Top quark production and properties at D0 E. Shabalina for the D0 collaboration RAS conference November 26-30, 2007

Top quark production Top quark production and properties at D0and properties at D0

E. Shabalina E. Shabalina for the D0 collaborationfor the D0 collaboration

RAS conferenceRAS conferenceNovember 26-30, 2007November 26-30, 2007

Top quark Top quark Special place in FNAL physics program Special place in FNAL physics program The only place where top quarks are The only place where top quarks are

producedproduced 12 years since the discovery12 years since the discovery The heaviest fundamental particleThe heaviest fundamental particle

• mmt t = 170.9±1.8 GeV (~1% precision)= 170.9±1.8 GeV (~1% precision)

• Close to a gold atomClose to a gold atom Mass close to scale of electroweak Mass close to scale of electroweak

symmetry breaking symmetry breaking • May shed light on EWSB mechanism

Decays as a free quark Decays as a free quark • = 5×10-25 s <<QCD

-1

• Passes spin information to its decay products

• Allows to test V-A structure of SM

Top quark propertiesTop quark properties Produced mostly in Produced mostly in tttt

pairs at the Tevatronpairs at the Tevatron• 85% 85% qq,qq, 15% 15% gggg

= 6.8 ± 0.6 pb at NLO= 6.8 ± 0.6 pb at NLO Dataset: ~1 fbDataset: ~1 fb-1-1, ~10 , ~10

times more than Run Itimes more than Run I Precise measurements of Precise measurements of

top quark properties are top quark properties are possible possible

Most of the current Most of the current knowledge comes from knowledge comes from the studies of top quark the studies of top quark pairs pairs

Production of single top Production of single top quarks: next talk by E. quarks: next talk by E. Boos Boos

p

p tb

W

q

q’

t b

W+

l

X

Production Production cross-cross-sectionsection

Resonant Resonant productioproductionn

Production kinematicsTop Top charge charge asymmetrasymmetryy

Top MassTop MassW helicityW helicity

|V|Vtbtb||

Branching Branching RatiosRatios

Rare/non SM Decays

Anomalous Couplings

CP violation

Top Spin

Top Charge

Top Width

_ _

_

_

Production mechanism

44

Top decay channelsTop decay channels

SM decaySM decay::

E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07

Top pair final states are classified based on W decays

Dilepton (ee, Dilepton (ee, μμμμ, e, eμμ))• Both W’s decay leptonicallyBoth W’s decay leptonically

Lepton (e or Lepton (e or μμ) + jets) + jets• One W decays leptonically, One W decays leptonically,

one hadronicallyone hadronically All-hadronicAll-hadronic

• Both W’s decay hadronicallyBoth W’s decay hadronically ττhadhad +X +X

BB-jet identification-jet identification Top, Higgs signal contain Top, Higgs signal contain bb--

jetsjets Most of backgrounds do notMost of backgrounds do not BB-hadron lifetine -hadron lifetine ~ 1 ps~ 1 ps BB-hadron travels L-hadron travels Lxyxy~1 mm ~1 mm

before decaybefore decay

Combine properties of reconstructed Combine properties of reconstructed secondary vertexes and displaced secondary vertexes and displaced tracks in 7-variable networktracks in 7-variable network• Working point: efficiency ~54%, fake rate Working point: efficiency ~54%, fake rate

~1% ~1% tt event tagging probability ~70% tt event tagging probability ~70%

11/26/0711/26/07E. Shabalina E. Shabalina

RAS conferenceRAS conference 55

Dilepton channelDilepton channel



57 candidates with nj>=2, 43.9 expected signal ,13.3 background

use events with 1 jet in emu channel to increase acceptance

22.5%For mtop = 175 GeV

ee,eµ,µµ: = 6.8 (stat) (syst) ± 0.4(lum) pb = 6.8 (stat) (syst) ± 0.4(lum) pb+1.2+1.2 1.11.1

+0.9+0.9 0.80.8

77

Dilepton combinationDilepton combination


Lepton+track channelLepton+track channel Looser selection – apply Looser selection – apply bb-tagging-tagging Orthogonal to dileptonOrthogonal to dilepton Use events with nj=1 and nj>=2 Use events with nj=1 and nj>=2

16 16 candidates with candidates with nj>=2 nj>=2

18 18 expected signal expected signal 3.2 3.2 background background

Combined (L=1 fb-1) @ mtop = 175 GeV

CDF best (L=1 fb-1) in lepton+track (no dilepton veto): 19%

dilepton: = 6.2 = 6.2 ± 0.9 ± 0.9 (stat) (syst) ± 0.4(lum) pb(stat) (syst) ± 0.4(lum) pb+0.8+0.8 0.70.7 19.5%

88

Hadronic Hadronic +lepton channel+lepton channel Important channel to expand top program (searches for non-SM top

decays, H+) Cross section: use all available top events, assume SM branching

Cross section×Br for ttµ(e)+h (other top contributions are considered to be background)

µ+ e+ Selected 29 18 Expected 5.6 4.7

µ+ e+ Selected 29 18 Expected 5.6 4.7

SM: ×Br = 0.126×Br = 0.126

CDF (350 pb-1)

5 events selected 2.7±0.4 bckg


= 8.3= 8.3 ± ± (stat) (syst) ± 0.5(lum) pb(stat) (syst) ± 0.5(lum) pb+1.4+1.4 1.21.2

+2.0+2.0 1.81.8

BrBr = 0.19 = 0.19 ± 0.08 ± 0.08 (stat) ± 0.07(syst) ± 0.01(lum) pb(stat) ± 0.07(syst) ± 0.01(lum) pb

Pretag

After tagging

99

Lepton+jets channel (topology)Lepton+jets channel (topology)

Update is expected soon, will include events with nj=3

Cross section (L=0.9 fb-1, mtop = 175 GeV)

Uncertainty 18.5%


1010

Top branching fractionsTop branching fractions

2222

2

||||||||

||

)(

)(tb

tdtstb

tb VVVV

V

WqtBr

WbtBrR

=1 in SM

Assumed value of R changes the fraction of events with 0,1 and 2 tags

Perform simultaneous fit of R and ttbar cross section


0 tags, >=4 jets

=3 jets >=4 jets

179 single, 58 double tags76 double tags

ResultsResults



R=1: 12%

Using unitarity of CKM matrix

Simultaneous fit result in good agreement with SM:

CDF best (L=1.1 fb-1): 13%

Cross section summaryCross section summary

09/28/0709/28/07E. Shabalina (UIC)E. Shabalina (UIC)

D0 collaboration meetingD0 collaboration meeting 1212

The most precise singlemeasurement in the world

1313

Cross sections ratioCross sections ratio Extract as much physics as possible from the Extract as much physics as possible from the

existing measurementsexisting measurements

dilepton

jetsl

ttpp

ttppR

)(

)(

Ratio is sensitive to the non-W decays of top beyond SM tXb

)(21.1 27.026.0 sysstatR

Simplified model: charged Higgs tH±b with a mass close to W and exclusive H±cs

(leptophobic higgs in MHDM: hep-ph/9509203, hep-ph/9401311, radiative corrections in MSSM: hep/ph/9907422)

FC limit: B(t→H+b) < 0.35 @95% C.L.

)(13.0 12.011.0 sysstatB


1414

Top resonancesTop resonances No resonant production is predicted in SMNo resonant production is predicted in SM Some models predict ttbar bound states: Some models predict ttbar bound states:

topcolor assisted technicolor predicts topcolor assisted technicolor predicts leptophobic Z’ with strong coupling to 3leptophobic Z’ with strong coupling to 3rdrd generationgeneration

Narrow width: width is dominated by detector Narrow width: width is dominated by detector effects effects

Use lepton+jets events with >=1 Use lepton+jets events with >=1 bb-tag-tag

Excluded mass range:Excluded mass range:MMZ’Z’ < 680 GeV @ 95% CL < 680 GeV @ 95% CLExpected MExpected MZ’Z’ < 740 GeV < 740 GeV CDF MCDF MZ’Z’ < 725 GeV < 725 GeV

No evidence for a narrow resonance Set upper limit on the X×B(Xtt)tt=6.8

pb

Z’ with mass 750 GeV


Search for bumps in ttbar Search for bumps in ttbar reconstructed mass spectrumreconstructed mass spectrum

Z’

1515

Top charge asymmetryTop charge asymmetry No asymmetry in QCD LO, 5No asymmetry in QCD LO, 510% at NLO, 10% at NLO,

even larger at NNLO even larger at NNLO Depends on the region of phase space, Depends on the region of phase space,

any extra jet productionany extra jet production Reconstruct ttbar pair using kinematic Reconstruct ttbar pair using kinematic

fitter fitter Extracted simultaneously with sample Extracted simultaneously with sample

composition from the likelihood fitcomposition from the likelihood fit Do not correct for acceptance and Do not correct for acceptance and

reconstruction effects reconstruction effects

A = (12 ±8 (stat) ± 1 (syst))%A = (12 ±8 (stat) ± 1 (syst))%

Large positive asymmetry is predicted for Z’ productionLeptophobic Z’ Not restricted to narrow Z’

Limits the fraction F of top pairs produced via Z’:


F<0.44 (exp), F<0.81(obs) F<0.44 (exp), F<0.81(obs) for for Mz’Mz’ = 750 GeV = 750 GeV

1616

W helicityW helicity

ff++ = 0.017±0.048 (stat)±0.047 (syst) f = 0.017±0.048 (stat)±0.047 (syst) f++

< 0.14 @95% C.L. < 0.14 @95% C.L. for ffor f00=0.7 (fixed to SM value) =0.7 (fixed to SM value)

• Helicity = the relative direction between the spin and the particle's motion

• SM V-A vertex dictates the fractions

• extract from cos(*) distribution

• combine dilepton (two measurements per event and l+jets channels)


dileptondilepton

l+jetsl+jets

Model-independent measurement: • simultaneously fit fsimultaneously fit f00 and fand f++

• measure angle between top quark and down-type fermion (lepton or d,s-quark)• allows to use hadronic W-boson decays

Coming soon

Coming soon

Top quark mass measurementTop quark mass measurementMany techniquesMany techniques

• TemplateTemplate• Matrix ElementMatrix Element• Ideogram (hybrid of the Ideogram (hybrid of the

above) above)

Follow the same pattern:Follow the same pattern: Measure the observable Measure the observable

sensitive to the top quark sensitive to the top quark massmass

Map the partons to Map the partons to reconstructed objects reconstructed objects (combinatorics!) (combinatorics!)

Calibrate with pseudo-Calibrate with pseudo-experimentsexperiments

Extract the mass from the Extract the mass from the maximum likelihoodmaximum likelihood



Require a clean mapping between Require a clean mapping between reconstructed objects and partons reconstructed objects and partons Jet energy scale calibration is crucialJet energy scale calibration is crucial

W boson decay products allow W boson decay products allow to use the known W mass as an to use the known W mass as an in-situ calibration tool in-situ calibration tool

1818

Dilepton mass Dilepton mass

Neutrino weighting Scan potential top quark masses

and rapidities Solve for the 4-vectors Assign weights based on

comparison of calculated and reconstructed MET

Form weight templates for each mass

Fit signal and background templates to data

Matrix weighting Scan potential top quark masses Solve for top quark momentum

• assume two leading jets are b-jets• 4 solutions per ttbar • Include detector resolution

Calculate weight as a function of mass for each event


Underconstrained kinematics given two neutrinos

172.5±5.8(stat)±3.5(syst) GeV172.5±5.8(stat)±3.5(syst) GeV

Dominant SystematicsDominant Systematics

JES ±2±2.5 GeVb JES ±2±2.0 GeVTemplate statistics ±0.9 GeV±0.9 GeV

4%

175.2±6.1(stat)±3.4(syst) GeV175.2±6.1(stat)±3.4(syst) GeV

CDF: 2.5% @1.8fb-1 ME

1919

Matrix Element method Matrix Element method Pioneered by D0 and provides the most accurate

measurement of the top quark mass • Calculate per-event probability density for signal and

background as a function of the top quark mass using 4-vectors of reconstructed objects

• Multiply the event probabilities to extract the most likely mass

• Maximizes statistical power by using all event information• Extremely CPU intensive (most recent result required >0.5 M

grid-hours for integration)E. Shabalina E. Shabalina

RAS conferenceRAS conference11/26/0711/26/07

2020

Lepton+jets mass (ME) Lepton+jets mass (ME)

LO matrix element is used to calculate probability

Transfer functions: map measured quantities (x) to parton-level ones

The jet energy calibration (JES) is a free parameter in the fit, constrained in-situ by the mass of hadronically decaying W

1.6% (CDF: 1.25% @1.7fb-1)E. Shabalina E. Shabalina


m=170.5± 2.4 (stat+JES) ±1.2 (syst) GeVm=170.5± 2.4 (stat+JES) ±1.2 (syst) GeV

Use b-tagging information to reduce combinatorics Weight each jet-parton assignment with b-tagging event probability

24 possible weighted assignments between jets and partons

0.9 fb-1

2121

Mass combination and summaryMass combination and summary

New Tevatron average top mass is planned for Moriond’08 Requires a lot of work on systematics together with CDF


Dilepton combined: 173.7±5.4 (stat)±3.4 (syst) GeV173.7±5.4 (stat)±3.4 (syst) GeV

D0 combined: 172.1 ± 1.5 (stat) ± 1.9 (syst) GeV172.1 ± 1.5 (stat) ± 1.9 (syst) GeV

3.7%

1.4%

1.3% CDF

2222

ConclusionConclusion Top quark is the least knows quark and the most

interesting for new physics

Top physics has entered the era of precision measurements, we (finally!) have plenty of top quarks

Many top properties measurements are just beginning to have sensitivity

There is still a lot to understand about top!


Backup slidesBackup slides



2424

Search for scalar topSearch for scalar top Stop is predicted by SUSY

• Consider mstop <= mtop

1+ and 0

masses - close to their experimental lower limits

• mstop: 145-175 GeV 1

+: 105-135 GeV Same final state as ttl+jets

•

• Use kinematic fitter to reconstruct events to ttbar hypothesis

• Build discriminant to separate top from stop

• regular kinematic variables

• fit output Limit is 7-12 times Limit is 7-12 times higher than MSSMhigher than MSSM


stop

2525

Single topSingle top

Since evidence ME Since evidence ME and BNN analyses and BNN analyses were updatedwere updated

All three show similar All three show similar sensitivitysensitivity

Results are combinedResults are combined

(tb+tqb)=4.7 pb

p-value 0.014%

significance 3.6


2626

Mass from cross sectionMass from cross section What mass do we measure?

Depends on the convention…• Pole mass? • MS? • Pmas(6,1) in Pythia? – probably

the closest given the analysis techniques

Cross sections are less dependent on the details of signal simulation

Extract for dilepton and l+jets channels independently

Depends on theory prediction:• Cacciari et al• Kidonakis, Vogt

(lepton+jets, Kidonakis and Vogt) E. Shabalina E. Shabalina


m=166.9± (stat+sys) (theory) GeVm=166.9± (stat+sys) (theory) GeV+5.9+5.9 5.25.2

+3.7+3.7 3.83.8

Documents

Top quark production and properties at D0 E. Shabalina for the D0 collaboration RAS conference November 26-30, 2007