Physics with Top Quarks

Physics with Top QuarksProf. Robin Erbacher

University of California, DavisLepton-Photon 2007

t

R. Erbacher - LP07 2

Something about top historyTop 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!


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 Properties• Top Mass

• Top Quark Width• Charge of Top Quark


Events Characterized by W Decays tWb ~ 100%


Dilepton Decay Mode


Lepton+Jets Decays


All-Hadronic Decays


-MIP signal

In calorimeter

Jet 2secondary

vertex

interaction point

Jet 1

secondary vertex

interaction point

Muon + jets event with 2 tagged b-quark jets

R. Erbacher - LP07 11One top pair each 1010 inelastic collisions at s = 1.96 TeV

Strong Pair Production at the

TevatronRarely!!

How is top produced?

~85%

~15%


Electroweak Single Top Production

NewResonanceProduction?

s-channel ~1 pb

t-channel ~2 pb


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

DØCDF

Chicago

Booster

Wrigley Field

p source Main Injector

Fermilab Tevatron


Top Quark Production• Production mechanism• Top pair cross section

• EWK production (single top)• Forward-backward charge asymmetry

• Resonances decaying to top


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)


pb7.6)175@( ≈=→ GeVMttpp top

Nevents - Nbackground(tt)=Luminosity * e

Top Pair ProductionCross Section:• As QCD predicts?• Only SM top?• By heavy particles?

t-tbar!


Top pairEvent topologyDiscriminant:

No b-jettagging

Requiring twoidentified b-jets:Ultra pure top

pair sampleTop pair


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 ± 1.5(stat)± 1.0(sys) pb

Lepton+Track+btag (1.0 fb-1) tt= 4.9 ± 1.5(stat)± 1.0(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

Tau Leptonchannelsdifficult!

*BR(ttl+t+2+2b) =0.19±0.08(stat) ± 0.07(syst) !


WbbWccWc

non-WMista

gs

tt

Z/Dib

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


neural network

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 limit on Vtb:0.68 <|Vtb|< 1 @ 95%CL

D0 Single top results

s+t= 4.9 ±1.4 pb

Expected sensitivity: 2.1

3.4!

Boosted decision trees

D0 Results: Search for Single Top


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 Top

s+t= 3.0 ± 1.2 pbs= 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 pbs= 1.1, t =1.3 pb


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)

Z. Sullivan, Phys.Rev. D70 (2004) 114012

t-channel

s-channel

D0 |Vtb|>0.68


Forward-Backward Production Asymmetry

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?

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)

First Afb Result from D0

F: fraction of top pair events produced via Z' resonance


Afb CDF First 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)


Resonances decaying to ttbarNew

ResonanceProduction?

Bump-hunting for Xttbar!

New D0 Result!See Talk by K. Tollefson Today:

Tevatron Striking Results


Stop Search

Single-variable separation poor;Construct multivariate discriminant.

Can SUSY stop hide alongside

top?

No evidence for stop(Limits on BR v. stop mass)


Top Properties• Top Mass

• Top Quark Width• Charge of Top Quark


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?

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)

New for summer 2007!

Top mass:Exciting Program of measurements at the Tevatron

Most precise!


Top mass dilepton

Matrix Element Weighting

Neutrino-weighting

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!

Cacciari, 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

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:

Mtop=170.9 ± 1.9 GeV/c2

See P. Petroff’s Talk (next)for electroweak implications

D0-CDF Joint Systematics Effort Underway!New combinations will follow…


Top quark widthTop QuarkWidth

Gt < 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.

f+ = 0.87P-value = 0.31

*Chang,Chang,Ma ‘99

Top Quark Charge: -2/3?


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(left-handed, longitudinal, right-handed)

t-W-b Coupling

cos* = angle between lepton and top in W rest frame


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 RatioSimultaneous measurement of and Branching Ratio

New measurement by D0!See Talk by K. Tollefson Today:Tevatron Striking Results


Limits on charged higgsRatio of Cross Sections:Limit on Charged Higgs!

R=(pptt)l+jets/(pptt)llR=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% CL

Previous CDF result with 200 pb-1 explores other parameter spaces.

Expected: B(tH+b)<0.35 @95% CL


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: FCNC

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

Tevatron Striking Results


Summary


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

trainingOverall 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


Cos(theta*) kinematics

Physics with Top Quarks

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