energy in the EM calorimeter
• Isolated in calorimeter
• Transverse and longitudinal shower shapes
  consistent with those expected for an electron
• Matched central track
• High value of electron discriminant (tracking and
  calorimeter information combined)

• Muon
• segments in muon system matched inside and
  outside of the toroid
• Non-cosmic (based on timing from associated
  scintillator hits)
• Matched central track
• Isolated in calorimeter and in the tracking
  system

loose
loose
tight
tight
Loose and tight lepton quality is used to
determine backgrounds
9
Leptonjets channel

• Signature

• Selection
• One isolated lepton (pTgt20 GeV e ?lt1.1, µ
  ?lt2)
• At least four jets (pTgt20 GeV, ylt2.5)
• gt20 GeV and not collinear with lepton
  direction in transverse plane
• Features
• Relatively high Br
• Manageable background
• Perfect for studies of top properties

jet
_
?
_
?
p
b
jet
jet
jet
10
Sample composition
Triggered data
Wjets
Multijet background
ttbar
Loose leptonic W selection ?4 jets
Tight leptonic W selection ?4 jets
Multijet
Loose to tight lepton
Ntight
Nloose
Combine topological event information into a
discriminant and perform fit to the data
ttbar
11
Discriminating variables
Provide the best separation between ttbar and
Wjets and the least sensitivity to the dominant
systematics
Top events are

• Energetic ? HT
• Central
• Centrality HT/H
• Spherical
• Aplanarity (large A ? spherical events)
• Sphericity (large S ? isotropic events)
• kTmin measure of minimum jet pT relative to
  another
• Use only 4 highest pT jets

12
Discriminant function
For uncorrelated variables

• Kinematic properties of multijet background are
  similar to Wjets after selection
• Use only ttbar and Wjets to build discriminant
  
• Extract Nttbar, Wjets and multijet events in
  the sample from fit to discriminant distribution
  in data

13
Results of the fit
14
Results combined
For leptonjets channel combination minimize the
sum of negative log-likelihood functions for
individual channels
240 pb-1
Sample composition 38 ttbar 44 Wjets 18
multijet
Statistical and systematic uncertainties are
comparable
hep-ex/0504043
combined _at_ m_top 175 GeV
15
Event kinematics
background dominated
signal dominated
Dlt0.5
Dgt0.5
16
Systematic uncertainties ??tt(pb)
By far the largest systematic uncertainty comes
from the Jet energy calibration, 90 of total
error
17
Leptonjets channel with b-tagging
b-tagging

• Selection as in topological analysis but
• Relax cut on jet transverse momentum pT gt 15 GeV
• Use events with njet?3
• Use events with one and two jets as control
  samples for background estimation
• Signature of a b decay is a displaced vertex
• Forms long lifetime of B-hadrons (c? 450µ)
• B-hadrons travel Lxy 3mm before decay with
  large charged track multiplicity

Wjets
ttbar

• event has 2 b-jets
• b-jets in background processes are rare
• Use this feature to discriminate signal from
  background
• Dramatically improves signal-to-background ratio

18
b-tagging algorithm - SVT

• Reconstructs secondary vertex
• ?2 tracks with pT?1GeV, ?1 SMT hit, impact
  parameter significance gt3.5
• Remove tracks associated with K0S, ?0 and photon
  conversions (? ? ee-)
• Positive tag
• Secondary vertex within a jet with a decay length
  significance Lxy/?Lxygt7
• Negative tag
• Secondary vertex within a jet with a decay length
  significance Lxy/?Lxylt?7 (due to resolution
  effects)

Impact parameter
K0S
19
Tagging rates

• b-tagging efficiency
• Measured in dijet data events for jets with muon
  inside
• Compare two samples with different heavy flavor
  content
• Tag jets with two tagging algorithms SVT and SLT
  (SLT soft muon with pTrelgt 0.7 GeV inside a
  jet)
• Solve system of 8 eqs in bins of jet pT and y to
  extract semileptonic b-tagging efficiency
• Use MC to correct measured efficiency to
  inclusive one

• Light tagging rate
• Measure negative tagging rate in dijet events
  (low missing ET)
• Correct for long-lived particles in light jets
• Heavy flavor contribution in dijet events
• c-tagging rate
• From MC corrected with the SF derived for
  b-tagging

20
Backgrounds

• Calculate QCD (non-W) contribution from Matrix
  Method
• Subtract small backgrounds (single top, diboson,
  Z???) using known cross sections
• Predict number of background events after tagging
  
• Interpret excess in observed tagged events with
  ?3 jets over predicted background as ttbar
  signal

small bkgr
Matrix method
21
Event tagging probability

• For Wjets, use the ALPGEN MC to estimate the
  fraction of the different Wheavy flavor
  subprocesses
• Limited knowledge of heavy flavor fractions is
  one of the largest sources of systematics

• Use MC to calculate event tagging probability
• Rely only on the flavor composition of the jets
  in the final state and overall event kinematics
• Apply the tagging rates measured in data to each
  jet in MC based on its flavor, pT and y

Wjets, average ttbar
4j, 1 tag 4 44
4j, 2 tag 0.4 15
22
Results
DØ RunII Preliminary, 363pb-1
for ?tt 7 pb
1 tag
2 tags
3j, 1tag 3j, 2tag 4j, 1tag 4j, 2tag
Expected bkg 71 9 7 1 22 3 1.50.3
S/B 0.6 1.6 2.3 12
Observed events 121 11 88 21
Background dominated
23
Kinematics of llets tagged sample
DØ RunII Preliminary, 363pb-1
24
Result and systematic uncertainties

• Perform fit in 8 channels
• Gaussian term for each source of errors (nuisance
  parameter method)
• Each source affects the central value of the
  cross section

• Results is combined statistical and systematic
  error
• Refitting after fixing all but one Gaussian term
  to obtain error from one source

363 pb-1

• Systematic and statistical uncertainties are the
  same 11
• Main sources
• JES and jet ID
• B-tagging efficiency in data
• W fractions
• Luminosity

25
Probing the assumption Br(t?Wb)1

• q b, s or d-quark
• CKM matrix element Vtb0.9990 to 0.9992 _at_90
  C.L.
• R0.9980 to 0.9984. True in SM
  assuming three generations of quarks
• For expanded CKM matrix Vtb0.07?0.9993 _at_90
  C.L.
• Measure of R to test SM
• CDF measurement
• 162 pb-1

26
Method

• Split selected sample into 3 categories 0,1 and
  ?2 tags
• Predicted number of ttbar events depends on R
• Fit R and ?tt from the number of observed tagged
  events and the event kinematics in 0 tag sample
  

• Compute probabilities to observe 0, 1 and ?2 tags
  for each final ttbar state
• Combine to obtain
• Pn-tag(R), n-tag0, 1, ?2
• Use topological discriminant in 0 tag sample with
  ?4 jets to determine ttbar content

2 b-jets
1 b, 1 light jet
2 light jets
light quark
27
Fitting procedure

• Perform binned maximum likelihood fit to data in
• 10 bins of discriminant of ljets 0 tag, Njet?4
• 2 bins of ljets 0 tag, Njet3
• 4 bins of ljets 1 tag, Njet3, ?4
• 4 bins of ljets 2 tag, Njet3, ?4
• Nuisance parameter method to include systematic
  uncertainties (similar to ljets/btag analysis)

Njet3
Njet?4
Br(t?Wb)1 and ?tt7 pb
28
Result
The most precise measurement to date
Model independent measurement

• Potential for improvement
• include dilepton events
• higher b-tagging efficiency

Statistical uncertainty dominates 90 of total
error on R
29
All hadronic channel

• Selection
• Njets ? 6, pTgt15 GeV
• Suppress multiple interactions
• Reject events with gt1 hard primary vertices gt3 cm
  apart
• At least 3 jets assigned
• Jet is assigned to PV if ?2 tracks from it come
  from PV
• Removes 32
• Reject bb background
• ?R(tagged jets)gt1.5
• Selected sample contains 0.3 of ttbar

• Signature 6 jets, 2 b-quark jets
• All decay products should be visible in the
  detector, no energetic neutrinos produced
• Six jet multijet production rate is many orders
  of magnitude larger than ttbar
• Impossible to extract signal without tagging
  b-jets SVT algorithm is used

30
Background estimate

• Select variables discriminating signal from
  background
• Avoid JES dependent variables
• Use the smallest possible number of input
  variables
• Optimized for the smallest systematic error
• Combine into Neural Network (NN)

• Use untagged selected sample
• Apply TRF (tag rate function) to predict
  background after tagging
• Derive TRF in selected sample in 4 bins of HT
  0?200, 200 ? 300, 300 ? 400, ?400 GeV
• Parameterize as a function of jet pT, ?, f,
  position of primary vertex along the beam

TRF 300ltHTlt400
3
31
Discriminating variables
NN
Variables are designed to address different
aspects of the background

• Energy Scale HT
• Event Shape aplanarity
• Soft non-leading Jets ET56 geometric mean of
  the transverse energies of the 5th and 6th
  leading jet

• Rapidity lth2gt - weighted RMS of h of 6 leading
  jets
• Top Properties
• Mmin3,4 the second smallest dijet mass
• Mass likelihood, ?2-like variable calculated from
  MW, ?W, ?top

32
Cross section calculation

• Background was estimated on the sample containing
  signal ?? correct cross section
• ?TRF probability to tag ttbar MC event using
  TRF
• ?btag probability to tag ttbar event using b,c
  and light tagging rates

?TRF 0.1250.002
?btag 0.600.01
?TRF/?TRF 0.2070.004
Signal region
33
Result
At mtop 175 GeV, 350 pb-1
New measurement
Potential for improvement optimize use of double
tagged events
JES error dominates 70 of total systematic
error
34
Dilepton channels
p
?
p
Presented at WC seminar on July 8th
?
Small branching fraction, small backgrounds
35
Dilepton channels

• Selection
• At least two jets (pTgt20 GeV, ylt2.5)
• Two charged opposite sign leptons (pTgt15 GeV e
  ?lt1.1 or 1.5lt?lt2.5 µ ?lt2)
• and further selections are optimized for
  each channel to account for difference in
  backgrounds in resolution

• Physics backgrounds
• Leptons from W/Z decay and missing ET from
  neutrinos WW/WZ, Z/?????ll
• Estimated from MC
• Instrumental backgrounds
• jet or lepton in jet fakes isolated lepton (QCD,
  Wjets)
• missing ET originates from resolution effects,
  misreconstructed jet or lepton or noise in
  calorimeter (Drell-Yan processes Z/??ee(µµ) (eµ
  channel is not affected)
• Estimated from data

p
?
36
Dilepton events properties
eµ
Electron likelihood distribution for data events
after full selection
combined
for ?tt 7 pb
37
Summary of channels
38
Combined result
ttbar signal
Background control bin
370 pb-1
Signal significance 4.8?
combined dilepton _at_ m_top 175 GeV
39
Systematic uncertainties ??tt(pb)
Comparable contributions from all sources
Still statistic error dominates 25
systematics 12
40
Summary
hep-ex/0505082, 230 pb-1
Accepted for publication in PLB
hep-ex/0504058, 230 pb-1
Best precision 16 ljets/btag at 363 pb-1
Work in progress on combination of the latest
results up to 370 pb-1

• What improvement do we expect from combination?
• CDF best result ljets/btag 16 total
  error (318 pb-1)
• CDF combined up to 350 pb-1 13 relative error

41
From TeV2000 to reality

• Do we meet expectations?
• For 363 pb-1
• predicted 180 b-tagged events (scaled from 500
  per fb-1)
• Observed 140 (241 tagged event, 101 expected
  background)
• We are close to the expected performance

• Can we do better?
• Data quality
• Improved calorimeter calibration
• Improved performance of SMT is crucial (Layer 0
  to be installed during the shutdown)
• Improved simulation
• Increase acceptance
• Better tools
• Neural network lifetime b-tagger is almost ready
• Fighting major sources of systematic
  uncertainties

42
Glance into the future
Total projected error / exp on ljets/btag
channel

• Assumptions
• Zero errors from limited MC statistics
• Luminosity dependent and constant terms
• JES
• b-tagging efficiency
• Lepton identification
• Limiting factors
• Luminosity (6.5)
• Heavy flavor fractions (6)
• Solutions
• Luminosity from Ws (2-3)
• Measure ratio of ttbar to W cross section
• Use large data sets to constrain model
  assumptions (W fractions, gluon radiation, )
• Combine channels

363 pb-1
43
Conclusion

• The precision of the latest top pair production
  cross section measurements rapidly approaches
  accuracy of theoretical prediction and will allow
  to sensitively probe the Standard Model
• With combination of measurements in different
  channels and using different methods we have an
  excellent opportunity to exceed the precision
  goal set by TeV2000 11 for 1 fb-1
• and the one for 10 fb-1 6 but with less
  luminosity!

This is a challenge but we are on track to make it