Top production at the tevatron
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Top Production at the Tevatron. Daniel Sherman Harvard/CDF. Experimental Seminar, SLAC December 7, 2006. Top. Fermilab celebrated the 10th anniversary of the discovery of the top quark last year General picture from Run I is consistent with the Standard Model

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Top Production at the Tevatron

Daniel Sherman


Experimental Seminar, SLAC

December 7, 2006


  • Fermilab celebrated the 10th anniversary of the discovery of the top quark last year

    • General picture from Run I is consistent with the Standard Model

    • A few (very) subtle hints of new physics in the top sample

  • Top physics is certainly interesting enough to justify a closer look…

And If You Don’t Care…

Top is VERY rare.

(1 in 1010 collisions)


We’ll never find




all of these


Proton-antiproton collider

Two multi-purpose detectors

Collider Detector at Fermilab

Run I (1992-1996)

√s = 1.8 TeV

Integrated Luminosity 110 pb-1

Top Discovery in 1995

Run II (2001-present)

√s = 1.96 TeV

Collected almost 2 fb-1 so far

Tevatron Complex

The Fermilab Tevatron is the only place in the world to study top

(for the next year)

Top Production at the Tevatron

  • Top quarks are mostly produced (strongly) in pairs

    • 30% higher than in Run I

  • In the Standard Model, they are also produced one at a time

~7.5 pb



σSM ~ 3pb

Not Yet Observed

Accelerator Performance

3 top pairs/hr

1.0 fb-1

  • Tevatron is performing quite well

    • Nearly 2 fb-1 delivered in Run II

    • Typically recording 20-25 pb-1 per week (new Run I-sized dataset every month!)

    • 15,000 Run II top pairs per experiment

  • Record instantaneous luminosity (2.2 x 1032 ~ 1 top event every 10 minutes)

    • Shooting for 6-8 fb-1 by the end of 2008

  • This talk: 700 pb-1 (with silicon)

CDF II Detector Overview




  • Silicon Tracker

    • |h|<2

  • Drift Chamber

    • |h|<1

  • Solenoid

  • Electromagnetic and Hadronic Calorimeters

  • Muon Chambers

CDF II Detector

Event Signature

  • Each top (should) decay to Wb, so we can sort final states based on the W decay products

  • Lepton+jets final states consist of 2 bottom quarks, a lepton + neutrino, and 2 light (charm) quarks

  • In the detector, an ideal event will be comprised of:

    • 4 jets (require ≥3 with ET>15 GeV)

    • Large missing ET (ET>20 GeV)

    • An energetic lepton (pT>20 GeV/c)

      • Essential for triggering

  • Top is still dominated by high-order QCD jet production with a real W

    • Top is distinguished by presence of heavy-flavor (mostly bottom) jets

    • Require jets to be b-tagged

    • Large gains in signal purity

~2 cm



…but how do we detect b’s?

b-Tagging Overview

  • We use two key properties of bottom quarks to “tag” them:

    • High semi-leptonic branching fraction (~10% per e/μ, plus cascades)

      • Algorithms identify soft electrons or muons inside jets

        • Electron tagging difficult at CDF (large conversion background)

      • Maximum b-jet efficiency limited by decay rates

    • Long lifetime (cτ~500 μm)

      • B hadrons will travel a macroscopic distance (several mm) before decaying

      • Two strategies at CDF:

        • Calculate heavy-flavor probability based on impact parameters of tracks

        • SecVtx algorithm: Explicitly reconstruct heavy-flavor decay vertices






Tracking is critical


Impact parameter resolution asymptotically approaches 25 μm

Multiple scattering dominant at low pT

Most of the work done by the innermost layers

Entire silicon system upgraded in Run II

Three-component system covering radii of 1.5 cm (LØØ), 2.5-11 cm (5-layer SVX), 20-30 cm (2-layer ISL)

Silicon Tracking





Intrinsic Beam Width ~30μm

CDF Silicon Detector




Event Reconstruction I: Beamlines




  • Starting point: ~50 tracks and average beam position & width

    • Measured online with SVX

30 m

Event Reconstruction II: Primary Vertices






10 m

30 m

  • Starting point: ~50 tracks and average beam position & width

    • Measured online with SVX

  • Vertex high-quality tracks near beam, extract interaction point

30 m

Event Reconstruction III: SecVtx Algorithm







  • Starting point: ~50 tracks and average beam position & width

    • Measured online with SVX

  • Vertex high-quality tracks near beam, extract interaction point

  • Select tracks with large impact parameter that point from primary vertex to jet

  • Make best 2-track ‘seed’ vertex and attach all nearby tracks

  • Iteratively remove those with large χ2

  • Decide whether or not secondary vertex is inconsistent with the primary

    • Tag based on significance of displacement, not L2D

    • Primary vertex error matters





Problem: Efficiency is sensitive to poorly-modeled detector quantities

Resolution tails, primary vertex errors, etc.

Solution: Derive a multiplicative “scale factor” to correct the simulated efficiency in low-pT lepton samples

Complementary methods for 8-GeV electrons and muons inside jets

Only need the b-fraction of lepton jets

Muons: Fit μ momentum relative to the jet for the heavy-flavor fraction

Electrons: Compare tag rates to control sample of conversions and apply a simple formula…

Well, not that simple

Calibration of b-Tagging Efficiency I

b-Fraction 70% (e), 80% (μ)

b-Tagged Jet (e)

We measure the data-to-Monte Carlo “scale factor” to be ~0.92 ± 0.06

±2% depending on the tagger

Two years ago: 0.8 ± 0.1

Dominant systematic derived from extrapolation to top-like energies

ET dependence is poorly constrained

Large source of error in allb-tagged analyses (including the cross section)

Calibration of b-Tagging Efficiency II

Electron Sample

Muon Sample

Tuning the Algorithm

  • For multi-b event signatures (top/Higgs/SUSY), the tagged light-flavor background is typically quite small

    • Efficiency gains dominate purity losses

  • New in Run II: Try to maximize yield of doubly-tagged events

    • Reduce combinatorics in top lepton+jets event reconstruction (esp. for mass)

  • Final specs for “loose” SecVtx:

    • b-Tag Efficiency up 20%

    • Light-flavor tag rate x2.5

b-Tagging Efficiency in Top Events

Expect a top candidate sample 10 times larger than Run I

(25 times larger for double-tags)

The b-tagged lepton+jets sample gives us a lot of things to explore

Where to begin?

We can’t study top properties without knowing how much signal and background we have

The cross section measurement is the foundation for all top physics analyses in this channel

Top Physics Program

W Helicity


Top Mass

Top Lifetime


Top Charge


Cross Section













Top Spin



Non-SM Decays




Non-Top in Lepton+Jets (Superjets)




Cross Section Calculation

  • The cross section is derived from the expression:

  • : Number of b-tagged events in data sample

  • : Expected number of background events

  • : Total integrated luminosity: 695/pb

  • : Acceptance (includes branching fraction): ~7%

  • : Event b-tagging efficiency (16-70%)

Backgrounds I: W+Jets

Fake Tags (Negative L2D)










  • Background dominated by events with a real W and jets

    • Tags can be real heavy flavor or mis-tagged light flavor

  • W+Light Flavor (~40%)

    • Mistag rate measured with negative tags

    • Normalization comes from data

  • W+Heavy Flavor (~35%)

    • Contributions from Wbb, Wcc, and Wc

    • “Scaled” leading-order Monte Carlo

    • Wbb dominates double-tag background

Backgrounds II: Non-W and Electroweak

  • Remaining background contributions are relatively small

    • Non-W (~15%)

      • W signature (lepton and or missing ET) faked

        • Lepton: Conversions, hadrons identified as muons, B decays, misidentified jets

        • Missing ET: Calorimeter resolution, incomplete detector coverage

      • Extrapolated from outside the W signal region

    • Low-Rate Electroweak Processes (~10%)

      • Single top, dibosons, etc. with additional jets

Electron Data


Ultimately, we expect S/B of ~3 for single-tags, ~10 for double-tags

Cross Section Results

≥1 Tag

≥2 Tags

Best single measurement (14% error)

Top “discovery” with double-tags!

See Phys. Rev. Lett. 97, 082004 (2006)

Systematic Uncertainties

  • Limiting systematic uncertainty in cross section comes from data-to-simulation scale factor on the b-tag efficiency

Interpreting the Cross Section

  • We measure 8.8 ± 1.2 pb for a top mass of 175 GeV/c2

    • CDF Dilepton: 8.3 ± 1.8 pb

    • CDF All-Hadronic: 8.3 ± 2.1 pb

  • Some dependence on assumed mt

  • Unscientific combination favors a lower mass (~166 ± 4 GeV/c2)

    • Measured: 170.9 ± 2.4 GeV/c2

  • Natural question: How significant is the difference between the measured cross section and theory?

    • Strictly speaking, not very interesting (O(1σ))

  • More exciting questions: How much does the sample look like top? Can we rule out new phenomena that would inflate the cross section?



  • We attribute the excess in tagged events to top, but does it really look like top?

    • YES!

  • Need to be a bit more quantitative with some hypotheses…



Searches I

  • Is there a non-QCD production mechanism for top pairs?

    • No further evidence for 500 GeV/c2 resonance

    • Also: Detached W (“top lifetime”) hunt consistent with zero

Searches II

  • Are we mistaking something else for top?

    • Dedicated searches for t’→Wq in lepton+jets using event kinematics

    • Exclude t’ with mass 258 GeV/c2

Searches III

300 GeV/c2 stop

  • Are we counting events from top pairs+X? (e.g. X=H, ET)

    • Heavy stop pairs (→tχ0) or top partners in little Higgs (→tAH)

    • Sensitive to O(0.5 pb) in 1 fb-1

  • The bottom line:there is no significant evidence for physics beyond the Standard Model in the top sample

Aside: Top as a Calibration Sample

Remember this?

Forget it.

  • If it’s all top, double-tag statistics can be exploited to tackle systematics

  • σ: Requiring σ1-tag=σ2-tag constrains the tagging “scale factor” directly

    • 20% error reduction with 700 pb-1 (benefits all tagging analyses)

    • Expect to reach total precision of 12% on cross section with 1.2 fb-1

  • mt: Forcing the untagged jets to the W mass constrains the jet energy scale

    • 40% error reduction

  • These approaches are becoming the standards at the Tevatron (& LHC?)

LHC Top Production

  • Tevatron center-of-mass energies are typically insufficient to produce a top pair (350 GeV/c2)

    • x > ~0.2

  • The 7-TeV beam at LHC can produce top at smaller values of x

    • Dominated by gluon fusion (90%)

  • Expected cross section increases by a factor of >100





Top Accessible @ LHC

Top Accessible @ Tevatron

The Lepton+Jets Sample

Mass of 3 leading

jets (ATLAS)





mt=175 GeV/c2

  • Enhancement in σ for top is larger than that for backgrounds (W+jets)

  • Without b-tagging, top may be visible in ~1 week at 1033 (150 pb-1)

  • With b-tagging, may reach a precision of 5-7% on σ (dominated by luminosity)

    • Combined with 2-GeV precision on mt, a stringent test of QCD (finally)


  • The CDF Run II top physics program is in great shape, benefiting from large improvements in accelerator and detector performance and in b-tagging capabilities

  • We have made the world’s best measurement of the pair production cross section in the lepton+jets decay channel with 700 pb-1, and we expect the result to improve significantly in the coming months (1.2 fb-1)

    • More sensitive searches for new physics in the top sample will follow

  • The LHC will bring us to a new level of understanding top, and the last few years of Tevatron data will help us get there


Radiative Corrections and Global Fits

Latest Higgs Results

Standard Model Top Decays

In the Standard Model, top pairs decay ~100% of the time as:

  • Signatures are distinguished by the W decay products:

    • All-hadronic: High yield (44% branching ratio), large QCD background

    • Dilepton (2 e/μ’s): High purity, low yield (5% BR)

    • τ channels: difficult to trigger on, reconstruct leptons

    • Lepton+jets(1 e/μ only): Good purity and yield (30% BR), manageable background, kinematically constrained

Detector Signatures

  • CDF’s design allows us to perform signature-based analyses with physics objects

    • Jets (quarks and gluons):Clusters of tracks pointing to EM/hadronic calorimeter deposits

    • Electrons:Track pointing to narrow EM deposit

    • Muons:Track with little calorimeter energy pointing to muon “stub”

    • Neutrinos: Undetected → observe imbalance in transverse energy

Loose-Tag Backgrounds

No b-tagging systematics included

Double-Tag Backgrounds

No b-tagging systematics included

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