Physics with Top Quarks
description
Transcript of Physics with Top Quarks
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Physics with Top QuarksPhysics with Top QuarksProf. Robin Erbacher
University of California, Davis
Lepton-Photon 2007
Prof. Robin Erbacher
University of California, Davis
Lepton-Photon 2007
t
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Something about top historyTop Discovery!Tevatron Run 1
1994-5
Top Discovery!Tevatron Run 1
1994-5
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Top Rediscovered in Run 2
Top Rediscovered in Run 2!
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Periodic Table of the Particles
5 orders of magnitude!
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Many top properties measurements just beginning to have sensitivity: lots about top still to understand!
Many top properties measurements just beginning to have sensitivity: lots about top still to understand!
New Physics?!?
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Top physics is one of the more sexy things to study at the Tevatron…
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Top Quark Production• Mechanism• Top Pair Cross Section• Ewk Production (single top)• Forward-backward asymmetry• Resonances decaying to top• stop production
Top Quark Production• Mechanism• Top Pair Cross Section• Ewk Production (single top)• Forward-backward asymmetry• Resonances decaying to top• stop production
Top Event Decays• W helicity (V-A)• Branching ratios
• Top to charged higgs• Top sample (W+c)
• FCNC
Top Event Decays• W helicity (V-A)• Branching ratios
• Top to charged higgs• Top sample (W+c)
• FCNC
Top Properties• Top Mass
• Top Quark Width• Charge of Top Quark
Top Properties• Top Mass
• Top Quark Width• Charge of Top Quark
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Events Characterized by W Decays tWb ~ 100% tWb ~ 100%
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Dilepton Decay ModeDilepton Decay Mode
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Lepton+Jets Decays Lepton+Jets Decays
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All-Hadronic DecaysAll-Hadronic Decays
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-
MIP signalIn calorimeter
Jet 2
secondary vertex
interaction point
Jet 1
secondary vertex
interaction point
Muon + jets event with 2 tagged b-quark jetsMuon + jets event with 2 tagged b-quark jets
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R. Erbacher - LP07 13One top pair each 1010 inelastic collisions at s = 1.96 TeVOne top pair each 1010 inelastic collisions at s = 1.96 TeV
Strong Pair Production at the
Tevatron
Rarely!!Rarely!!
How is top produced?How is top produced?
~85%
~15%
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Electroweak Single Top Production
Electroweak Single Top Production
NewResonanceProduction?
NewResonanceProduction?
s-channel ~1 pb
t-channel ~2 pb
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Fermilab, Chicago, IL U.S.A.Fermilab, Chicago, IL U.S.A.
Tevatron
DØCDF
Chicago
Booster
Wrigley Field
p sourceMain Injector
Fermilab Tevatron
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Top Quark Production• Production mechanism• Top pair cross section
• EWK production (single top)• Forward-backward charge asymmetry
• Resonances decaying to top• Search for stop production
Top Quark Production• Production mechanism• Top pair cross section
• EWK production (single top)• Forward-backward charge asymmetry
• Resonances decaying to top• Search for stop production
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Does t-tbar production match NLO prediction?Does t-tbar production match NLO prediction?
(ggtt)/(pptt) = 0.07 ± 0.16
(NLO: gluon~15%, quark~85%)
(neural net analysis: fg < 0.33 @ 68% CL)
(ggtt)/(pptt) = 0.07 ± 0.16
(NLO: gluon~15%, quark~85%)
(neural net analysis: fg < 0.33 @ 68% CL)
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pb7.6)175@( ≈=→ GeVMttpp top pb7.6)175@( ≈=→ GeVMttpp top
Nevents - Nbackground(tt)Luminosity *
Top Pair ProductionCross Section:• As QCD predicts?• Only SM top?• By heavy particles?
Top Pair ProductionCross Section:• As QCD predicts?• Only SM top?• By heavy particles?
t-tbar!t-tbar!
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Top pairTop pairEvent topologyDiscriminant:
No b-jettagging
Event topologyDiscriminant:
No b-jettagging
Requiring twoidentified b-jets:Ultra pure top
pair sample
Requiring twoidentified b-jets:Ultra pure top
pair sample
Top pairTop pair
€
th (p p → tt ) ≈ 6.7 pb
€
th (p p → tt ) ≈ 6.7 pb
tt=8.1 ± 0.9(stat)± 0.5(sys) pb
tt=8.2 ± 0.5(stat)± 0.9(sys) pb
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Many new dilepton measurements!
Dilepton Selection Cross Section (Mtop =175 GeV)
2 Tight Leptons (1.2 fb-1) tt= 6.2 ± 1.1(stat)± 0.7 (sys) pb
Lepton+Track (1.1 fb-1) tt= 8.3 ± 1.3(stat)± 0.9 (sys) pb
Lepton+Track+btag (1.0 fb-1) tt= 10.1 ± 1.8(stat)± 1.3(sys) pb
2 Tight Leptons (1.0 fb-1) tt= 6.8 ± 1.2(stat)± 0.9(sys) pb
2 T+ L+T+btag comb (1.0 fb-1) tt= 6.0 ± 0.9(stat)± 0.8(sys) pb
Lepton+Tau+btag (1.0 fb-1) tt= 8.3 ± 1.9(stat)± 1.4 (sys) pb
€
theory ( p p → tt ) ≈ 6.7 pb
€
theory ( p p → tt ) ≈ 6.7 pb
Tau Leptonchannelsdifficult!
Tau Leptonchannelsdifficult!
*BR(ttl++2+2b) =0.19±0.08(stat) ± 0.07(syst) pb !
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WbbWccWc
non-W
Mistags
tt
Z/D
ib
Backgrounds!• Best channels S/B~1/20
• Signal smaller than background uncertainty!
Single Top Production:• Rate |Vtb|2 in SM• Sensitive to H+, 4th gen,
W’, FCNC, …• Signature ~ SM Higgs• SM cross section ~3 pb
Single Top Production:• Rate |Vtb|2 in SM• Sensitive to H+, 4th gen,
W’, FCNC, …• Signature ~ SM Higgs• SM cross section ~3 pb
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neural network
multivariate techniques can coax signal out from large backgrounds
multivariate techniques can coax signal out from large backgrounds
Boosted decision tree
boosted decision trees, matrix element reconstruction, bayesian neural networks, likelihood discriminants
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DØ Combination: 3.6
Expected significance: 2.3
First direct measurement of Vtb:0.68 <|Vtb|< 1 @ 95%CL or
|Vtb| = 1.3 ± 0.2
First direct measurement of Vtb:0.68 <|Vtb|< 1 @ 95%CL or
|Vtb| = 1.3 ± 0.2
D0 Single top results
s+t= 4.9 ±1.4 pb
s= 1.0, t =4.0 pb
s+t= 4.9 ±1.4 pb
s= 1.0, t =4.0 pb
Expected sensitivity: 2.1
3.4!
Boosted decision trees
D0 Results: Search for Single Top D0 Results: Search for Single Top
PRL 98 18102 (2007)
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Observed p-value = 0.09% / 3.1Expected p-value = 0.13% / 3.0
Results for Single Top from CDF
New CDF Results: Search for Single TopNew CDF Results: Search for Single Top
s+t= 3.0 ± 1.2 pb
s= 1.1, t =1.9 pb
s+t= 3.0 ± 1.2 pb
s= 1.1, t =1.9 pb
Expected sensitivity: 2.9Observed significance: 2.7
Expected sensitivity: 3.0
3.1 Evidence
s+t= 2.7 ± 1.2 pb
s= 1.1, t =1.3 pb
s+t= 2.7 ± 1.2 pb
s= 1.1, t =1.3 pb
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Using the Matrix Element cross section measurement, CDF determines |Vtb| assuming |Vtb| >> |Vts|, |Vtd|
Using the Matrix Element cross section measurement, CDF determines |Vtb| assuming |Vtb| >> |Vts|, |Vtd|
CDF Run II Preliminary L=1.5 fb-1
|Vtb|= 1.02 ± 0.18 (expt) ± 0.07 (theory)|Vtb|= 1.02 ± 0.18 (expt) ± 0.07 (theory)
Z. Sullivan, Phys.Rev. D70 (2004) 114012
t-channel
s-channel
D0 |Vtb|>0.68, |Vtb| = 1.3 ±0.2
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Forward-Backward Production Asymmetry
Forward-Backward Production Asymmetry Afb
Forward-Backward Production Asymmetry Afb
Diagram interferences for qq
No asymmetry expected at LO4-6% expected at NLO in parton frame
J. Kuhn, et al.
Reduced Asymmetry in tt+jet -- Uwer, et al.
Smaller asymmetry in lab frame
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< 0< 0
Afb
How would new physics
look?
How would new physics
look?
For MFor MZ'Z' = 750 GeV: = 750 GeV:
F < 0.44 (expected)F < 0.44 (expected)
F < 0.81 (observed)F < 0.81 (observed)
> >
00
Afb= 12 ± 8(stat) ± 1(syst) %
(Uncorrected for reconstruction)
Afb= 12 ± 8(stat) ± 1(syst) %
(Uncorrected for reconstruction)
First Afb Result from D0First Afb Result from D0
F: fraction of top pair events
produced via Z' resonance
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Afb CDF First Afb Results from CDFFirst Afb Results from CDF
Compare with D0 result:
Afb(bkg sub)=(14.4 ± 6.7(stat) )
%
NLO: (4-7%) in y*Ql
Afb=(28 ± 13(stat) ± 5(syst) ) %
(Fully corrected)
Afb=(28 ± 13(stat) ± 5(syst) ) %
(Fully corrected)
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Resonances decaying to ttbarNew
ResonanceProduction?
NewResonanceProduction?
Bump-hunting for Xttbar!
New D0 Result!See Talk by K. Tollefson Today:
Tevatron Striking Results
New D0 Result!See Talk by K. Tollefson Today:
Tevatron Striking Results
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Stop Search
Single-variable separation poor;Construct multivariate discriminant.
Can SUSY stop hide alongside
top?
Can SUSY stop hide alongside
top?
No evidence for stop(Limits on BR v. stop mass)No evidence for stop
(Limits on BR v. stop mass)
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Top Properties• Top Mass
• Top Quark Width• Charge of Top Quark
Top Properties• Top Mass
• Top Quark Width• Charge of Top Quark
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Top Quark Mass: Important EWK ParameterTop Quark Mass• Important EWK parameter • Key role in BSM physics models• Constrains the Higgs mass• Heavy: Unexpected role in EWSB?
Top Quark Mass• Important EWK parameter • Key role in BSM physics models• Constrains the Higgs mass• Heavy: Unexpected role in EWSB?
What a theorist sees… What an experimentalist sees
Challenges: combinatorics, b-tagging efficiencies, jet energy scale.
Solutions: sophisticated analyses,in-situ Wjj calibration
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New for summer 2007!
(not included in March combination)
New for summer 2007!
New for summer 2007!
Top mass:Exciting Program of measurements at the Tevatron
Top mass:Exciting Program of measurements at the Tevatron
Most precise!
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Top mass dilepton
Matrix Element Weighting
Neutrino-weighting
Many newtop dilepton mass results!
Many newtop dilepton mass results!
Lepton Pt
Combining channels helps:D0 matrix weighting + D0 neutrino weighting ~4% better for same luminosity
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New IdeasTop mass with cross
section constraint: trades stat uncertainty for theory
Top mass extracted from cross sections: Compare to theory and across channels! Consistent with kinematic measurement?
dilepton
New Ideas!New Ideas!
Cacciari, Mangano, et alhep-ph/0303085Cacciari, Mangano, et alhep-ph/0303085
LEPTON+JETS
€
M top =166.1 −5.3+6.1 (stat + sys) -6.7
+4.9 (theo) GeV
€
M top =174.1 −8.4+9.8 (stat + sys) -6.0
+4.2 (theo) GeV
DILEPTON
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Top mass Best per channelSnapshot:
most precise per channel
Snapshot:most precise per channel
Most precise!
Mtop=171.6 ± 2.0 GeV/c2
Mt=170.4 ± 3.1(stat) ± 3.0(sys)GeV/c2
all-hadronic fromwinter 2007:
Mt=170.4 ± 3.7(stat+JES) ± 2.1(sys)GeV/c2
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Top mass summary and combination
World Average March 2007:World Average March 2007:
Mtop=170.9 ± 1.9 GeV/c2
See P. Petroff’s Talk (next)for electroweak implicationsSee P. Petroff’s Talk (next)
for electroweak implications
D0-CDF Joint Systematics Effort Underway!New combinations will follow…
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Top quark widthTop QuarkWidth
Top QuarkWidth
t < 12.7 @ 95% CLMt = 175 GeV
t < 12.7 @ 95% CLMt = 175 GeV
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Top Quark Charge: -2/3?
D0 result with 300 pb-1:See no evidence for exoticmodel so far…
Exclude top charge exotic model XM of -4/3* with 87% C.L.
Exclude top charge exotic model XM of -4/3* with 87% C.L.
f+ = 0.87P-value = 0.31
*Chang,Chang,Ma ‘99
Top Quark Charge: +2/3?Top Quark Charge: +2/3?
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Top Event Decays• W helicity (V-A)
• Toplight charged higgs• Branching Ratios
• Top sample (W+c)• FCNC
Top Event Decays• W helicity (V-A)
• Toplight charged higgs• Branching Ratios
• Top sample (W+c)• FCNC
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t-W-b CouplingV-A?
The V-A character of the decay
makes the helicity of the W only
F0 = 0.70, F- = 0.30, F+ = 0(longitudinal, left-handed, right-handed)
t-W-b Couplingt-W-b Coupling
cos* = angle between lepton and top in W rest frame
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W helicityt-W-b Coupling: W Helicityt-W-b Coupling: W Helicity
1.7 fb-1
1-d fit: Fix F0=0.7, fit for F+
D0(1 fb-1) : f+=0.02 ± 0.05 ± 0.05 f+<0.14 @ 95%CL
CDF1(1.7 fb-1) : f+=0.01 ± 0.05 ± 0.03 f+<0.12 @ 95%CL
CDF2(1.7 fb-1) : f+=-0.04 ± 0.04 ± 0.03 f+<0.07 @ 95%CL
2-d fit: Fit for F0, F+ togetherCDF1(1.7 fb-1) : f0=0.38 ± 0.22 ± 0.07
f+=0.15 ± 0.10 ± 0.04
CDF2(1.7 fb-1) : f0=0.61 ± 0.20 ± 0.03 f+=-0.02 ± 0.08 ± 0.03
V-A: F0=0.7, F-=0.3V+A: F0=0.7, F+=0.3
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tt = 8.10-0.82 (stat+syst) ± 0.49
(lumi) pb
+0.87
R= 0.991-0.085
(stat+syst)
+0.094
Simultaneous measurement of and Branching Ratio
Simultaneous measurement of and Branching RatioSimultaneous measurement of and Branching Ratio
New measurement by D0!See Talk by K. Tollefson Today:Tevatron Striking Results
New measurement by D0!See Talk by K. Tollefson Today:Tevatron Striking Results
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Limits on charged higgsRatio of Cross Sections:Limit on Charged Higgs!Ratio of Cross Sections:Limit on Charged Higgs!
R=(pptt)l+jets/(pptt)ll
R=1.21 ± 0.26 (stat+sys)
Assume tH+b, H+cs only.If MH=MW (not ruled out by LEP):
B(tH+b)<0.35 @95% CLB(tH+b)<0.35 @95% CL
Previous CDF result with 200 pb-1 explores other parameter spaces.
Expected: B(tH+b)<0.25 @95% CL
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First: W+c Cross Section
g+s : ~ 90%, g+d: 10% Algorithm• Use soft lepton tagger (h.f. jets)
• Wc: TL, SLT charges are fully anti-correlated, large charge asymmetry A
• Backgrounds: W+l.f., nonW QCD, DY, etc mostly charge symmetric except DY
• Observable: same sign subtracted events N(OS) – N(SS)
OS SS
OS SS
N NA
N N
−=
+
( )( ) ( )4.0
4.3
( 8,| | 3)
28.5 8.2(stat) syst 1.7 lum pb
Wc c cpt BR Wσ η ν+−
> < × →
= ± ±
l
W+c Cross Section
W+c Cross Section
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Search for tZc: FCNCNo FCNCs in SM at tree level
▪Allowed in higher order penguins
Light quark penguins observed▪e.g. b→sγ observed by CLEO in 1995, BR O(10-4)
Not yet observed for top ▪SM BR: O(10-12)
New Physics models predict BRs up to O(10-2)
▪SUSY, Higgs doublet, Warped extra dimensions (J. A. Aguilar-Saavedra, Acta Phys. Polon. B35 (2004) 2695)
Tree level FCNC
Penguin
Search for tZc: FCNCSearch for tZc: FCNC
CDF: First Run 2 limits, better than LEP!See Talk by K. Tollefson Today:
Tevatron Striking Results
CDF: First Run 2 limits, better than LEP!See Talk by K. Tollefson Today:
Tevatron Striking Results
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SummarySummary
•The top quark is the least known quark, and the most interesting for new physics.
•The top physics program is very active at the Tevatron, with both precisionmeasurements and first results appearing all the time.
•Beginning to have sensitivity to the unexpected in particle properties and inthe data samples!
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ConclusionsConclusions
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Backups
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D0 single top expected and observed
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How to extract top width
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Afb and higher orders
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CDF Single Top: What Changed ?Likelihood
• improved treatment of kinematic ambiguities
• Bug fix in matrix element
• More MC statistic allows refined training
Overall expected sensitivity gain:
(as measured on 955 pb-1 analysis) :35%
Matrix Element• Separate treatment of single and
double tag events
• More refined transfer functions
• Overall expected sensitivity gain:
(as measured on 955 pb-1 analysis) :~10%
Common Improvements• new ALPGEN Monte Carlo
• W + Heavy Flavor normalization from W + 1 jet eve nts
• Define event selection on hadron level jets:
-CDF Top group wide change
-More meaningful to theorists
-Better understood (MET resolution, nonW model..)
Causes event migrations:• LF loses 1 gains 7 • ME loses 5 gains 4 for highest discriminant region
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Event Yield of DPF and LP Analyses
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•Method inherited from CDF Run I (G. Unal et. al.)
•Measure fraction of W+jets events with heavy flavor (b,c) in Monte Carlo
•Normalize fractions to W+jets events found in data
•New improvement: get normalization from W + 1 jet bin (instead of generic dijet sample)
Correct data for non W+jets events
W + Heavy Flavor Estimate
€
NWbb
data = (NWbb
NW + jets
)MC ⋅K ⋅NW + jetsdata
€
NW + jetsdata = NCandidates
data − Nnon−W − NEWK
Heavy flavor fractionsand b-tagging efficiencies
from LO ALPGEN Monte Carlo
Calibrate ALPGEN heavy flavor Fractions from W + 1 jet bin
Note: Similar for W+charm background
Large uncertainties from Monte Carlo estimate and heavy flavor calibration (~25-30%)
CDF Run II Reference for standard method:PhysRevD.71,052003
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Heavy Flavor Normalization• Improve heavy flavor estimate by
calibrating it in W+1 jet side band
• Take advantage of NN based flavor separator
• Compare Loose Secondary Vertex mass and NN flavor separator output:– consistent results within errors
• K-factor for heavy flavor:
1.4 ± 0.4
• Applied to predict W + Heavy Flavor content of W + 2 jets bin
mistags / charm ………. beauty
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Q* and Mlb cross-checks
EPD > 0.9EPD > 0.9 EPD > 0.9EPD > 0.9
EPD > EPD > 0.9660.966
EPD > EPD > 0.9660.966
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Sensitivity to New Physics and WH•Single top rate can be altered due to the presence of New Physics:- t-channel signature: Flavor changing neutral currents (t-Z/γ/g-c couplings)
- s-channel signature: Heavy W boson, charged Higgs H+, Kaluza Klein excited WKK
Tait, Yuan PRD63, 014018(2001)
Z
ct
W,H+
s (pb)
1.2
5
t (p
b)
•s-channel single top has the same final state
as WHlbb=> benchmark for WH search!
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Jet1
Jet2
Lepton
Central Electron CandidateCharge: -1, Eta=-0.72 MET=41.6 GeVJet1: Et=46.7 GeV Eta=-0.6 b-tag=1 Jet2: Et=16.6 GeV Eta=-2.9 b-tag=0QxEta = 2.9 (t-channel signature)EPD=0.95
Single Top Candidate Eventt-channel single top production has a
kinematic peculiarity:- Distinct asymmetry in Q x distribution:
lepton charge (Q) x pseudo-rapidity =-log (tan/2) of untagged jet
Run: 211883, Event: 1911511
u d
EPD > 0.9EPD > 0.9
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Best-Fit Point:
1.41.1
1.21.0
1.1
1.3
s
t
+−
+−
=
=
Each nuisance parameterfit for at each test valueof the cross section.
The Likelihood 2D Fit
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Cos(theta*) kinematics