1 top-quark physics at the tevatron wolfgang wagner 1.introduction top quarks, tevatron, cdf and dØ...

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Top-Quark Physics

at the TevatronWolfgang Wagner

1. IntroductionTop quarks, Tevatron, CDF and DØ

2. Measurement of top quark properties2.1 Cross section 2.2 Top quark mass2.3 W helicity in top decays2.4 Single-top searches2.5 Window for “new physics”

3. Conclusions

Contents:Mini-Workshop

“Massive particle production at the LHC”

Berlin, October 31, 2007

Universität KarlsruheCentrum für Elementarteilchen-

und Astroteilchenphysik

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Why Do Top-Quark Physics?

special top dynamics?

Mt ~ scale of electroweak symmetry breaking

Is the observed particle the SM top quark?

charge, branching ratios, spin, polarisation, …

top quark samples are still small plenty of room for new phenomena

Mt >> Mb > Mc >> Ms

large contribution of top loop-diagrams

5 orders of magnitude between quark masses!

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The Tevatron at Fermilab

proton-antiproton collisions at 1.7 MHz

most energetic collider until the turn on of the LHC

s = 1.96 TeV

(Run 1: 1.8 TeV)

ring length: L = 6.28 kmrevolution time: T = 20.95 stypical storage time: t = 10 bis 20 h

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Tevatron Performance in Run 2

record luminosity: L = 2.9 1032 cm-2 s-1 (goal: 1.6 – 2.7 1032)

event rate: dN/dt = L L = luminosity

number of events: N = L dtintegrated luminosity

Since April 2002 3.2 fb-1 were delivered by the accelerator.

coming out of shutdown right now

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Detectors at the Tevatron

CDF

DØ

multi-purpose detectors: tracking, b-tagging, calorimeter, muon system, …

strength of CDF: momentum resolution and particle ID (K,)strength of DØ: muon coverage and jet energy resolution

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CDF Silicon Tracker

3 silicon subsystems

L00 R = 1.35, 1.6 cmSVXII R = 2.5 to 10.7 cmISL R = 20, 22, 28 cm

SVXII Barrel

ISL

Layer 00

SVXII

• 720 double-sided sensors

• 5 layers

• 12 -wedges

• r and rz measurements

L00

• 72 single-sided sensors

• 2 layers 6 -wedges

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Data Taking Efficiency

~84 %

Downtime sources:

• Detector/trigger/DAQ ~5%

• Beam, start/end stores ~5%

• Trigger deadtime ~5% (our choice)

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Physics at the Tevatron

H

(barn)

WZ

with 1 fb-1

1.4 x 1014

1 x 1011

3 x 107

1 x 107

14,0008,0006,000

2000 … 200200 … 20

Low Mass SUSY

observed

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Top-Antitop-Quark Production

quark-antiquark annihilation gluon-gluon fusion

Tevatron ~85% ~15%

LHC ~15% ~85%

predicted total cross-section

Tevatron 6.7 0.8 pb

LHC 830 50 pb

top quark factory

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Top Quark Decay

standard model prediction:

top quark decays as quasi free particle

spin information and polarization is accessible

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Classification of Top-Antitop Events

lepton (e, ) + jets channel = “golden channel”

+ large branching fraction (30%)

+ manageable backgrounds

+ allows full event reconstruction

classification according to the decay modes of the W bosons

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2. Top Quark Properties

tt productioncross sectionspin correlationscharge asymmetryX ttbar

top quark massconsistency test ofelectroweak interaction

top quark decayW helicityFCNC decayst H+ + b top quark charge

single-top quarks

|Vtb|2

single-top production via FCNC

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2.1 Selection of Top-Antitop Candidates

event signature:

- one isolated lepton- missing transverse energy- at least 4 Jets,

2 of them b jets

selection cuts: - 4 jets mit ET > 20 GeV

|| < 2.0 - lepton: ET > 20 GeV

- MET > 20 GeV - 1 b tag

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Secondary Vertex Reconstruction in b Jets

b hadron lifetime: 1.5 ps c 450 m

typical decay length in CDF: O(mm)

requirement of a secondary vertex: large reduction of generic W + jets events

jargon: reconstruction of a secondary vertex = b tag

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Event with Two Secondary Vertices

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Top-Antitop Cross Section

e.g. counting experiment with secondary vertex reconstruction

combination of different measurements:

good agreement with theory curve (NLO + NLL resummation)

includes luminosity uncertainty

inclusive measurementusing excess in W + 3 jets

LHC will allow to investigate spectrum

compare to: P. Uwer et al.: Phys. Rev. Lett. 98:262002, 2007

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Dilepton and Hadronic Channel

+ very clean- lower statistics

challenge: fake rate of leptons

+ large statistics: BR = 44%- huge backgrounds

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Top-Antitop-Kinematic Reconstruction

Measured objects must be matched to elementary particles.

missing transversemomentum

charged lepton

jet 1, with b tag

jet 2

jet 3

jet 4

lepton + jets event with one b tag:6 permutations

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Kinematic Fit

Which one is the best assignment?

test all assignments lepton and jet momenta are varied within their uncertainties, such that 2 is minimized.

Choose combination with the smallest 2:

estimate of mtop for this event

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2.2 Top Mass Templates

Templates for mtreco taken

from Monte Carlo events

Distributions are parametrised as continuous function.

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Fit of Templates to Data

mtreco

Mtop = 171.6 ± 2.1 (stat) 1.1(syst) GeV/c2

1 b tagged jet 2 b tagged jets

mjj

In-situ calibration

of the jet energy

scale

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Systematic Uncertainties

likelihood contours in Mtop- JES plane Systematic Mtop

(GeV/c2)

Residual JES 0.6

b-jet energy scale 0.6

Background JES 0.4

ISR 0.4

FSR 0.2

PDFs 0.2

Generators 0.3

Background shape 0.2

Backgr. composition 0.2

QCD modeling 0.1

MCstatistics 0.1

TOTAL 1.1

Mtop = 171.6 ± 2.1 (stat) 1.1(syst) GeV/c2

Best single best measurement in the world!

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Matrix Element Method

Use dependence of matrix element on Mt

calculate weight P(x | Mt) für jedes Ereignis als Funktion von Mt

matrix element transfer functionfor jets and „unclustered energy“

parton distribution function

Wirkungsquerschnitt

mit CompHEP

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Matrix Element Result

Mtop = 170.5 ± 2.4 (stat) 1.2(syst) GeV/c2

• increase purity with NN b tagger

• use in-situ JES calibration

• determine signal purity with likelihood discriminant

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Combination of Mass Measurements

Mtop = 170.9 ± 1.1 (stat) ± 1.5 (syst) GeV/c2

not all new measurements included yet

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Implications on the Higgs Mass

mH < 182 GeV/c2

@ 95% C.L.(including direct lower limit)source: LEPEWWG, 2007

minimum at:

mH = 76 +33-24 GeV/c2

Fit to electroweak precision measurements:

Top physics is Higgs physics !

S. Heinemeyer, W. Hollik, D. Stockinger, A.M. Weber, G. Weiglein ´07

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2.3 W Helicity in Top Decay and cos *

cos * is the angle between the charged lepton and the negative direction of the top quark in the W rest frame.cos * is highly sensitive to the helicity of the W boson

hW = -1 hW = 0 hW = +1

f- = 0.3 f0 = 0.7 f+ = 0.0

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W Helicity: Measurement Results

maximum likelihood fit to reconstructed cos * distribution

unfolding

F0 = 0.65 ± 0.10 (stat) ± 0.06 (syst)

F+ = 0.01 ± 0.05 (stat) ± 0.03 (syst)

F+ = 0.02 ± 0.05 (stat) ± 0.05 (syst)

uses lepton + jets and dilepton events

F+ < 0.14 @ 95% C.L.F

+ < 0.12 @ 95% C.L.

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2.4 Single Top-Quark Production

Theoretical cross section predictions at s = 1.96 TeV

t = 1.98 0.25 pb s = 0.88 0.11 pb

top quark production via the weak interaction

B.W. Harris et al. Phys. Rev. D 66, 054024 (2002), Z. Sullivan, Phys. Rev. D 70, 114012 (2004)compatible results: Campbell/Ellis/Tramontano, Phys. Rev. D 70, 094012 (2004), N. Kidonakis, Phys.Rev. D 74, 114012 (2006)

Vtb

Vtb

t-channel s-channel

Experimental Signature:charged lepton + missing ET + 2 energetic jets

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Single-Top Sample at CDF

• 1 isolated high-PT lepton (e,)pT > 20 GeV, |e| < 2.0 and || < 1.0

• MET > 25 GeV

• Jets: Njets= 2, ET > 15 GeV, || < 2.8

1 b tag (secondary vertex tag)

backgrounds are the challenge

main backgrounds:

after event selection: S/B = 5.5%

total predicted background

1040 ± 220

predicted single-top

60.9 ± 11.5

total prediction 1100 ± 220

observation 1078

using CDF II data with Lint = 1.5 fb-1

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First Evidence for Single-Top

Matrix Element Method Boosted Decision TreesNeural Networks

Matrix Element Method Neural Networks Likelihood Discriminants

3.1

excess:

excess: 2.7 0.8

3.2 2.2 3.4

We are looking forward to analyse more Run II data soon!

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2.5 Top-Quarks as a Window to Physics Beyond the Standard Model

SUSY

left-right-symmetric

modelsTop-Antitop-Resonances

t c + Z

massive t‘

t H+ + b

charge

asymmetry

axigluons

top charge

u + g t

top color

W‘ tb

extra-dimensions

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Search for FCNC in Top-Decays

In SM FCNC are strongly suppressed in the top sector:BR 10-14

Construct mass ² to measure tt-likeness

New world‘s best limit ! previously L3: BR < 13.7% @ 95% CL

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Top-Antitop Resonances

Search for a narrow width resonance:

• Analyze lepton+jets events

• b jet ID: NN b tagger

Interpretation in frame of topcolor-assisted technicolor model

narrow lepto-phobic Z´ excluded

with M(Z´) < 680 GeV/c² and (Z´) = 0.012 M(Z´) at the 95% CL

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Charge Asymmetry in Top-Antitop Events

quasi forward-backward asymmetry

Top quarks are more likely to be produced in proton direction, antitop quarks are more likely produced in antiproton direction.

observed total asymmetry:0.28 0.13 0.05

QCD expectation: 6 – 8 %

limit on axigluon mass: < 1.2 TeVarXiv:0709.1652 [hep-ph]

J. Kühn & G.Rodrigo

Interference effect at NLO measured quantity: rapidity difference of top quarks (Lorentz-invariant)

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• Top quark physics enters a precision era.– Cross Section: tests NLO QCD calculations

– Top mass: Mtop = 170.9 1.9 GeV/c2 Mtop/Mtop = 1.1%

– W helicity: F0 = 0.65 ± 0.10 (stat) ± 0.06 (syst)

F+ < 0.12 @ the 95% C.L.

Conclusions / Prospects

• Evidence for single-top production at CDF and DØ (3.2 and 3.4 sigma).More data are being analyzed. Stay tuned for new results.

• Top-Quarks as window for physics beyond the SM:FCNC decays, resonance production, charge asymmetry, FCNC single-top production, W´ search, top charge measurement

• Top quark physics will play an important role during LHC start:calibration of reconstruction, precision measurements, background for searches, …

Thanks to Jan Lück for this artistic impression of Fermilab.

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Backup

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Improved b Jet Identification

Fit to NN output for W + 2 jets events with one secondary vertex (955 pb-1)

jet and track variables, e.g. vertex mass, decay length, track multiplicity, …

neural network powerful discriminant Replace Yes-No by

continuous variable

New possibility:In situ measurement of the flavor composition in theW + 2 jets sample

mistags / charm ……….…………. beauty

About 50% of the background in the W + 2 jets sample do NOT contain b quarks even though a secondary vertex was required!

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2.2 Helicity of the W Boson in Top Decay

bW+

sW=1Spin : st=1/2 sb=1 /2

t

Helicity: standard model prediction:

hW =0

hW =−1

assume b quark to be massless

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Background Processes

W + heavy flavor: Wbb, Wcc, Wc

top-antitop pairs

non-W: multijet production, bb production

W+light jets (mistags)

diboson: WW, WZ, ZZ

Z+jets