Top Production Cross Section from CDF

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Top Production Cross Section from CDF

• Anyes Taffard

University of Illinois
On behalf of CDF Collaboration
32nd International Conference on High Energy
Physics
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Why Study Top Quark Physics ?

• Experimentally, still know very little about the
  top quark
• Run I 110 pb-1, 100 top candidates
• Overall consistency with SM prediction, statistic
  limited
• Only know fermion with mass near electroweak
  scale
• Theoretical models proposed to solve the problems
  of the SM often have top playing a leading role
• SUSY Large top mass causes EWSB
• In many dynamical symmetry breaking models, top
  interactions are modified (e.g technicolor)
• Probe for physics beyond the SM
• Non SM production (X?tt, X?lljetsET)
• Non-SM decay Heavy enough to decay to exotic
  particles (t?Xb)
• On-sheel charged Higgs, SUSY
• Signal of today, background of tomorrow

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Tevatron CDF

• Proton-antiproton collisions
• ?s1.96 TeV
• Main injector
• 150 GeV proton storage ring

• New DAQ
• New Track Trigger
• New Silicon ?lt2
• Improved b-tagging
• New Drift Chamber
• New Plug Calorimeter
• Increase acceptance
• Upgrade Muon Detectors

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Tevatron CDF Luminosity

• Record Peak Luminosity
• July 16, 2004 10.3 x 1031 cm-2sec-1
• With mixed pbar (recycler accumulator)

Acquired luminosity in 2004 already surpassed
2003 total CDF 450 pb1 total on tape In this
talk160 pb-1 lt?Ldt lt200 pb-1
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Top Quark Production Decay

• Pair Production
• 85 qq, 15 gg (fractions reversed _at_ LHC)
• Central, spherical events
• Large transverse energy
• Cross section increases ?30 with Tevatron ?s
  increase to 1.96 TeV
• Single top production is a factor of 2 smaller

• BR(t?Wb)? 100
• Both Ws decay via W?l? (l e or ? 5)
• final state l? l? bb dilepton
• One W decays via W?l? (l e or ? 30)
• final state l? qq bb leptonjets
• Both W decays via W?qq (44)
• final state qq qq bb all hadronic

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Measuring tt Cross Section

• Starting point for all top physics
• Events triggered on one high momentum lepton or
  multi-jets
• Optimized event selection for top physics and new
  physics
• Define top sub-samples by counting lepton and
  jets
• ? 2 jets in dilepton channel
• ? 3 jets in ljets channel
• ? 6 jets in all hadronic channel

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Dilepton Channel

• Small sample, but highest S/B
• Backgrounds
• Z/g ?ll-
• WW, WZ, ZZ
• Wjets (fake leptons)

Background can be further reduced with an HT
cut. Ht Scalar summed ET of jets, leptons, and
missing ET
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Counting Experiments
Standard Run I method (ee,mm,em)
Looser selection e/m track

• Lower Purity, higher acceptance (20 from t)

• Higher purity, lower statistical significance

13 candidates 1 ee, 3 mm, 9 em
1st Run II paper Combined Result hep-ex/0404036

• With higher statistics in Run II
• observe good agreement with SM

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Inclusive Dilepton Analysis

• No cuts other than two identified leptons
• If same flavor, Z ?ee-/mm- dominates require
  significant Etmiss
• Advantages increase acceptance
• Goal search for new physics
• Fit data to tt,WW, Z ?tt- contribution in 2D
  (Etmiss, Njet) plane

Top Cross Section
WW Cross Section
J.M.Campbell and R.K.Ellis, Phys.Rev.D60 113006
(1999)
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Ljets Channel
Candidate Event Display

• Larger sample due to BR, but less pure
• Improve SB by requiring b-tagging
• 2nd vertex

m
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Using Vertex Tagging

• B decay signature displaced vertex
• Long life time c? 450 ?m
• Travels Lxy3mm before decay
• Top event tagging efficiency 52
• False tag rate per QCD jets 0.5

_at_least 1 b-tag HTgt200 GeV
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Using Double b-Tag

• Double b-tag event essential for top mass
  measurement reduces combinatorics
• Improve b-tagger
• Increase per jet tagging e 10.8 ? 12.0
• False tag rate x 3.4
• Significance up by 18
• tt expectation increases from 8.7 ? 13.0
• 3x more background

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Using Soft Lepton Tag Tagging

• B may decay semileptonically
• Leptons id challenges
• softer spectrum than leptons from W/Z
• non-isolated (cannot use calorimetry information)
• Id low pT muon
• Background dominated by fake tag
• Punch though, decay in flight
• Top event tagging efficiency 15
• False tag rate (QCD jets) 3

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Using Kinematics Fits

• Determined signal fraction using kinematics shape

Fit leading jet ET, require _at_ least 1 b-tag
Fit NN output (7 kin. var.)
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All Hadronic Channel

• Final State 4 jets from W, 2 b-jets (data
  sample multi-jets trigger)
• Large statistics, but huge QCD background
• SB?12500
• Increased SB by requiring
• ?ETgt320GeV
• Topological cuts (aplanarity, centrality)
• Event selection efficiency 6.2
• SB ? 124
• _at_ least 1 b-tag jet SB ? 14
• Measure Xs with ? 6 jets ? 1 b-tag jet

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Single Top

• Probe EW coupling, direct determination of Vtb
• Sensitive to new physics
• t-channel anomalous couplings, FCNC
• s-channel new charged gauge boson

• Strategy
• Isolate Wexactly 2 jets 1 b-tag jet
• Non-top 89 Wjets (62), false tags (25), QCD
  (10)
• Top 11
• Likelihood Fit to Ht (combined) to discover
• Likelihood Fit to Qh (t-channel) to see new
  physics
• Q of lepton, h of light quark jet

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Single Top cont.
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Summary

• x2 Run I dataset
• Observed consistent with SM prediction _at_
  mtop175 GeV/c2
• 1st paper out, more coming soon.
• Near future already x2 more
• data on tape