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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
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