# Presentation: 325712

## Top Mass Measurements at the CDF Experiment

- Luca Scodellaro
- Instituto de Fisica de Cantabria
- for the CDF Collaboration

- QCD 05
- 12th International QCD Conference
- 4-9th July, Montpellier (France)

## Outline

- Motivations for measuring the top mass
- Top production at the Tevatron
- Top mass measurements at CDF
- Summary and Perspectives

- Measurement Challenges
- b quark tagging
- Jet energy corrections

- Template Method
- Matrix Element Method

## Motivations for Measuring Top Mass

- Special Role in SM due to the high mass
- Precise top mass measurement provides:

- Dominant parameter in
- radiative corrections for
- Electroweak predictions

- Consistency tests for the Standard Model
- Constraint on unknown parameter (Higgs boson mass)
- Sensitivity to physics beyond the Standard Model

## Top Production at the Tevatron

- Tevatron: pp collisions at √s=1.96 TeV
- Pair production is the
-    dominant mechanism
-    BR(t->Wb)  ≈ 100%
- Observed final states given by W boson decays:

- _

- Dilepton:       tt -> lνlνbb   (5%)
- Lepton+jets: tt -> lvqqbb   (30%)
- Hadronic:      tt -> qqqqbb (45%)

- }

- l=electron or muon

## Measurement Challenges 

- Lepton+Jets channel:
-    Neutrino not detected:
-         Px and Py from transverse energy conservation
-         Pz from Mw=Mlv constrain (two solutions)
-    Four expected jets in the final states:
-         Ten possible choices for parton-jet assignments
- Dilepton channel:
-    Low statistics
-     Two neutrinos in final states: under-constrained kinematic system
- Jet Energy Measurement:
-    Largest contribution to top mass uncertainty

## b Quark Tagging

- Tagging the b quark improves parton-jet assignments:
-    lepton+jets: 1 tag-> 6 combinations, 2 tags->2 combinations
- Requiring a b tagged jet also reduces background contamination
- B hadrons are long lived
-     Jet tagging
-     efficiency -> 42%
-     False  tag rate -> 0.5%

## Jet Energy Corrections

- Big effort in understanding jet energy measurement
-  ~3% jet pT  uncertainty
-  in  top events
-   Big improvements
-   for plug jets (|η|>1)
-   with respect to RunI
-   due to new detector
- Significant improvement with respect 2004 analyses

- Central Jets (|η|<1)

- --  RunII
- --  RunI
- --  Winter 2004

## The Template Method

- Reconstruct Mtop per event
-    kinematic fit to determine the most probable jet-parton assignment
- Top mass measurement
-    best likelihood fit of  mass distribution in data to background and signal templates for different top mass  values

## Lepton+Jets: Jet Energy Scale Fit

- W  mass reconstruction used to fit the JES
- Mass templates for top and W as a function of top mass and JES
- Data divided in 0-tag, 1-tag and 2-tags sub samples
- Fit data simultaneously for Mtop and JES:
-    20% improvement in  uncertainty due to JES

**M****top **** = 173.5 ****+2.7**** ****-2.6**** (stat.)±2.5(JES)±1.7(syst.) GeV/c****2**

- Most precise single top mass measurement in the world

## Lepton+Jets: Additional b Tagging

- Increasing S/B ratio and parton-jet assignment
-    by using further b tagging techniques
- No W constrain on Jet Energy Scale applied

**M****top **** = 173.0 ****+2.9**** ****-2.8**** (stat.)±3.3(syst.) GeV/c****2**

- The Jet Probability algorithm
- computes the probability for
- a set of tracks inside a jet to
- come from the primary
- interaction of the event

## Template Method in Dilepton Events

- Two neutrinos in the final states:
-     additional assumptions to constrain the event kinematic
- Neutrino weighting approach:
- Mtop maximizing probability W used to build templates:

**M****top **** = 170.6 ****+7.1**** ****-6.6**** (stat.) ± 4.4 (syst.) GeV/c****2**

- Missing ET constraints

- Probability for assumed neutrinos’ η

- Integration over parton assignments and neutrinos’ η

- Assumed theoretical value of MW

-            W(Mtop,Mw)   =

## Matrix Element Method

- Not to look only to a particular parton-jet assignment
- Compute a probability as a function of Mtop:
- Sum over all the unknown in the event:
-    parton-jet assignments and neutrino solutions
- Top mass from maximum likelihood fit:

- Matrix element

- Parton distribution function

- Transfer functions connect
- parton to jet variables

- L = ПiP(xi;Mtop)

## Matrix Elements in Lepton+Jets Events

- Only signal probability considered
- Background effect described
-    by a mapping function
-    obtained by pseudo-experiments
-    for different background fractions

- One inclusive b tag required
- Mapping function at 21% applied

**M****top **** = 173.8 ****+2.7**** ****-2.5**** (stat.)±3.3(syst.) GeV/c****2**

## Matrix Elements in Dilepton Events

- Background probabilities included in the likelihood
-       P(xi; Mtop) = psPs(xi; Mtop) + ΣjpbgjPbgj(xi)

**M****top **** = 165.3 ± 6.3 (stat.) ± 3.6 (syst.) GeV/c****2**

-  33 candidate events
-    observed  in 340 pb-1
-    of data
-   Top Mass measured
-     from the expectation
-     value of the posterior
-     probability density

## Summary and Perspectives 

- TevEWWG has produced a new average:
-    Mtop = 174.3±3.4 GeV/c2
- Currently using most precise single measurement per decay channel from each experiment
- Working ongoing to combine results within each channel (including CDF results from RunI)

## Summary and Perspectives

- Electroweak fit to Higgs boson mass using latest Tevatron combination
-    New central value
-    MH=98+52-36GeV/c2
-    New upper limit
-    MH<208GeV/c2 @95%CL

## Summary and Perspectives

- Fitting the jet energy scale from W decays will reduce systematic uncertainty with luminosity:
-    Uncertainty on Jet Energy Scale less than 1 GeV achievable
- Can reach our RunII goal ΔMtop =2-3GeV/c2

## Backup Slides

## The Tevatron Collider

-  Tevatron RunII:
-     pp at √s=1.96 TeV
-  Peak luminosity:
-     L   = 1.2∙1032

- _

## The CDF Detector

## Electroweak fits

## Comparison with EW Fits