Post on 31-Dec-2015
description
Tevatron Top Physics
Meenakshi Narain
Brown University(for the D0 and CDF collaborations)
Fermilab User’s Meeting – June 4, 2008
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top at Fermilab• 13 years ago…
…we observed a few handfuls of top quark decays
• today…
…we are performing detailed studies of 1000s of top decays
4.2 fb-1
3.7 fb-1
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outline• strong production
– cross section– branching fractions
• mass• couplings
• new physics?– FCNC decays– tt resonances– tb resonances– H+
– ….
• electroweak production– |Vtb|
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W W
t
b
W W
H0
mtop2
log(mH)
why is the top quark important?• most massive elementary particle
– dominant contributor to radiative corrections
– how is its mass generated?• topcolor?
– does it couple to new physics?• massive G, heavy Z’, H+, …
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top-antitop production • strong interaction top-antitop pairs
= 7.6±0.6 pb (Kidonakis & Vogt, arXiv:0805.3844)• mt = 171 GeV, NNLO(approx)+NNNLL, CTEQ6M
= 7.6 ±0.6 pb (Cacciari et al., arXiv:0804.2800) • mt = 171 GeV, NLO+NLL, CTEQ6.5
= 7.8 ±0.6 pb (Moch & Uwer, arXiv:0804.1476) • mt = 171 GeV, NNLO(approx.), CTEQ6.5
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standard model top 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
dileptons6%had+e/μ
4%
lepton+jets34%
had+jets10%
all jets46%
primaryvertex
secondary vertex
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dileptons lepton+jets
why measure the ttbar cross section?• cross section analysis
– basic understanding of signal and background necessary for further study
– consistency between channels– decay branching fractions– are there non-standard decays?
B(tH+b) = 0 B(tH+b) = 0.1 B(tH+b) = 0.2 B(tH+b) = 0.3 B(tH+b) = 0.4 B(tH+b) = 0.5 B(tH+b) = 0.6 all jets
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ttbar cross section in dilepton channel• characteristics
– small branching fraction– two neutrinos– kinematically underconstrained + signal:background = 3:1
• selection– two isolated leptons/lepton+track 2 jets
– missing 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
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ttbar cross section in dilepton channel• CDF (2 fb-1)
– b-tag = 9.0±1.1(stat)±0.7(syst)±0.5(lum) pb
– no b-tag = 6.8±1.0(stat)±0.4(syst)±0.4(lum) pb
• D0 (1 fb-1)– dileptons
= 6.8+1.2-1.1(stat)+0.9
-0.8(syst)0.4(lum) pb
– lepton+track = 5.1+1.6
-1.4(stat)+0.9-0.8(syst)0.3(lum) pb
– combined = 6.2 0.9(stat) +0.8
-0.7(syst)0.4(lum) pb
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ttbar cross section in channels• interesting because of
tH+b, H+• 3 types of hadronic decays
• require– 1 , 1 e/, ≥2 jets, missing pT
– 1 jet is b-tagged
• neural networks distinguish decays from background
1 trackno em cluster12%
1 trackem cluster38%
≥2 tracks15%
= 7.4+1.4–1.3(stat)+1.2
–1.1(syst)±0.4(lum) pb (2.2 fb-1)
B(ttl) = 0.190.08(stat)0.07(syst)0.01(lum) pb (1 fb-1)
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ttbar cross section in all-jets channel• characteristics
+ large branching fraction+ no neutrinos+ complete reconstruction– signal:background 1:16
• selection– 6-8 well separated jets– no significant miss pT
– at least one b-tag– neural network
ET, event shape, angles
• backgrounds– pretag data x tagging
probability from 4-jet control region
jet
b-jetb-jet
jet
jet jet
All-hadronic(BR≈46%)
= 8.3±1.0(stat)+2.0-1.5(syst)±0.5(lum) pb
1 fb-1
CDF PRD 76 072009
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ttbar cross section in l+jets channel• characteristics
– large branching fraction– one neutrino– kinematically overconstrained – signal:background = 1:2, 2:1(b-tag)
• selection– one isolated lepton – 4 jets – missing pT jet
e,
b-jet
b-jet
jet
Lepton+jets(BR~34%)
MET
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ttbar cross section in l+jets channel• extract top fraction using event topology
– angles, momentum sums, and event shape variables– dominated by statistical uncertainties
D0 (0.9 fb-1) likelihood discriminant=6.60.8(stat)0.4(syst)0.4(lum) pb
CDF (0.8 fb-1)neural network=6.00.6(stat)0.9(syst)0.3(lum) pb
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ttbar cross section in l+jets channel• count number of events with at least one b-tagged jet
– smaller statistical uncertainty– large systematic uncertainty from jet energy calibration and b-tagging
D0 (0.9 fb-1)
=8.1±0.5(stat)±0.7(syst)±0.5(lum) pb
CDF (2 fb-1)
=8.2±0.5(stat)±0.8(syst)±0.5(lum) pb=8.7±1.1(stat)+0.9
-0.8(syst)±0.6(lum) pb=7.8±2.4(stat)±1.5(syst)±0.5(lumi) pb
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implications of cross section• compare different channels
– B(tWb)>0.79 @ 95% CL– B(tH+b)<0.35 @ 95% CL
for mH+ = mW and H+cs
• compare with theory– combine likelihood and b-tag
cross section measurements
– σtt = 7.62 ± 0.85 pb
for mtop = 172.6 GeV
– mt = 170 ± 7 GeV
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single top production• electroweak production of top quarks
─ s channel (tb) ─ t channel (tqb)
– event selection• 1 isolated e/
• missing pT
• 2-4 jets, ≥1 b-tag
NLO = 0.9±0.1 pb = 2.0±0.3 pb PRD70 (2004) 114012
event yields (CDF 2.2 fb-1)
Njets 2 3
single top 103 32
backgrounds 1492 745
data 1535 712
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single top production• D0 results (0.9 fb-1)
• CDF results (2.2 fb-1)
= 4.7 ± 1.3 pbsignificance: 2.3 σ (expected)significance: 3.6 σ (observed)PRL 98, 181802 (2007) PRD (accepted)
= 2.2 ± 0.7 pbsignificance: 5.1 σ (expected)significance: 3.7 σ (observed)
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single top production• measure |Vtb| from total cross section
– D0: |Vtb|>0.68 @ 95% CL CDF: |Vtb|>0.66 @ 95% CL
• disentangle s and t channels
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top mass measurement• template fits
– mass estimator (eg best mt from kinematic fitter)
– fit probability density functions from simulated tt events generated to data
• event-by-event likelihood – for each event determine
likelihood as a function of mt (eg by integrating over LO matrix element)
– extract mass from peak of joint likelihood
Event 3Event 2Event 1
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dilepton channel • D0 (1 fb-1)
– compute weight curve as a function of top mass for each event
– template fit to mass distribution
• CDF (2 fb-1)– selection uses evolutionary
neural network that optimizes resolution
– compute event weight using LO matrix element
matrix weighting
173.7±5.4(stat)±3.4(syst) GeV 171.2±2.7(stat)±2.9(syst) GeV
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lepton+jets• matrix element analysis
– integrate over LO matrix element to get likelihood for event as a function of top quark mass
– in situ jet energy calibration using Wqq decay – peak of joint likelihood = top quark mass
– CDF: 171.4±1.5(statjes)±1.0(syst) GeV (1.9 fb-1) – D0: 172.2±1.1(stat)±1.6(systjes) GeV (2.1 fb-1)
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all jets (CDF)• kinematic fitter
– leading 6 jets
– jj/jjj masses, jet pTs
• 2-dimensional template fit– top/W masses with smallest 2
jet
b-jetb-jet
jet
jet jet
mtop = 177.0 ± 3.7(statjes) ± 1.6(syst) GeV
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combination
http://tevewwg.fnal.gov/top/http://lepewwg.web.cern.ch/LEPEWWG/plots/winter2008/
for the first time m/m<1% Run II goal: m 1 GeV
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top quark couplings• if top plays a special role in ewk symmetry breaking its
couplings to W bosons may differ from predictions
• sm predicts V-A coupling at Wtb helicity of W boson– F0 = 0.7, F = 0.3, F+ = 0.0
(longitudinal, left-handed, right-handed)– different distributions of *
angle between lepton and top in W rest frame
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top quark couplings• D0 (1 fb-1)
F0 = 0.42 0.17(stat) 0.10(sys)
F+= 0.12 0.09(stat) 0.05(sys)
• CDF (1.9 fb-1)
Phys Rev Lett 100, 062004 (2008)
68% CL95% CL
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searches for non-standard physics• quarks with charge 4/3e • FB ttbar asymmetry • 4th generation t’ quarks • scalar top production • charged Higgs bosons• tb resonances • ttbar resonances• FCNC decays of top quarks
disfavored
consistent with sm
m > 284 GeVno evidence limits on H+ tb,tH+b
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tb resonances• vector resonance: W’tb• left-handed couplings
– DØ include interference with SM W
• right-handed couplings – decay to Rl/qq, depending on m(R)
• lower limits on MW’: 731-825 GeV
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tt resonances • D0 (2.1 fb-1)
– technicolor Z’ttHill & Parke, PRD 49 (1994) 4454)
• CDF (1.9 fb-1)– massive gluon Gtt
MZ’>760 GeV
for Z’/MZ’ =1.2%
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FCNC decays of top quarks• flavor changing neutral currents
– highly suppressed in sm– select Z(ee/) + ≥4 jets
– fit to
B(tZq) < 3.7% @ 95% CL
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conclusion• top physics has come a long way since 1995
– Tevatron is still the only place to do it– ttbar cross section measured to 11%– evidence for single top production
• 5 this summer?
– top quark mass measured to 0.8%• will reach uncertainties below 1 GeV
– top properties and possible non-standard physics studies in great detail
– in the last year: D0 7 papers and 13 theses– in the last year: CDF 8 papers and 16 theses
• it still looks like top
http://www-d0.fnal.gov/Run2Physics/top/http://www-cdf.fnal.gov/physics/new/top/top.html
thank you
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charged Higgs bosons• MH+> mt H+ tb
• tb resonance
• MH+< mt t H+b
• H+cs
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top quark charge• is it
– tW+b (Qtop = 2/3 e)– tW-b (Qtop = -4/3 e)
• Exotic model – doublet (–1/3e,–4/3e) ?– D. Chang et al., PRD59 (1999) 091503
• D0 PRL 98, 041801 (2007) – 4/3e excluded at 92% CL– fraction of exotic quark pairs
< 0.80 (90% CL)
• CDF result with 1.5/fb– p-value for SM: 0.31– exotic model XM excluded with
87% CL
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FB charge asymmetry • Asymmetry:
– LO – no asymmetry– NLO – small (3-5%) asymmetry predicted in sm– new physics could lead to larger values (eg Z’)
Phys. Rev. Lett. 100, 142002 (2008)
D0 AFB in parton rest frame:AFB = 0.12±0.08(stat)±0.01(syst)consistent with sm expectation
CDF unfold reconstructed AFB to go to parton levelAFB
pp = 0.17± 0.07(stat) ± 0.04(syst)
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t’ quark• 4th generation heavy quark
– pair-produced via strong interaction– more massive than top– decays to Wb, Ws, Wd
Mt’ < 284GeV at 95%C.L.
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mass syst• CDF dilepton l+jets
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xsec syst
source uncertainty
vertex 0.15 pb
e id 0.11 pb
μ id 0.08 pb
jet id 0.12 pb
non-W bkg 0.06 pb
jet response 0.30 pb
MC model 0.29 pb
b-tagging efficiency 0.48 pb
total 0.69 pb
source selection fit total
vertex 0.13 pb 0.13 pb
e id 0.10 pb 0.10 pb
μ id 0.06 pb 0.06 pb
jet id 0.10 pb 0.02 pb 0.12 pb
non-W bkg 0.10 pb 0.10 pb
jet response 0.35 pb 0.26 pb 0.11 pb
MC model 0.13 pb 0.09 pb 0.11 pb
template stats 0.15 pb 0.15 pb
total 0.36 pb
D0 l+jets b-tag kinematic likelihood