Diapositiva 1

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Top Mass and Cross Section at the Tevatron
Ambra Gresele
Trento University INFN
IFAE, Pavia 19-21 April 2006
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Tevatron Run II

• 1fb-1 per experiment on tape
• 1.3 fb-1 delivered luminosity
• Peak luminosity 1.7 x 1032cm-2s-1
• Presented here 700 pb-1

Goal of Run II per experiment
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Top production and decay

• Production mainly

• Decay BR(t ? W b) 100

• All-hadronic 44
• Lepton jets 30
• Dilepton 5

di-lep
all-had
lepjets
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Why measuring the top mass?

• Top mass is related to
• W, Higgs
• (and other observables)
• When all W, t, H measured
• test SM
• (and to test you have to measure well)
• Radiative corrections affect observables
• Game of ElectroWeak fits

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Mass Measurement Methods

• Template Method Matrix Element
  Method

Build likelihood from matrix element(s), PDFs
and transfer functions (connect quarks and jets)
Pick a test statistic (e.g. recontructed mass)
Create templates using events simulated with
different mt values (background)
Integrate over unmeasured quantities (e.g. quark
energies)
Perform maximum likelihood fit to extract
measured mt
Calibrate measured mt and uncertainty using
simulation
Less assumptions / robust measurement
Better statistical precision expected w/ using
more info
All methods in all channels are well validated by
a blind sample
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CDF Template Method ljets

• W-gtjj dijet mass distribution is a resonance
• Resonance peaks stands out at 80.4 GeV/c2
• Sensitive to shifts in jet energy scale (JES)

• Datasets
• Data
• Background MC
• ttbar MC

mjj template
Mtop template mtopreco
Mass fitter

• Mass Fitter
• Finds best top mass and jet-parton assignment
• One per event based on overconstrained system
• Additional selection cut on resulting c2

Parameterize
Likelihood Fit

• Likelihood Fit
• Fit mtopreco and wjj distributions in data to
  sum of signal and background parametrizations
• Constrain background and JES with prior knowledge

• Parametrizations
• For both templates, as a function of top mass
  and JES
• For both signal and background

gives JES, top mass!
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CDF Template Results (I)
680 pb-1
mtreco templates w/ fit overlaid
mjj templates w/ fit overlaid
Mtop 173.4 2.5 (stat. DJES) GeV/c2
Miscalibrationin units of sc,external calib.
DJES -0.3 0.6 (stat. Mtop)
sc 318 pb-1 Mtop 173.5 3.9-3.8 (stat. DJES)
GeV/c2
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CDF Template Results (II)
Systematic ?Mtop(GeV/c2)
Residual JES 0.7
B-jet energy scale 0.6
Bkgd JES 0.4
Bkgd Shape 0.5
ISR 0.5
FSR 0.2
Generators 0.3
PDFs 0.3
MC stats 0.3
B-tagging 0.1
TOTAL 1.3
Likelihood contours in Mtop-DJES plane
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CDF Matrix Element Method dileptonic channel
The complete information contained in an event
(x) regarding the top mass can be expressed as
the conditional probability
If the momentum of each parton could be exactly
deduced from final-state particles, the
calculation of d?/dx would be simple. Instead we
must integrate over quantities which are unknown
and, in addition, quark energies are not
directly measured. The total expression for the
probability of a given pole mass for a specific
event can be written
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CDF Matrix Element results
750 pb-1

• Best measurement in challenging dilepton channel
• Could reach 2 GeV (stat)sensitivity by end of
  run II

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Template Method ljets
370 pb-1

• Event-by-event Mtop by c2 fit
• Use 69 candidate events with ?1 b-tagged jet

Mtop 170.6 ? 4.2 (stat) ? 6.0 (syst) GeV/c2
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Combination of CDF and D0 Top Mass
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Top Pair Production Cross Section
Mtop (GeV/c2)
170 7.8
175 6.7
178 6.1
sinel70mb so 7M events/s at 1032/cm2s but 1 tt
in 1010 events
M. Cacciari et al. JHEP 0404068 (2004) N.
Kidonakis and R. Vogt, Phys. Rev. D 68 114014
(2003)
Need better quality

• s tt is crucial
• Check of perturbative QCD
• Window to NP
• Look at all possible channels
• Starting point for all properties analysis
• tt is background of searches

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Dilepton Channel

• Selection
• 2 leptons ETgt20GeV with opposite sign
• gt2 jets ETgt15GeV
• Missing ETgt25GeV (and away from any
• jet)
• HTpTlepETjetMETgt200GeV
• Z rejection

• Backgrounds
• Physics WW/WZ/ZZ, Z ? tt
• Instrumental fake lepton

s(tt)  8.3 1.5 (stat) 1.0 (syst) 0.5
(lumi) pb
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LeptonJets Channel Kinematics

• Backgrounds
• Wjets
• QCD

• Selection
• 1 lepton with pTgt20GeV/c
• gt3 jets with pTgt15GeV/c
• Missing ETgt20GeV

energetic
discriminate
central
spherical

• 7 kinematic variables in neural net

binned likelihood fit
s(tt)  6.0 0.6 (stat) 0.9 (syst) pb
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All Hadronic Channel

• Selection
• gt6 jets with pTgt15GeV/c
• gt1 b tagged
• NN discriminant gt 0.9

• Huge QCD background !

discriminate

• 6 kinematic variables in neural net

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Summary of Top Pair Production Cross Sections
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Conclusions

• The top mass is now know
• with an accurancy of 1.3, limited by the
  systematic uncertainties which are dominated by
  the jet energy scale.
• With in situ JES calibration, dominant
  systematic now scales as 1/sqrt(N).
• Expect 2 GeV/c2 precision by LHC turn-on.
• All the cross section measurements are consistent
  with SM

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Back up slides
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(No Transcript)
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History of Mtop measurement

• Top first observed
• at CDF D0 in 1995.
• Tevatrons Run I 110pb-1
• Run I Average

Mtop 178.0 ? 4.3 GeV/c2
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Jet Energy Corrections

• The jet energy scale (JES) is the major source
• of uncertainty in the top quark mass
• measurement and inclusive jet cross section
• Absolute scale the jet energy measured in the
• calorimeter needs to be corrected for any
• non-linearity and energy loss in the
  un-instrumented
• regions of each calorimeter (from MonteCarlo)
• Relative scale since the central calorimeters
• are better calibrated and understood, this scales
  
• the forward calorimeters to the central
  calorimeter
• Scale (is obtained using Pythia and data di-jet
  events)
• Multiple interactions the energy from different
• ppbar interactions during the same bunch crossing
  
• falls inside the jet cluster (from minimum bias
  data)
• Underlying event is defined as the energy
• associated with the spectator partons in a hard
• collision event.
• Out-of-cone corrects the particle-leve energy

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• The total systematic uncertainties in the
  central calorimeter
• are the same order than RunI (significant
  improvement with
• respect 2004 analyses)
• Big improvements for plug jets with respect to
  RunI due
• to new detectors

3 jet pT uncertainty in top events
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Matrix Element Technique dilept

• Harder to reconstruct Mtop in dilepton events
  two neutrinos make system underconstrained
• Determination of probability is similar to ljets
• No W resonance ? no fit for JES
• Approximations havesignificant effect
• MC calibration essential
• Correct fitted mass for slope 0.85
• Correct for pull width of 1.49

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Matrix Element Technique ljets

• Calibrate method against MC samples
• Shows unbiased measurement
• Error are rescaled to account for observed pull
  widthdue to approximations in integration

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Matrix Element Results
750 pb-1

• JES here is constant multiplicative factor
• Edata EMC/JES
• JES 1.02 0.02
• Consistent with template method
• Virtually identical sensitivity with fewer events!

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Combination of CDF results

• Use BLUE (Best Linear Unbiased Estimator)
  technique
• NIM A270 110, A500 391
• Accounts for correlations in systematics
• Stat correlations in progress
• So far only combine measurements on independent
  datasets.

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Decay Length Technique
680 pb-1
B hadron decay length ? b-jet boost ? Mtop

• Difficult, measure slope of exponential
• But systematics dominated by tracking effects ?
  small correlation with traditional measurements!
• Statistics limited now
• Can make significant contribution at LHC