Physics with Top Quarks

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

R. Erbacher - LP07 2

Something about top historyTop Discovery!Tevatron Run 1

1994-5

Top Discovery!Tevatron Run 1

1994-5


Top Rediscovered in Run 2

Top Rediscovered in Run 2!


Periodic Table of the Particles

5 orders of magnitude!


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?!?


Top physics is one of the more sexy things to study at the Tevatron…


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




Events Characterized by W Decays tWb ~ 100% tWb ~ 100%


Dilepton Decay ModeDilepton Decay Mode


Lepton+Jets Decays Lepton+Jets Decays


All-Hadronic DecaysAll-Hadronic Decays


-

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

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%


Electroweak Single Top Production

Electroweak Single Top Production

NewResonanceProduction?


s-channel ~1 pb

t-channel ~2 pb


Fermilab, Chicago, IL U.S.A.Fermilab, Chicago, IL U.S.A.

Tevatron

DØCDF

Chicago

Booster

Wrigley Field

p sourceMain Injector

Fermilab Tevatron



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


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)


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!


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


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 !


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


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


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)


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


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


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


< 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


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)


Resonances decaying to ttbarNew

ResonanceProduction?


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:



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)







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


New for summer 2007!

(not included in March combination)



Top mass:Exciting Program of measurements at the Tevatron

Top mass:Exciting Program of measurements at the Tevatron

Most precise!


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


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


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


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…


Top quark widthTop QuarkWidth

Top QuarkWidth

t < 12.7 @ 95% CLMt = 175 GeV

t < 12.7 @ 95% CLMt = 175 GeV


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?


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


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


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


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


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


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


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:


CDF: First Run 2 limits, better than LEP!See Talk by K. Tollefson Today:



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!


ConclusionsConclusions


Backups


D0 single top expected and observed


How to extract top width


Afb and higher orders


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


Event Yield of DPF and LP Analyses


•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


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


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


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!


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


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


Cos(theta*) kinematics

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Physics with Top Quarks