top quark mass measurements at the Tevatron

top quark mass measurements at the Tevatron

Ulrich Heintz

Boston University(for the D0 and CDF collaborations)

XLIII Rencontres de Moriond, QCD and High Energy Interactions, La Thuile, March 8-15, 2008

3/11/2008 Ulrich Heintz - Moriond QCD 2

introduction• top is the most massive fundamental particle

• dominant contributor to radiative corrections

W W

t

b

W W

H0

mtop2

log(mH)


top production and decay• production

– pp tt with ≈ 7 pb

• decay – tWb with B ≈ 100% – Wqq with B ≈ 67 %– Wℓ with B ≈ 11%

e with B ≈ 17 %

• final state signatures for top-antitop pairs

• b-tagging

lepton+jets38%

dilepton 6%

all jets56%

primaryvertex

secondary vertex


jet energy scale/combinatorics/radiation• calibrate jet response

– external calibration– in situ calibration

• W qq from top decays

• many jets– combinatorics

• how to associate them with partons from top decay?

– initial state and final state radiation

• which jets are from top decay?


template fits• mass estimator

– typically best mtop from kinematic fitter

• templates– probability density functions from simulated tt events

generated with a range of top quark mass values

• maximum likelihood fit– fit templates to data distribution

• ensemble test validation– perform analysis many times on pseudodata sets

generated to have same signal/background as expected in collider data

– average and rms of measured mass– correct for biases


matrix element method• probability density for event o if top quark mass is mt

• signal probability

• combine all events in a joint likelihood

• and maximize wrt mt jes ftop

jesbkgtopjestsigtoptopjestevt opfmopffmoL |1,|,,|

topjesti

n

i

topjestn fmoLfmooL ,,|ln,,|ln1

1

top fraction jet scale parameter

jestttttt

jestsig qoWmqdxfxfxdxdm

mop

,|,1

,|

normalization pdf |M|2 dLIPS

detector response function


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

Dilepton(BR~6%)

e,

b-jet

e,

b-jet

miss pT

Dilepton(BR~6%)

e,

b-jet

e,

b-jet

e,

b-jet

e,

b-jet

miss pT


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

57/13 eventsmatrix weighting


dileptons (D0 – 1.05 fb-1)• statistical correlation factor = 0.67

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


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

344/192 events

144/34 events


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 – 4 jets

– miss pT

– at least one b-tag jet

e,

b-jet

b-jet

jet

Lepton+jets(BR~38%)

miss pT

jet

e,

b-jet

b-jet

jetjet

e,

b-jet

b-jet

jet

Lepton+jets(BR~38%)

miss pT


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) ± 1.6 GeV (2.1 fb-1)

271/109 events


lepton+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 (statjes)±1.2(syst) GeV

318/70 events


lepton+jets (CDF – 1.9 fb-1)• template fit to mtop and mjj from kinematic fit

mtop= 171.8 ± 1.9 (statjes) ± 1.0 (syst) GeV

332/47 events


all jets (CDF)• characteristics

+ large branching fraction+ no neutrinos+ complete reconstruction– signal:background 1:16– 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

• kinematic fitter– leading 6 jets– jj/jjj masses, jet pTs– use top/W masses with smallest 2

• background model– pretag data * tagging probability

jet

b-jetb-jet

jet

jet jet

All-hadronic(BR≈56%)


all jets (CDF – 1.9 fb-1)• 2-dimensional template fit

mtop = 177.0 ± 3.7(statjes) ± 1.6(syst) GeV 3112/2570 events


combinationRun IIMeasurement

CDF di-l

D0 di-l

CDFl+j

D0l+j

CDFall-j

CDFlxy

worldaverag

e

∫Ldt (fb-1) 2.0 1.1 1.9 2.1 1.9 0.7

Result 171.2 173.7 172.7 172.2 177.0 180.7 172.6

Jet Energy Scale 2.5 3.1 1.5 1.3 2.0 0.3 0.9

Signal 0.7 0.8 0.6 0.7 0.6 1.4 0.5

Background 0.4 0.6 0.6 0.4 1.0 7.2 0.4

Fit 0.6 0.9 0.2 0.1 0.6 4.2 0.1

MC 0.7 0.2 0.4 0.0 0.3 0.7 0.2

Systematic 2.8 3.4 1.7 1.6 2.4 8.5 1.1

Statistical 2.7 5.4 1.2 1.1 3.3 14.5 0.8

Total Uncertainty 3.9 6.4 2.1 1.9 4.1 16.8 1.4

•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


combination

GeV 1.38.169

GeV 5.14.172

GeV 9.33.177

TeV-EWWG

http://tevewwg.fnal.gov/top/


winter 2008 global electroweak fit• LEP EWWG procedure

• results for winter 2008 (summer 2007) had = 0.02767±0.00034

s = 0.1185±0.0027

– MZ = 91.1874±0.0021 GeV

– mtop = 172.8±1.4 GeV (171.3±1.7 GeV)

– MH = 87 +36 -27 GeV (76 +33 -24 GeV)

2/dof = 17.2/13 (18.2/13)– p-value = 19%


winter 2008 global electroweak fit

mH < 160 GeV @ 95% C.L. (<190 GeV incl. LEP-2 limit)

LEP-EWWG procedure

http://lepewwg.web.cern.ch/LEPEWWG/plots/winter2008/


conclusions• 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 are we measuring?

• MC top mass parameter• pole mass?

http://www-d0.fnal.gov/Run2Physics/top/http://www-cdf.fnal.gov/physics/new/top/top.html

Documents

top quark mass measurements at the Tevatron