Tevatron Searchesin Top Decays
Junji Naganoma
Waseda University
For the CDF and D∅ Collaborations

DIS 2008, London, April 09, 2008

Why Searches in Top Sample?
• Top quark is really massive.
– Mt = 172.6 ± 1.4 GeV/c2: arXiv:0803.1683• Mass of order electroweak scale– Special role in Electroweak Symmetry Breaking?

• Smallest data size in the quark sector– Plenty of room for unexpected phenomena.
– Currently, only Tevatron can make top quarks.

• Many models include new physics coupling to top quark– Could be sensitive to physics beyond SM.

April 09, 2008

J. Naganoma, DIS 2008

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Tevatron Searches in Top Quark••

In SM, top quarks are dominantly produced inpairs at the Tevatron, and decay quickly to Wb.
But there are lots of theoretical predictions
beyond the Standard Model:• Resonances decaying to top– tt decay (next talk by J. Meyer)– tb decay: CDF & D∅
• Exotic top quark decay– Flavor Changing Neutral Current: CDF– Charged Higgs: D∅• Mimics in top decays (Admixture in top sample)– Heavy Top-like Quark (t’): CDF– Scalar Top: D∅
April 09, 2008

J. Naganoma, DIS 2008

?
t?

tt,b?
qWqx
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Top Flavor Changing Neutral Currents•

No FCNC at tree level in SM.

Top FCNC Decay
via Penguin Diagram– Further suppressed by GIM mechanism
and CKM suppression•

Occur rarely via penguin diagrams– SM: BR(t→Zq)=O(10-14)•

Beyond SM models predict higher
branching fractions– up to O(10-4)•Best published limit from LEP (L3)
– BR(t→Zq): 13.7%•Any signal at the Tevatron→ New Physics

April 09, 2008

[after J.A. Aguilar-Saavedra,
Acta Phys. Polor B35 (2004) 2695]

J. Naganoma, DIS 2008

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Top FCNC Search in CDF• Search in tt→ZqWb– Z→ee,µµ (clean signature)– 4 jets (large BR of W→qq’)• SM backgrounds– Dominant: Z + jets
– Small: tt and diboson• Full event kinematics reconstructedmass χ2 variable is used as discriminant.

April 09, 2008

J. Naganoma, DIS 2008

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Top FCNC Search: Results• Limit on B(t→Zq) obtained
from template fit to mass χ2.

1.9 fb-1

failed at leastone cut– Simultaneous fit to two signal
regions and one control region– Feldman-Cousins limit withsystematic uncertainties• World’s best limit on B(t→Zq):
B(t→Zq) < 3.7% @ 95% C.L.
• Best published limit (13.7%)
improved by factor of 3.5.

April 09, 2008

J. Naganoma, DIS 2008

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Charged Higgs Search in Top Decay• MSSM predicts large BR(t→H+b)>10% for small and largetanβ, if M(H+)<M(t).
• H+ decays differently than W+:– H+→τ+ντ if large tanβ
– H+→t*b→W+bb for high M(H+) if low tanβ
– H+→cs if small tanβApril 09, 2008

J. Naganoma, DIS 2008

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Charged Higgs Search in D∅
In SM, cross section ratio expectation:

1 fb-1Rσ confidence interval

Measurement in agreement with SM:

BR(t→H±b) confidence interval•Translate Rσ into BR(t→H±b) by assuming– M(H±) ~ M(W): not ruled out by LEP– H± decays exclusively to cs(in a general multi-Higgs-doublet model)

BR(t→H±b) < 35% @ 95% C.L.

Expected limit: BR(t→H±b) < 25%April 09, 2008

J. Naganoma, DIS 2008

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Heavy Top-like Quark (t’) Search•Consider possible contribution to top sample from top-like particle (t’)•Examples– 4th chiral generation consistent with precision EWK data
[Phys. Rev. D64, 053004 (2001)]– “Beautiful Mirrors” Model: additional generation of quarks that mix with3rd generation[Phys. Rev. D65, 053002 (2002)]– Little Higgs model with T-parity conservation: suggests a heavy top[Phys. Lett. B563, 191 (2003)]•Assumptions– t’ is pair-produced strongly– t’ is heavier than top quark– t’ decays promptly to Wq• Happens when M(t’) < M(b’) + M(W)• Precision EWK data suggests small mass splitting between b’ and t’

April 09, 2008

J. Naganoma, DIS 2008

9/17

t’ Search in CDF•

2.3 fb-1

Search in lepton+jets channel– No b-tagging requirement•

Background– Dominant: W+jets– Smaller: top pair, QCD multi-jets•

2D fit in HT vs Mreco– HT = ΣPT of all objects– Mreco = Wq mass reconstructedwith a χ2 fitter used in Mt measurementsApril 09, 2008

J. Naganoma, DIS 2008

10/17

t’ Search: Result•

2.3 fb-1

Set 95% C.L. upper limits on σ(t’t’)

M(t’) > 284 GeV/c2@ 95% C.L.

Model Independent Search• Counting experiment in n×n box• Find a region with a max. excess• 4.7 events expect, 11 observed• Apply a trial factor to obtain anoverall p-value• p-value = 2.8% (~2σ)

April 09, 2008

J. Naganoma, DIS 2008

11/17

Scalar Top (stop) Search••••M(stop)=170 GeV @ tan β=5M(stop)=230 GeV @ tan β = 45

Search for superpartner of top
At Tevatron, stops are produced inpairs,
adecay modecan be mixed in top sample withadditional Met by neutralino.
Consider:– M(stop) ≤ M(top)– MSSM parameters with100%
– M(stop) = 145 ~ 175 GeV– M(chargino) = 105 ~ 135 GeV– M(neutralino) = 50 GeV•Search in lepton+jets channel– Dominant bkg: ttbar– Smaller bkg: W+jets, etc.
April 09, 2008

red = final particles
J. Naganoma, DIS 2008

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Stop Search in D∅•

Define likelihood discriminant:

0.9 fb-1

Mrecoµ+jets
175/135•

Chose appropriate variables
at each stop/chargino mass toseparate signal from background.

µ+jetsµ+jetsApril 09, 2008

J. Naganoma, DIS 2008

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Stop Search: Result•••

0.9 fb-1

No evidence for scalar top is observed.
Set the cross section upper limit at 95% C.L.
for electron, muon channel separately.
The limits are a factor of 7-12 higher than MSSM expectations.

April 09, 2008

J. Naganoma, DIS 2008

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1.9 fb-1•••••

Search for W’→tb

0.9 fb-1

W’ occurs in some extensions of the SM withhigher symmetry.
Complementary to searches in W’→e(µ)ν
(e.g. W’ of leptophobic nature).
Same event selection and backgroundestimate as single top analysis.
Use M(lνjj) as discriminant
W’L : Interference with SM W boson.
W’R : No interference

April 09, 2008

J. Naganoma, DIS 2008

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1.9 fb-1

W’ Search: Results

0.9 fb-1

Observe no evidence for resonant W’ production.
Mass limits: Based on the theoretical cross section prediction(Z. Sullivan, Phys. Rev. D 66, 075011, 2006)

Mass limits:M(W’)>800 GeV if M(W’R)>M(νR)
M(W’)>825 GeV if M(W’R)<M(νR)
M(W’)>739 GeV if M(W’R)>M(νR)
M(W’)>768 GeV if M(W’R)<M(νR)
M(W’)>731 GeV if W’L
D0 Results: arXiv:0803.3256 (submitted to Phys. Rev. Lett.)
April 09, 2008
J. Naganoma, DIS 2008

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Conclusions• Many searches in top decays have been done at Tevatron– FCNC, Charged Higgs, t’, stop, W’• No evidence is obtained so far.
– Just pushing the world’s best limits• Tevatron will provide more than a factor of 3 data for– more precise measurements– high energetic tail region– unique opportunity in qq→tt mode, unlike LHC• Top quark have lots of motivation for physics BSM– could open the window to physics BSM with more data

• Other searches not covered in this talk
– Charged Higgs (CDF: 190 pb-1)– Single top via FCNC (D0: 230 pb-1)
April 09, 2008

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April 09, 2008

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CDF FCNC Event Selection
Base selection:• Z + ≥4 jets
• OS lepton + track with 76GeV< M(Z) < 106 GeV• jets with ET>15 GeV & |η|<2.4

April 09, 2008

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CDF FCNC Acc. & Backgrounds

parameters aresmaller bkg:

Z+jets: Float in the fit

April 09, 2008

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CDF FCNC Systematic Uncertainties
Signal Acceptance:

Background Rate:

Shape Uncertainties:• Jet Energy Scale Uncertainties: Signal & Background• Z+jets Q2 scale (Alpgen): Z+jets processes
April 09, 2008

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D0 Charged Higgs• Measured cross sections:• Lepton+Jets: 8.27 +0.96-0.95 (stat+syst) ± 0.51 (lumi) pb• Dilepton: 6.8 +1.2-1.1 (stat) +0.9-0.8 (syst) ± 0.4 (lumi) pb• How to calculate

where,Then,

April 09, 2008

J. Naganoma, DIS 2008

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CDF t’: Selection & Systematics
Mass χ2• Event selection• lepton (e or mu) pT > 20 GeV• MET > 20 GeV• ≥ 4jets (ET>20GeV)• For QCD veto• ∆φ(lepton,met) ≥ 4.408 – (1/6.11)Met• ∆φ(1st jet, met) ≥ 1.888 – (1/21.6)Met• ET(1st jet) ≥ 60 GeV
Systematics

PDF uncertainties
Shape uncertainties• Jet Energy Scale• Q2 Scale (Alpgen)

April 09, 2008

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CDF t’: Counting Experimentcounting in NxN bin in HT vs. Mreco

April 09, 2008

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D0 Stop: MSSM Parameters

April 09, 2008

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D0 Stop: Selection&Bkg&Syst.
Event selection:systematics• e+jets• pT(e)>20GeV, |eta|<1.1• ≥4 jets (pT>15GeV, |eta|<2.5)• pT (1st jet) > 40 GeV• Met>20GeV, ∆φ(e,Met)>0.7π - 0.045Met• mu+jets• pT(mu) > 20 GeV, |eta|<2.0• ≥4 jets (pT>15GeV, |eta|<2.5)• pT(1st jet) > 40 GeV• Met>25 GeV, ∆φ(mu,met)>2.1-0.035Met
background

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D0 Stop: Variable Choice, etc.

µ+jetsMost discrepant variable

April 09, 2008

J. Naganoma, DIS 2008

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Searches in Single Top Sample• Interesting signal -- s and t-channel rates are• Can search for FCNC’s involving top quarks• Single top quarks are ~100% polarized
in the SM• Can test this with angular distributionsof decay products

1.25 σt (pb)differently sensitive to new interactions• Can test CP-violation -- single t vs. single tbar
• A check of the b PDF of the proton• Can search for heavy W′ bosons (L or R-handed),
contributing to s- and t-channel production modes
April 09, 2008

J. Naganoma, DIS 2008

σs (pb)T. Tait and C. P. Yuan,
Phys.Rev.D63:014018 (2001)

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