# Presentation: 657814

## Emanuela Barberis
Northeastern University, Boston
for the CDF and DØ Collaborations

*Northeastern University, Boston*
- for the CDF and DØ Collaborations

- Top Quark Mass and Kinematics

-   Top quarks at the Tevatron
-   General techniques for
-     measuring the top mass
-   Mass in the l+jets channel
-   Mass in the dilepton channel
-   Anomalous kinematics
-   ttbar mass spectrum

- Les Rencontres de Physique de la Vallee D'Aoste

- 3/9/2006

## Introduction

## Top Quark Production and Decay

-  In  proton anti-proton collisions
-    at Tevatron energies, top quarks
-    are primarly produced in pairs via
-    strong interactions

-  EW single top production:
-    not yet observed

- 15%

- 85%

- t-channel

- s-channel

- LO diagrams

-  Br(t→Wb) ~100%

## The Top Quark mass

-     Fundamental parameter of the Standard Model
-  Affects predictions of  SM via radiative corrections
-            mt can be related, with mW, to the Higgs mass
-  mt is roughly 1⁄2 the
-            vacuum expectation
-            value of the Higgs field
-            ****** **probing the EWSB
-            mechanism (new physics?)
** **
**  ** Precision measurement  2 fb-1 projection: mt~1.5 GeV (MH/MH=30%)

## CDF and DØ, the Tevatron detectors

-  Precise tracking and vertexing: new silicon vertex detectors.
-     Tracking chambers and TOF (CDF)/ fiber tracker (DØ).
-  Upgraded calorimeters, preshowers.
-  Upgraded Muon systems.
-  Upgraded DAQ/trigger systems.
- Data taking efficiency: 85%
- Run II results presented here: 160-750 pb-1

## Top mass measurement ingredients

-  Main mass extraction techniques:
-  Template methods: typically, one mass
-    per event from kinematic fit,
-    compare data to MC templates.
-  Dynamical methods: event by event
-    weights according to quality of agreement
-    with Standard Model top and background
-    differential cross-sections.

- Precision measurement  maximize statistical significance with sophisticated mass extraction techniques + minimize systematic uncertainties  (jet energy scale, signal/background modeling).

- differential cross-section (LO matrix element)

- PDF’s

- Transfer function: mapping from
- parton level variables (y) to
- reconstructed level variables (x)

## Top mass measurement ingredients

- Handles on systematic uncertainties:
-  JES systematic can be reduced with in-
-    situ calibration of the hadronic W mass
-    in top decays  simultaneous
-    determination of Mtop and JES from
-    reconstructed mtop and MW templates.
-  b-tagging can be used to reduce physics
-    backgrounds as well combinatorial.
-  Many systematical uncertainties are
-    expected to decrease with larger data
-    samples.

## Mass in the l+jets channel

**b**

****

**p**

- t(→W±b) t(→W±b)

- e±,±

- qq

*jet*

*jet*

*jet*

*jet*

## CDF: Template Method, 680 pb-1

- Reconstructed mtop and mjj from data are compared to templates of various true Mtop and JES (jet energy calibration shift) using an unbinned likelihood fit
-  uses *in-situ *mW constraint to calibrate the jet energy scale.

- Event selection:
-  1 e or  with pT > 20 GeV
-  ETmiss > 20 GeV
-   4 exclusive samples with
-     different jet pT selections:
-     0, 1 (L/T), 2 b-tags

- 360 candidates, 4 samples with different S/B
- and sensitivity to Mtop:
-  0 b-tag :  4 jets ET > 21GeV
-  1 b-tag (loose) : 3 jets ET > 15GeV, 4th jet
-    8GeV < ET < 15GeV (S:B = 1:1)
-  1 b-tag (tight) : 4 jets ET > 15GeV (S:B = 4:1)
-  2 b-tags: 3 jets ET > 15GeV, 4th jet ET > 8GeV
-    (S:B = 11:1)

- Backgrounds:

## CDF: Template Method, 680 pb-1

- mtop reconstruction: from kinematic fit that yields the lowest 2 (with 2 < 9)

- mjj reconstruction: uses all
- combinations of untagged jets

- Template fits: parametrization of mtop and mjj for input values of Mtop and JES
- (signal) + parametrization of mtop, mjj background shapes  probabilities
- Unbinned Likelihood fit: extraction of Mtop, JES, ns, nb (constructed from the
- probabilities determined with the Template fits)

## CDF: Template Method, 680 pb-1

- Likelihood for each sample:

- Combined Likelihood:

- Gaussian constraint onJES

- Systematic uncertainties:

- Combined Likelihood JES vs Mtop:

-  most precise
- single measurement

- External JES calibration uncertainty: 3 GeV
- Mjj in-situ calibration leads to a 40% improvement on the external JES

## CDF: Template Method, 680 pb-1

- CDF: Template Method, 680 pb-1

- mtop and mjj reconstruction in data:

- Additional
- kinematics
- distributions:

## DØ: Template Method, 230 pb-1

- DØ: Template Method, 230 pb-1

- Event selection:
-  1 e or  with pT > 20 GeV
-  ETmiss > 20 GeV
-   ≥ 4 jets, ET > 20 GeV
-  Topological selection: discriminant
-    of input variables uncorrelated with mtop
-  Lowest 2 solution for reconstructed
-    mtop, data compared to MC templates
-    with a likelihood fit.
-  94 candidates, S:B = 1:1

- reconstructed mtop:

- background

- signal

## DØ: Template Method, 230 pb-1

- DØ: Template Method, 230 pb-1

- Event selection:
-  1 e or  with pT > 20 GeV
-  ETmiss > 20 GeV
-   ≥ 4 jets, ET > 15 GeV
-   ≥ 1 b-jet(s)
-  Lowest 2 solution for reconstructed
-    mtop. Data compared to MC templates
-    with a likelihood fit.
-  69 candidates, S:B = 3:1

- reconstructed mtop:

- Systematic uncertainties:

-                     untagged        tagged
- JES                 +6.8/-6.5       +4.7/-5.3
- Jet resolution      0.9                0.9
- Gluon radiation   2.6                2.4
- Signal Model       2.3               2.3
- Backg. Model      0.7               0.8
- b-tagging              --                 0.7
- Calibration          0.5                0.5
- Trigger bias        0.5                0.5
- MC statistics       0.5                0.5
- Total            +7.8/-7.1 GeV     6 GeV

## DØ: Ideogram Method, 160 pb-1

- Same kinematic fitting and discriminant as the Template analysis.
- Event-by-event likelihood, each event gives a distribution of masses.

- discriminant

- combinatorics weight

- Signal Breit-Wigner’s

- Background shape

- Gaussian resolutions

## CDF: Decay Length Method, 695 pb-1

- Uncorrelated to other measurements:
- relies on tracking (no dependency on JES)
- Boost of b-quarks is correlated to mtop:
- use b-lifetime, or         , to infer mtop
- Event Selection
-  1 e or  with pT > 20 GeV
-  ETmiss > 20 GeV
-   ≥ 3 jets, ET > 15 GeV
-   ≥ 1 b-tagged jet

## CDF: Decay Length Method, 695 pb-1

- CDF: Decay Length Method, 695 pb-1

-  Signal and background Lxy
-    distributions used as PDF’s
-    for pseudo-experiments ensembles
-  375 candidates, S:B = 2:1

- Systematic uncertainties:

## DØ: Matrix Element Method, 320 pb-1

- Pioneered by DØ with re-analysis of Run I data  uses *in-situ *MW constraint
- to calibrate the jet energy scale.
- Makes maximal use of information in each event by calculating event-by-event
- probability to be signal or background, based on the respective matrix elements:

-  x: reconstructed lepton and jets kinematics
-  JES from MW constraint.
-  Signal and background probabilities: from differential cross-sections
-  All events are combined in a likelihood
-  Likelihood is maximized as a function of mtop and JES.

## DØ: Matrix Element Method, 320 pb-1

- Event selection:
-  1 e or  with pT > 20 GeV
-   ETmiss > 20 GeV,  (ETmiss,l) cut
-   = 4 jets, ET > 20 GeV
-  Signal and background fractions
-    cross-checked with a likelihood fit
-  150 candidates, S:B = 1:2

- Simultaneous fit of mtop, JES, and ftop:

- mtop and JES
- projections 

## DØ: Matrix Element Method, 320 pb-1

- Systematic uncertainties:

- Cross check on W mass:
- (scaling jets by fitted JES)

## CDF: Matrix Element Method, 680 pb-1

- Event selection:

- Simultaneous fit of mtop, JES, and ftop

- 118 candidates, S:B = 5:1

- Combined Likelihood JES vs Mtop:

## CDF: Dynamical Likelihood Method, 318 pb-1

- Event selection:
-  1 e or  with pT > 20 GeV
-   ETmiss > 20 GeV
-   = 4 jets, ET > 15 GeV
-  ≥ 1 b-tag
-  63 candidates, S:B = 6:1
- Likelihood based on event-by event probability to be signal from ttbar LO
- Matrix Element.
- Shift on mtop estimated as a function of
- background fraction (14.5% in the data)

## Mass in the ll+jets channel

- t(→W+b) t(→W-b)

- e+,+

- e-,-

## DØ: Template Methods, Matrix Weighting 370 pb-1

- General: the dilepton channel is underconstrained. Template methods assume
- values for certain variables  in order to extract a solution, and assign weights
- to the different solutions.

- The matrix Weighting method scans over top masses and assigns a weight to
- the solution, based on the Matrix Element predictions for the lepton pT’s:
-  binned maximum likelihood fit to signal and background templates

- Event selection:

- echannel
-  1 e, pT(e)>15 GeV
-  1 , pT()>15 GeV
-  HT > 122 GeV
-  ≥ 2 jets, ET > 20 GeV

- eechannel
-  2 e’s, pT(e)>15 GeV
-  mee< 80(>100) GeV
-  ≥ 2 jets, ET > 20 GeV
-  ETmiss>40(35) GeV
-       [mee<80(>100) GeV]
-  sphericity > 0.15

- channel
-  2 , pT()>15 GeV
-  contour cut on ETmiss
-    and 1,ETmiss)
-  ≥ 2 jets, ET > 20 GeV
-  Z fitter 2 test

## DØ: Template Methods, Matrix Weighting 370 pb-1

-                     untagged        tagged
- JES                     3.5                3.5
- PDF                    0.9                0.9
- Gluon radiation   0.8                0.8
- background         0.7                0.2
- calibration           0.6                0.6
- template stat.      0.3                0.3
- Total                  3.8 GeV     3.8 GeV

- DØ: Template Methods, Matrix Weighting 370 pb-1

-  Untagged analysis: 21 candidates, S:B = 4:1
-  b-tagged (≥ 1 b-tag) analysis: 14 candidates, S:B = 48:1

- untagged

-  tagged

- Likelihood fits:

- Systematic uncertainties:

- untagged

- tagged

## DØ: Template Methods, Neutrino Weighting 370 pb-1

- The Neutrino Weighting method scans over top masses  and the’s of the
- two neutrinos and assigns a weight (as a function of mtop) to the solution, based on the agreement of the calculated neutrino pT’s and the observed ETmiss
-  maximum likelihood fit to signal and background templates

- Event selection: similar to the matrix weighting measurement, only
- exception: simpler electron likelihood cut, ETmiss > 25 GeV,
- and HT(l,jets) > 144 GeV in the ee channel.
-  Untagged analysis: 21 candidates, S:B = 4:1

## DØ: Template Methods, Neutrino Weighting 370 pb-1

- Likelihood fit:

- Systematic uncertainties:

- JES                          5.3
- Jet resolution           0.5
- Muon resolution       0.4
- PDF                         0.7
- Gluon radiation        2.0
- background              1.3
- template stat.           0.9
- Total                        6.0 GeV

## CDF: Template Methods, Neutrino Weighting 358.6 pb-1

- JES                          3.4
- b-jet energy              0.6
- MC generator           0.5
- PDF’s                       0.5
- ISR                           0.6
- FSR                          0.5
- Signal templates      0.2
- Backg. templates     1.3
- Backg. shape           2.6
- Total                         4.6 GeV

- Systematic uncertainties:

- Event selection: l+track

-  1 e() with pT >20 GeV
-  ETmiss > 20 GeV
-   ≥ 2 jets, ET > 15 GeV
-  isolated track, pT >20 GeV
-  46 candidates, S:B=2:1

## CDF: Template Methods,  and Pz Weighting 340 pb-1

- CDF: Template Methods,  and Pz Weighting 340 pb-1

- Weight on the neutrinos’s

- Integrate over the value of ttbar Pz

## CDF: Matrix Element Method 750 pb-1

- CDF: Matrix Element Method 750 pb-1

- Uses a per-event probability for the mass as a weighted sum of the differential cross-section for LO top quark pair production and of the differential cross section for background processes:

- signal/backg fractions

-  Posterior probability density:
-   product of a flat prior and the joint
-   likelihood, (mean,  (Mtop,,Mtop).
-  Event Selection:
-    - 2 e() with pT >20 GeV
-    - ETmiss > 25 GeV
-    - (ETmiss,l or j)>20o
-    - ≥ 2 jets, ET > 15 GeV

## CDF: Matrix Element Method 750 pb-1

- CDF: Matrix Element Method 750 pb-1

-  most precise dilepton measurement

- Systematic errors:

- Joint probability density

-  64 candidates, S:B=2:1

- Dynamical Likelihood Method (340 pb-1):

## Mass summary & combination

## Top quark mass: Summary

- New Run II measurements achieving better uncertainties than Run I world average.
- EPS 2005 Run I+ Run II world average:
- 	Impact on SM Higgs boson:
- 	uncertainty now dominated by MW

- CDF average

## Search for new physics

## Anomalous ttbar kinematics

- Estimate consistency of observed kinematics of the ttbar system with SM
- (in Run I, several CDF events at high ETmiss and pTl)

-  four chosen variables: ETmiss, pTl , (l,ETmiss), dilepton “topology” (T)

- CDF, 193 pb-1
- consistent with
- the SM with
- a probability of
- 1-4.5%

- Model independent
- search for new
- physics

## DØ: ttbar mass spectrum

- Search for new particles in Top production, leading to a resonance
- in the ttbar mass spectrum

- Limit on a narrow leptophobic Z’ (total width Z’=1.2%MZ’):

- MZ’>680 GeV at 95% CL

-  l+≥4 jets candidate events (≥1 b-tag), 370pb-1
-  ttbar invariant mass reconstructed with ttbar production hypothesis (kinematic fitting)
-  Model independent limits on xBr(X→ttbar)

## CDF: ttbar mass spectrum

-  l+≥4 jets candidate events, 682pb-1
-  ttbar invariant mass reconstructed with ttbar production hypothesis (likelihood
-    incorporating LO matrix element for ttbar)
-  Model independent limits on xBr(X→ttbar)

- Limit on a narrow leptophobic Z’ (Z’=1.2%MZ’):

- MZ’>725 GeV at 95% CL

- CDF: ttbar mass spectrum

## Summary and conclusions

- Results on Top mass and kinematics presented for datasets up to 750 pb-1,
- with single mass measurements already exceeding past world average values.
- The excellent performance of the Tevatron and the CDF and DØ experiments
- are the key to precision measurements in top physics and to the search for new
- physics coupled to the top quark.
- It's good to look behind us, but the real excitement is in what lies ahead!