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Top quark production and properties at D0. E. Shabalina for the D0 collaboration RAS conference November 26-30, 2007. Top quark. Decays as a free quark = 5×10 -25 s
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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
11/26/0711/26/07E. Shabalina E. Shabalina
RAS conferenceRAS conference 66
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
= 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%
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
0 tags, >=4 jets
=3 jets >=4 jets
179 single, 58 double tags76 double tags
ResultsResults
11/26/0711/26/07E. Shabalina E. Shabalina
RAS conferenceRAS conference 1111
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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’:
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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)
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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
11/26/0711/26/07E. Shabalina E. Shabalina
RAS conferenceRAS conference 1717
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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
RAS conferenceRAS conference11/26/0711/26/07
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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!
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
Backup slidesBackup slides
11/26/0711/26/07E. Shabalina E. Shabalina
RAS conferenceRAS conference 2323
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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
E. Shabalina E. Shabalina RAS conferenceRAS conference11/26/0711/26/07
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
RAS conferenceRAS conference11/26/0711/26/07
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