Search for New Phenomena in the CDF Top Quark Sample

1
Search for New Phenomena in the CDF Top Quark
Sample

• Kevin Lannon
• The Ohio State University
• For the CDF Collaboration

2
Why Look in Top Sample?

• Top only recently discovered
• Top turned 10 in 2005
• Samples still relatively small
• Still plenty of room for unexpected phenomena
• Top is really massive
• Comparable to gold nucleus!
• Yukawa coupling near unity
• Special role in EWSB?
• Many models include new physics coupling to top

5 orders of magnitude between quark masses!
3
What Might We Find?

• Its not Standard Model top at all!
• Charge not 2/3? Phys.Rev.D59091503,1999
  Phys.Rev.D61037301,2000
• Spin not 1/2?
• Its not only Standard Model top
• Additional heavy particles decaying to high pt
  leptons, jets and missing energy (t )
  Phys.Rev.D64053004,2001 Phys.Rev.D65053002,200
  2
• Heavy resonance decaying to tt Phys.Lett.B266419
  ,1991
• t?Hb
• ttH production Phys.Rev.D68034022,2003
• Nothing but the Standard Model . . . .
• Not as bad as it sounds
• Test our abilities to calculate signal and
  background properties
• Important at the LHC ? top becomes background to
  other searches
• Constrains models that put new physics in the top
  sample

hep-ph/0504221
4
The Tevatron and CDF

• Tevatron accelerator
• Highest energy accelerator in the world (Ecm
  1.96 TeV)
• World record for hadron collider luminosity
  (Linst 2.72E32 cm-2s-1)
• Only accelerator currently making top quarks

Muon Detectors
Central Cal
Plug Cal

• CDF Detector
• Trigger on high pT leptons, jets and missing ET
• Silicon tracking chamber to reconstruct displaced
  vertices from b decays

CentralTracker
Silicon Tracker
5
Tevatron Performance
Integrated Luminosity
Peak Luminosity
Todays Presentation 200 pb-1 1 fb-1
Analyzed by Summer

• Integrated luminosity at CDF
• Total delivered 2.3 fb-1
• Total recorded 1.9 fb-1 ( 17? Run I!)
• So far for top analyses, used up to 1 fb-1
• More analyses with 1.0-1.2 fb-1 in progress for
  winter and spring
• Doubling time 1 year
• Future 4 fb-1 by 2007, 8 fb-1 by 2009

6
Triggering on Top

• Need high efficiency, low fake rate trigger for
  high pT leptons
• Relies on track trigger (XFT)
• Fake rate increases with occupancy
• Occupancy increases with luminosity
• 3x higher than original design because Tevatron
  didnt reduce bunch spacing (392 ns ? 132 ns)

Z ? ee at low lum.9 add. Int./crossing
Fake tracks can be made from segments of
different real physical tracks.
Instrumenting additional layers reduces fake
rate. Efficiency stays high.
fake
Reduction factor 4
Missing segments

• Upgrade put into operation in October
• Efficiency 96 for high pT tracks
• Fake track rejection factor 5-7

Trigger ? for muons without upgrade
7
Top Quark Production at Tevatron
(and LHC)

• QCD pair production
• ?NLO 6.7 pb
• First observed at Tevatron in 1995

85
15
833 pb
87
13
s-channel
t-channel

• EWK single-top production
• s-channel ?NLO 0.9 pb
• t-channel ?NLO 2.0 pb
• Not observed yet

10.6 pb
247 pb
Associated tW
???

• Other?

62 pb
8
Top Production Rates
Needle in haystack (approx.)

• Efficient Trigger
• 90 for high pT leptons
• Targeted event selection
• Distinctive final state
• Heavy top mass
• Advanced analysis techniques
• Artificial Neural Networks

• Like finding a needle in a haystack . . . .

One top pair each 1010 inelastic collisions at ?s
1.96 TeV
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SM Top Quark Decays
BR(t?Wb) 100

• Particular analysis usually focuses on one or two
  channels
• New physics can impact different channels in
  different ways
• Comparisons between channels important in search
  for new physics

10
Top Signatures
Dilepton
Lepton Jets
All Hadronic
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Top Event Yields

• To give an idea of CDF sample sizes . . . .
• Based on top cross section of 6.7 pb
• Background and signal numbers based on event
  yields from current analyses, scaled by
  luminosity
• Assume no changes in event selection, efficiency,
  etc.

Luminosity 1 fb-1 1 fb-1 1 fb-1 4 fb-1 4 fb-1 4 fb-1
Total Top Events 6700 6700 6700 26,800 26,800 26,800
Decay Mode Dil. L J L J (b-tag) Dil. L J L J (b-tag)
Before Event Selection 330 1985 1985 1325 7940 7940
Selected Signal Events 50 480 290 190 1910 1140
Expected Background 40 2290 160 150 9150 670

• LJ 2k signal events with 4 fb-1
  (signalbackground 1 5)
• LJ (b-tag) 1k signal events with 4 fb-1
  (signalbackground 21)

12
Searching for New Physics

• Precision study of top properties
• Non-SM behavior from top quark
• Evidence of something other than top in sample
• Direct search for new phenomena in top sample
• Resonant production
• Non-SM decays
• New particles with top-like signature
• New particles produced in association with top

Vtb
13
Top Properties Working Group
Vtb

• Studying all properties of top quark (except
  mass)
• 150 faculty, postdocs, students
• 15 papers (so far)
• 50 active analyses

14
Precision Study Cross Section

• Cross section
• Measured in different final states
• New physics can affect different final states
  differently
• Different techniques used in same final state
• Results combined at end for most precise answer
• tt production calculated to NLO
• Central value 6.7 pb 6.8 pb
• Uncertainties 5.8pb 7.4 pb
• For mtop 175 GeV/c2
• Combined result
• 7.3 ? 0.9 pb

NTop Nobs- Nbackground, or from fit
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Two Best Measurements

• Both in Lepton Jets Channel
• Vertex Tag (weight 0.50, pull 0.88)
• Uses b-tagging to increase ratio of signal to
  background
• Counting experiment
• Count Wjets events with a b-tag
• Subtract expected background
• Excess attributed to top
• Kinematic Artificial Neural Net (weight 0.32,
  pull -1.14)
• Uses kinematic variables to separate signal from
  background
• Combines several variables in a neural network to
  increase sensitivity
• Fit for the number of top events
• Does not use b-tagging (lower signal to
  background ratio)

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B-Tagging

• b-tagging Identifying jets containing a b quark
• Take advantage of long b lifetime
• Look at precision tracking information for tracks
  within jet
• Reconstruct secondary vertices displaced from
  primary
• Efficiency
• Per jet
• 40 for b jet
• 9 for c jet
• 0.5 for light jet

• Per event (tt )
• 60 for single tag
• 15 for double tag

17
Sample Composition
Number of events with an identified W ? 1 jets
695 pb-1
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Lepton Jets Vertex Tag Result

• One Tag HT Cut
• 8.2 0.6 (stat.) 1.0 (sys.) pb

• Two tags, no HT Cut
• Cross check
• 8.8 1.2 -1.1 (stat.) 2.0 -1.3 (sys.) pb

HT scalar sum of lepton, jet, and missing ET
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Using Kinematics to Identify Top

• Look for central, spherical events with large
  transverse energy
• Signal PYTHIA tt monte carlo
• Background ALPGEN HERWIG W 3p monte carlo

• Normalized to unit area
• HT ? scalar sum of lepton, jet, and missing ET
• Aplanarity uses lepton, jet and missing ET
• Max jet ? uses 3 highest ET jets all others use
  5 highest

20
Statistical Sensitivity

• Evaluate expected fit fractional error using
  MC-based pseudo experiments
• Single variable fits fit signal fraction using
  distributions of a single kinematic variable
• Plotted
• Points median fit fractional error
• Error bars 68 interval

21
Multivariate Approach Neural Nets

• Structure
• Composed of nodes modeled after neurons in
  nervous system
• Organized into layers
• Input layer initialized by input variables
• Hidden layer takes information from each input
  node and passes to output layer
• Output node new discriminating variable with
  range 0,1

• Training
• Neural net output determined by exposure to
  training data
• Iteratively adjust parameters to minimize error
• Training accomplished through JETNET
  program(Peterson et al. CERN-TH/7135-94)

7 kinematic variables ? 7 input nodes
Output noderange 0,1signal 1
1 hidden layer, 7 hidden nodes
Information flow
22
Statistical Sensitivity

• Evaluate expected fit fractional error using
  MC-based pseudo experiments
• Single variable fits fit signal fraction using
  distributions of a single kinematic variable
• NN fit NN output of data to NN templates
• Plotted
• Points median fit fractional error
• Error bars 68 interval
• NN Fit performs significantly better than single
  variable fits

23
Using NN to Fit Data

• Basic Approach
• Train NN to distinguish tt signal from
  backgrounds
• PYTHIA tt MC as signal model
• ALPGEN HERWIG W 3p MC as background model
• Use this NN to make templates for fitting the
  data
• Use same signal model as above
• Also extract QCD multijet template from data
• Supplement electroweak template with
  contributions from other processes WW,WZ, Z
  jets, single top
• Fit templates to NN distribution from data
• Binned maximum likelihood fit
• Three component fit
• Signal and electroweak float
• QCD constrained to value estimated using
  isolation vs missing ET method

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Lepton Jets Kinematic ANN Result
Sample Events Fitted tt ?(tt )
W ? 3 Jets 2102 324.6 ? 31.6 6.0 ? 0.6 ? 0.9 pb
W ? 4-Jet 461 166.0 ? 22.1 5.8 ? 0.8 ? 1.3 pb
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Kinematics of b-Tagged Events

• Looks like top!

26
Systematic Uncertainties

• Main Systematic Uncertainties uncorrelated
• Lepton Jets Vertex Tag
• b-tagging efficiency 6.5
• Background estimation 3.4
• Kinematic ANN
• Background shape modeling 10.2
• Jet Energy Scale 8.3
• For both results, uncertainty dominated by
  systematics
• Both are working to reduce for 1.2 fb-1
  publications

27
Search for t ?Hb
Phys.Rev.Lett. 96 (2006) 042003

• Compare top yield in four different channels
• Measurements consistent with SM
• Consider correlated effect of t?Hb decays on
  four channels
• Exclude when changes make expectation
  inconsistent with data
• Limits for 6 sets of MSSM parameters and less
  model-specific scenarios

Varying model parameters changes BR(t?Hb) BR(H
???) BR(H?cs) BR(H?tb) BR(H?Wh0) BR(H?WA0)
Shown here Variations as a function of tan?
particular set of MSSM parameters
28
MSSM Limits

• Calculate BR(t?Hb) and H BRs as a function of
  MH and tan(?)
• Use 6 different MSSM benchmarks
• Results for Benchmark 1 shown below

29
Less Model Dependent Limit

• Tauonic Higgs Model
• Assume BR(H???) 1
• i.e. MSSM with high tan(?)

• Worst Limit
• Find arbitrary combination of H BRs that give
  least stringent limit

30
t Production

• Consider possible contribution to top sample
  from heavier particles with top-like signature
  (t)
• Examples
• 4th chiral generation consistent with precision
  EWK data Phys. Rev. D64, 053004 (2001)
• Beautiful Mirrors Model additional generation
  of quarks that mix with 3rd generation Phys.
  Rev. D65, 053002 (2002)
• Consider decay of t?Wq
• Happens when mt lt mb mW
• Precision EWK data suggests mass splitting
  between b and t small
• Search for by fitting HT vs Mreco
• HT sum of transverse momenta of all objects in
  event
• Mreco Wq invariant mass reconstructed with a ?2
  fitter (same technique used in top mass
  reconstruction)

31
t Search Results

• No evidence for t observed
• Set 95 confidence level limits on ?t?BR(t?Wq)2

• Exclude mt lt 258 GeV for BR(t?Wq) 100
• Interesting behavior in high mass tails

32
Summary
There are many more CDF results than I could show
here.

• Even More results on the public webpage
• http//www-cdf.fnal.gov/physics/new/top/top.html
• No deviations from Standard Model so far
• Many results statistically limited
• More results with 1-1.2 fb-1 coming soon
• Results for 2fb-1 by this summer

• Many new and updated analyses in progress
• Improved cross section measurements
• Single-top
• Top charge
• Flavor changing neutral currents
• Direct search for t?Hb
• http//www-cdf.fnal.gov/physics/new/top/top.html

33
The Future Top at LHC

• Top physics will be easy at the LHC
• Top cross section increases by factor of 100
• Background cross sections increase by factor of
  10
• Probe for new Physics
• Mtt distribution
• Associated Higgs production ttH
• Even used for LHC detector calibrations
• High precision results from Tevatron important
• Discover new physics
• 1-2 GeV/c2 precision on mass
• Production and decay well understood

precision physics
34

Extra Slides
35
Top Cross Section vs Mass
36
Search for Resonant Production

• Motivation
• Some models predict particles decaying to top
  pairs
• Should be visible as resonance in tt invariant
  mass spectrum
• Example model Topcolor assisted technicolor
• Extension to technicolor that includes new strong
  dynamics
• Couples primarily to 3rd generation
• Includes new massive gauge bosons topgluons and
  Z

37
Search for Resonant Production

• Look for generic, spin 1 resonance (X0) decaying
  to top pairs
• Assume ?X0 1.2?MX0
• Test masses between 450 GeV and 900 GeV in 50 GeV
  increments
• Results
• No evidence for resonance
• Set 95 confidence level limit for ?X0 at each
  mass
• Exclude leptophobic Z with Mz lt 725 GeV

38
W Helicity in Top Decay

• Helicity of W determined by V-A structure of EWK
  interaction
• 70 longitudinal
• 30 left-handed
• Right handed forbidden

V-A Forbidden
W- Left-Handed fraction F-
W0 Longitudinal fraction F0
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W Helicity in Top Decay

• Can be tested by measuring W helicity angle ?
• ? angle of the lepton relative to negative the
  direction of the top in the W rest frame.
• Can also use Mlb2 ? 0.5(mt2-mW2)cos ?

40
W Helicity Results

• Two CDF results with 955 pb-1
• Use different kinematic fitters to reconstruct tt
  system cos?
• Very consistent measurements of F0 and limits on
  F
• F0 0.61 ?0.12(stat) ? 0.04 (syst) and F lt 0.11
  at 95 C.L.
• F0 0.59 ? 0.12(stat) 0.07-0.06(syst) and F lt
  0.10 at 95 C.L.
• One measurement with 750 pb-1
• Uses Mlb and measures fraction of VA
• FVA lt 0.29 at 95 C.L.
• Assuming F0 0.7? F lt 0.09 at 95 C.L.

41
Top Quark Lifetime

• Measure impact parameter of lepton from Lepton
  Jets top decay
• Evidence of displaced top suggests
• Production via decay of long-lived particle
• New long-lived particle in top sample
• Anomalous top lifetime

Templates for SM processes
Result c? lt 52.5 ?m at 95 confidence level
42
Sample Composition
Number of events with an identified W ? 1 jets
Event count before applying b-tagging
Wlight flavor From pretag using mistag matrix
Wheavy flavor From pretag using MC for HF
fraction and b-tagging eff.
Difference between observed and predicted
background attributed to top
Single Top and Diboson Estimated using
theoretical cross section
Non-W QCD Estimated from MET and lepton
isolation side-bands
43
The Search for Single Top

• Standard Model
• Rate ? Vtb2
• Spin polarization probes V-A structure
• Background for other searches (Higgs)
• Beyond the Standard Model
• Sensitive to a 4th generation
• Flavor changing neutral currents
• Additional heavy charged bosons
• W or H
• New physics can affect s-channel and t-channel
  differently

Tait, Yuan PRD63, 014018(2001)
44
Signal and Backgrounds
Backgrounds
Other EWK
Single-top Signature
tt
e or ? pT gt 20 GeV
? METgt 20 GeV
Multi-jet QCD
W Heavy Flavor
W Light Flavor (Mistags)
2 jets ET gt 15 GeV, ? 1 b-tag
Must use multivariate, kinematic techniques to
separate signal from background

• Total Background 646?96 events
• Expected Single-Top 28 ? 3 events
• Signal / Background 1/20

45
Multivariate Discriminants
ZOOM

• Improve signal discrimination by combining
  several variables into a multivariate
  discriminant
• Neural Network and multivariate likelihood
  function both used
• Variables l?b and dijet invariant masses, HT,
  Q??, angles, jet ET and ?, W-boson ?, kinematic
  fitter quantities, NN b-tag output

46
Single Top Multivariate Likelihood Result

• Best fit result for s- and t-channel separately
• s-channel
• t-channel
• 95 CL upper limit on combined s- t-channel

47
Single Top Neural Network Result

• Separate search
• s- and t-channel vary separately
• Best Fit
• t-channel
• s-channel
• 95 CL Limit
• t-channel
• s-channel

• Combined search
• s-channel t-channel combined in SM ratio
• Best fit
• 95 CL Limit

48
Single Top Matrix Element Result
49
Summary

• This is an exciting time to be at the Tevatron
• 1.2 fb-1 sample currently in hand and being
  analyzed
• Top sample has grown from 30 events in Run I to
  several hundred
• Larger samples coming soon (almost 2 fb-1) by
  summer
• Analysis techniques becoming increasingly mature
  and sophisticated
• Look forward to ? 1 fb-1 publications this winter
• No evidence for new physics in top sample so far
• Have many more top measurements than covered in
  this talk (see CDF public results webpage)
• Increasing precision continues to test
  consistency of measurements in different channels
• Many new analyses on their way (as well as
  updates of current results)
• Improved cross section measurements
• Single-top
• Top charge
• Flavor changing neutral currents
• Direct search for t?Hb