Top Quark Mass Measurement at CDF Run II

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Top Quark Mass Measurementat CDF Run II

• Jean-Francois Arguin
• University of Toronto

• HEP Seminar
• University of Toronto
• February 1st, 2005

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Outline

• Introduction to top quark physics
• Why measuring Mtop?
• Experimental setup
• Mtop measurement using kinematic information
• Reducing the systematics
  calibration
• Conclusion and Outlook

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The Top Quark

• Discovered only recently (1995 at CDF, DØ)
• No real surprise existence is required for
  viability of the Standard Model (SM)
• Guarantee absence of triangle anomalies
  renormalizability
• Absence of FCNC, e.g.
• Agreement with measured decay rate

Most striking characteristics huge mass!
How large? 40 times Comparable to gold
nucleus
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Top Production and Decay

• Concentrate on QCD production (also single-top
  EWK production exists)
• Production at Tevatron
• 85
• 15
• Decay in SM
• Thus,

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Top Production and Decay

• W bosons decay either hadronically or
  leptonically.
• W decays define channel
• Dilepton 5
• Leptonjets 30
• All-hadronic 44
• In this talk always
• leptonjets channel

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Top Quark Physics

• Large mass ?very different behavior than other
  quarks
• Only known particles decaying to a real boson
• So short-lived
• (tt5x10-25sec)
• ? It decays before hadronizing!
• Last feature has interesting
• experimental consequences
• can observe bare quark!

Top quark measurements
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Motivation for Mtop inside SM framework

• At tree level, EW theory
• depend on 3 quantities, e.g. can choose
• Fine structure constant a
• Fermi constant GF
• MZ
• However, radiative corrections
• must be included

• Example parameter ?
• Receives corrections dominated by Mtop
• quadratic dependence on Mtop!

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Motivation for Mtop inside SM framework

• Many precision EWK measurements depend on Mtop
• Therefore, high accuracy Mtop measurement is
  crucial for
• Consistency check of SM
• Constrain unknown model parameters ? MHiggs
• Sensitivity to new physics

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Motivation for Mtop beyond SM

• Top quark mass so large, can it be a special
  particle?
• Beyond SM top quark plays a role in many
  theories
• mSugra Large Mtop provoke electroweak symmetry
  breaking (EWSB) at the weak scale
• In some dynamical EWSB theories (topcolor) top
  quark sees additional gauge interaction,
  resulting top condensates responsible for EWSB.
• MSSM in a similar situation than SM superpartner
  are constrained by Mtop
• Precise measurements of the top quark
• mass will help constrain these theories

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Summary of Mtop Measurements

• Direct Mtop meas. In Run I world ave.
• Higgs mass fit
• Run II Goals

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The Tevatron

• Proton-antiproton collisions at
• Run II c.m. upgrade (1.8 TeV to 1.96 TeV)
• ? 35 increase in

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The CDF Detector

• Calorimeters
• Central, wall, plug calorimeter
• Coverage
• EM reso.
• HAD reso.

• CDF II general purpose
• solenoidal detector
• 7 layers of silicon tracking
• Coverage
• B-tagging eff. 30
• COT drift chamber
• coverage
• Resolution
• Muon chambers
• Scintillator, proportional chamber interspersed
  with absorber
• Provide muon ID up-to

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Challenges of Mtop Measurement

• Statistical limitations
• Small statistics 25 b-tag leptonjets ev. / 100
  pb-1
• Complicated final state to reconstruct
• Observed final state
• Complications
• 12 possible jet-parton assignments
• B-tagging helps a lot
• Neutrino momentum diluted by missing ET
  measurement
• Jet resolution is poor (130/ET-1/2)

• Systematics limitations
• Currently completely dominated by jet energy
  scale uncertainty
• Current understanding set a lower limit of 3 GeV
  on the top mass uncertainty

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The Kinematic Analysis

• Advantage
• Simple, robust
• Less assumptions
• Published Run I ?baseline for Run II
• Disadvantage
• Use of information not optimal

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Selecting Leptonjets Events

• Final state
• Lepton selection
• One isolated e or µ ET (pT)gt20GeV with ID
• Electron EM cluster with expected shower shape,
  matched track
• Muon track matched to muon stub, expected
  deposited E in calo.

• Large missing ETgt20GeV
• Measured in calorimeters
• Infer pT(neutrino)
• gt4 jets
• Reconstructed with cone algorithm from
  calorimeter towers
• ETgt15 GeV (8 GeV on 4th jet)

After these selections S/B 1
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B-Tagging and Sample Division

• B-tagging
• Require B-tagging
• S/B 3 (gt1-tag)
• S/B 20 (2-tags)

• Leptonjets sample division
• Consider four subsamples
• ? Better statistical power by fitting events
  with different sensitivity to Mtop

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Top Quark Mass Reconstruction

• Chi-square kinematic fit

• Keep combination with lowest ?2
• B-tag improves a lot Mtop resolution

More correct combinations with b-tags!
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Mtop Templates

• Histograms are fitted with analytic functions
  parameterized by the true top mass
• Obtain probability density functions of
  reconstructed top mass as a function true Mtop

Reco. Mass and p.d.f.s vs true Mtop
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Background contamination
Background templates

• Background sources
• gt1-tag events
• Wjets mistag
• QCD multijets
• Wheavy flavor
• Others (WW/WZ, single top)
• 0-tag events
• QCD multijets
• Wjets

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Extracting Mtop

• Unbinned likelihood fit
• Compare data reconstructed mass distributions
  with p.d.f.s extracted from Monte Carlo
• Background constrained to expectations
• Combined fit multiply subsamples likelihood

• Likelihood fit sanity check

Unbiased fit!
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Systematic Uncertainties
JES dominates!

• Systematics sources
• Jet energy scale Completely dominates (come back
  later)
• ISR, FSR confuse jet from top decays with
  initial and final state radiation jets
• Other syst. PDFs, generators, background
  modelling, jet resolution are small

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Results on Data

• Fit applied to data
• Final result

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Dominant Systematics Jet Energy Scale (JES)

• Are jets observed in data well-modeled by MC?
• Source of uncertainties
• Fragmentation governed by non-perturbative QCD.
  Affect JES
• Particles momentum distribution
• Out-of-cone energy
• Detector response
• Non-linear for hadronic particles in CDF
• Underlying event
• deposits energy in jet cone

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Possible solution calibration

• JES, future limitation for Mtop
• Uncertainties from JES already dominate
  statistical uncertainty
• Knowledge of JES will improve, but has
  limitations
• Understanding of QCD
• LHC top measurements will be even more
  compromised by JES

• Extrapolation to ttbar events
• Standard JES calibration requires extrapolation
  to ttbar environment
• Calibration samples Photonjet, dijet, min. bias
• Differences with ttbar Jet flavor composition,
  Q2 scale, color flow, less busy

Fully in situ JES calibration using can provide
an elegant solution!
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Principles Calibration

• How does it work?
• Identify jets coming from W
• Reconstruct their invariant mass mjj
• mjj strongly dependent on JES (GeV)
• MW uncertainty is completely negligible (lt 50
  MeV)
• mjj mostly independent of Mtop

• How does it solve the two previous problems?
• No additional uncertainty from extrapolating JES
  from photon-jet, dijet events to ttbar
• JES uncertainty becomes statistical, will scale
  with luminosity!

mjj distribution can be used to constrain
jet energy scale!
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Reconstructing

• How to reconstruct Mjj?
• Which jet come from W?
• No ambiguity when 2 b-tags
• Otherwise keep all possible mjj and consider
  them equally
• This method works well because
• Less combinatorics for mjj than Mtop
• 25 better uncert. than kinematic fit
• More correct comb. considered
• More sensitivity to JES

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Adding a Dimension to the Measurement

• Show 2-D templates Mtop, JES

• Extend the machinery
• Determine simultaneously Mtop and JES
• mt and mjj templates
• Each template depend on Mtop and JES
• For optimal constraint on JES combine a priori
  uncertainty and

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Constraint by

• mjj templates
• Approximately independent of Mtop
• Measuring mjj gives constraint on JES independent
  of true top mass
• mjj provides an orthogonal determination of JES!

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B-jets Energy Scale

• Can we assume b-jets energy scale be set by
  W-gtjj?
• Estimated differences between W-jets and b-jets
• Harder fragmentation ?Mtop 0.2GeV/c2
• Semileptonic decays ?Mtop0.4GeV/c2
• Color flow ?Mtop0.3GeV/c2
• Additional uncertainties arising from b-jets
  is 0.5 GeV/c2
• Check through future in situ measurements of

LEP constraints on B-quark Fragmentations
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JES measurement with

• Mjj will be use for two purposes
• Independent cross-check of JES
• Calibration in Mtop measurement
• If cross-check is satisfying, will combine
  information on JES
• improve s(JES) by 30!

s(JES) with addition of
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Mtop measurement using

• Combine fit of mt and mjj improve s(Mtop) by 15!
• Future of the technique
• Both stat. and JES uncert. will scale with
  luminosity
• Expect ?Mtop2-3GeV/c2
• with
• reach CDF goals

Total Mtop uncertainty with of combined fit
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Conclusion

• Top quark mass measurement is one of highest
  priority of Tevatron
• Related to Higgs mass through radiative
  corrections
• Important role for beyond SM physics

• Top mass measurement is a complicated task
• Few events available
• Event topology is complicated
• Large uncertainty from jet energy scale
• We demonstrated
• Kinematic analysis provide robust Mtop
  measurement
• calibration can provide crucial
  improvement in understanding of jet energy scale
• very important for the future of the
  measurement at the Tevatron and LHC

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More Research Achievements

• Many services to the CDF collaboration
• Develop improved top-specific jet corrections
  (original work)
• Develop underlying event and multiple interaction
• Participated to CDF primary vertex finder
• Develop conversion photonjet balancing technique
  (original work)

• Responsible for the jet reconstruction software
• Largely responsible for jet corrections software
  structure
• Maintain and develop many pieces of code
  (including calorimetry)
• Wrote calorimeter software documentation
• Plus
• Beauty, charm physics experience from BaBar

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Potential for Physics Outreach

• Significant TA experience
• Significant undergraduate student supervision (in
  HEP context)
• Educational experience outside
• Graduate formation effective communication for
  physicists
• Strong communication skills