# Presentation: 684239

## top quark mass measurements

- Ulrich Heintz
- Boston University
- (for the D0 and CDF collaborations)

## introduction

- top is the most massive fundamental particle
- dominant contributor to radiative corrections

-  mtop2

-  log(mH)

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## top production and decay

- production
- pp  tt with  ≈ 7 pb
- decay
- tWb  with *B **≈** *100%
- Wqq with *B** ≈** *67 %
- Wl with *B** ≈** *11%
- e with *B** ≈** *17 %
- final state signatures for top-antitop pairs

- b-tagging

- lepton+jets

- dilepton

- all jets

- primary
- vertex

- secondary vertex

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## jet energy scale/combinatorics/radiation

- calibrate jet response
- external calibration
- +jet, dijet events  calorimeter response
- pile-up, noise, showering, bias corrections
- uniform response
- in situ calibration
- W  qq from top decays
- additional overall scale factor
- many jets
- combinatorics
- how to associate them with partons from top decay?
- initial state and final state radiation
- which jets are from top decay?

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## template fits

- mass estimator
- typically best mtop from kinematic fitter
- templates
- probability density functions from simulated tt generated with a range of top quark mass values
- maximum likelihood fit
- fit templates to data distribution
- ensemble tests
- perform analysis many times on pseudodata sets generated to have same signal/background as expected in collider data
- average and rms of measured mass
- average and rms of pull

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## matrix element method

- probability density for event *o* if the top quark mass is *m**t*
- signal probability
- combine all events in a joint likelihood
- and maximize wrt *m**t**, ****jes**, f**top*

- top fraction

- jet scale parameter

- normalization

- pdf

- |M|2 dLIPS

- detector response function

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## dileptons

- characteristics
- small branching fraction
- two neutrinos
- kinematically underconstrained
- signal:background = 3:1
- combinatorics: 2
- selection
- two isolated leptons
- 2 jets
- miss pT
- Z rejection in ee/ channels

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## dileptons (D0 – 1.05 fb-1)

- matrix weighting
- for each top mass determine possible top/antitop momenta
- weight by pdf, probability for lepton energy
- template fit
- top mass with largest weight

- neutrino weighting
- for each top mass loop over neutrino pseudorapidities
- weight by consistency with observed missing pT,
- template fit
- mean and rms of weight curves

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## dileptons (D0 – 1.05 fb-1)

- statistical correlation factor = 0.67

- matrix weighting: 	175.2±6.1(stat)±3.4(syst) GeV
- weighting:		172.5±5.8(stat)±3.5(syst) GeV
- combined:		173.7±5.4(stat)±3.4(syst) GeV

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## dileptons (CDF)

- matrix element (2.0 fb-1)
- selection uses evolutionary neural network that optimizes resolution

-  weighting (1.9 fb-1)
- template fit to mass with largest weight and to HT

- 171.2±2.7(stat)±2.9(syst) GeV

- 171.6±3.3(stat)±3.8(syst) GeV

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## lepton+jets

- characteristics
- large branching fraction
- one neutrino
- kinematically overconstrained
- signal:background = 1:2, 2:1(b-tag)
- combinatorics: 12, 6(1 b-tag), 2(2 b-tags)
- selection
- one isolated lepton pT>20 GeV
- 4 jets pT>40/20/20/20 GeV
- miss pT >20(25) GeV
- at least one b-tag
- background model
- W+jets (matched ALPGEN)
- multijets (data)

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## lepton+jets (D0 – 2.1 fb-1)

- lepton+jets (D0 – 2.1 fb-1)

- matrix element analysis – calibrate with pseudodata sets
- Run IIa:	170.5 ± 2.5(stat jes) ± 1.4 GeV (0.9 fb-1)
- Run IIb:	173.0 ± 1.9(stat jes) ± 1.0 GeV (1.2 fb-1)
- combined:172.2 ± 1.1(stat jes) ± 1.6 GeV (2.1 fb-1)

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## l+jets (CDF – 1.9 fb-1)

- matrix element signal likelihood
- NN discriminant gives event-by-event background fraction
- no integration over background matrix elements
- subtract expected background contribution from likelihood

- 172.7±1.8 (statjes)±1.2(syst) GeV

- 318 events

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## l+jets (CDF – 1.9 fb-1)

- l+jets (CDF – 1.9 fb-1)

- template fit to mtop and mjj from kinematic fit

- mtop= 171.8 ± 1.9 (statjes) ± 1.0 (syst) GeV

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## l+jets+dilepton template CDF

- simultaneous template fit

- mtop= 171.9 ±1.7 (statjes) ± 1.0 (syst) GeV

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## all jets (CDF)

- characteristics
- large branching fraction
- no neutrinos
- complete reconstruction
- background
- combinatorics 90/30(1 tag)/6(2 tags)
- selection
- 6-8 well separated jets
- no significant miss pT
- at least one b-tag
- neural network
- jet ETs, jj, jjj masses, aplanarity, sphericity
- kinematic fitter
- leading 6 jets
- jj/jjj masses, jet pTs
- use top/W masses with smallest 2
- background model
- pretag data * tagging probability

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## all jets (CDF – 1.9 fb-1)

- all jets (CDF – 1.9 fb-1)

- 2-dimensional template fit

- mtop = 177.0 ± 3.7(stat/jes) ± 1.6(syst) GeV

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## combination

| Run II
Measurement        |  CDF
 di-l   |  D0 
di-l  |  CDF
l+j    |  D0
l+j   |  D0
l+jb   |  CDF
all-j  |  CDF
lxy    |
| --- | --- | --- | --- | --- | --- | --- | --- |
| ∫Ldt (fb-1 |  |  |  | 2.1 |  | 1.9 |  |
| Result  | 171.2 | 173.7 | 172.7 | 172.2 | 173.0 | 177.0 | 180.7 |
| Jet Energy Scale | 2.5 | 3.1 | 1.5 | 1.3 | 1.6 | 2.0 | 0.3 |
| Signal  | 0.7 | 0.8 | 0.6 | 0.7 | 0.5 | 0.6 | 1.4 |
| Background      | 0.4 | 0.6 | 0.6 | 0.4 | 0.3 | 1.0 | 7.2 |
| Fit     | 0.6 | 0.9 | 0.2 | 0.1 | 0.2 | 0.6 | 4.2 |
| MC      | 0.7 | 0.2 | 0.4 | 0.0 | 0.0 | 0.3 | 0.7 |
| Systematic | 2.8 | 3.4 | 1.7 | 1.6 | 1.7 | 2.4 | 8.5 |
| Statistical | 2.7 | 5.4 | 1.2 | 1.1 | 1.3 | 3.3 | 14.5 |
| Total Uncertainty | 3.9 | 6.4 | 2.1 | 1.9 | 2.2 | 4.1 | 16.8 |

- statistical: uncorrelated
- jet energy scale: several subcategories with different correlations
- signal: ISR, FSR, pdf, b-id – correlated for all
- background: correlated within channels
- fit: MC stats – uncorrelated
- MC: Monte Carlo generator – correlated for all

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## combination

- Ulrich Heintz - Moriond QCD

- 3/11/2008

## conclusion

- with 2 fb-1
- mtop = 172.6 ± 1.4 GeV
- m/m = 0.8%
- with full data set (8 fb-1)
- experimental precision  below 1 GeV
- push on systematics
- are we missing anything?
- color reconnections
- what mass are we measuring?
- PYTHIA top mass parameter
- pole mass?

- Ulrich Heintz - Moriond QCD

- 3/11/2008