Top Quark Mass Measurements and Decay Properties

1
Top Quark Mass Measurements and Decay Properties
CDF and DØ collaborations Robert Zitoun Stony
Brook and LAPP

• Outline 3 new (preliminary) measurements
• Mass CDF Run II
• Improved mass DØ Run I
• W helicity CDF Run I

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Motivation

• Discovered 8 years ago. Still little known
  (100 events background)
• Mass Mt is a basic standard model parameter
• Affects observables (LEP/SLD, nN) through
  radiative corrections
• 2 GeV on Mt is worth 10 MeV on MW

• Mt ltf gt vv2 175 GeV
• EW symmetry breaking understanding might benefit
  from study of its decay properties

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Top Mass Published Results

• 2l 2 jets
• 1l(e/m) 4 jets
• 6 jets
• combinatorics jet energy scale
• b-tagging reduces background and combinatorics

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Mt measurement
q1

• Case of 1l 4 jets event
• 2-constraint kinematical fit ? Mt
• pn unknown 3
• Impose pT balance 2
• M(q1q2) MW 1
• M(ln) MW 1
• M(Wb) M(Wb) 1
• 4 jets 12 combinations per event
• (2 depending on kinematicsambiguities) ? keep
  best c2
• Fit Mt distribution with modeledproduction and
  background

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• CDF Run II Data
• Mass Measurement
• l 4 jets channel

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Data

• Integrated luminosity 72 pb-1
• Event selection (similar to Run I)
• One isolated high pT central e or ?
• 4 jets ETgt15 GeV ?lt2.0
• ETgt20 GeV
• Z veto
• 33 candidates
• Background (13 events)
• W 4 jets
• fake leptons, diboson, Drell-Yan, single top

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Result

• Systematicsbreak down

• was 4.4 GeV in Run I
• should improve quickly with improved detector
  understanding

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Using b-tagging

• Reduce background with b-tagging (56 pb-1 only)
• event tagging efficiency ? 45 1 5
  (from data MC)
• Relax 4th jet gt 8 GeV
• 11 candidates (background 1 event)
• Not yet fitted, but looks likea mass peak

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• DØ Run I Data
• Mass Reanalysis
• l 4 jets channel

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Outline of the method

• Kondos method (uses full set of event
  observables)
• Define a signal event probability
• Define a background probability
• Build an event probability
• Build a likelihood
• Minimize to get Mt (c1 and c2)

i-th event final state
parameters (Mt, c1, c2)
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Signal and Background

• Signal probability
• Background probability
• Only W 4 jets background
• considered
• 80 of total
• found adequate to represent multijet background
• Calculated with VECBOS
• Keep events with Pbkglt1011

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Result

• DØ RunI statistics (125 pb1) PRD 58 (1998),
  052001
• events 91 ? 77 with exactly 4 jets ? 22 after
  prob cut
• Preliminary result Mt 179.9 ? 3.6 ? 6.0 GeV

likelihood vs Mt
-log(likelihood) vs Mt
was 5.6 GeV eq. 2.4 increase in stat
next slide
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Systematic Uncertainty

• Main contribution from jet energy scale
• Improvement in systematics coming soon
• Further improvement by controlling the W?jj mass

Jet Energy Scale 5.6 GeV
Parton Distribution Function 0.2 GeV
Acceptance Correction 0.5 GeV
Signal model 1.5 GeV
Background model 1.0 GeV
Noise and multiple interactions PRD 58 52001, (1998) 1.3 GeV
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• CDF Run I Data
• W helicity
• in top decay

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Top quark Decay

• In VA theory, top produces no h 1 W only
• h 0 70
• h 1 30
• Angle accessible through associated mass
• M2lb ½ (M2t M2W)(1 cosyl)

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New CDF W helicity study (Run I)

• Use dilepton and leptonjets events with 1 and 2
  SVX b-tagged jets)
• Preliminary (f 0 in SM f 1 if all VA)
• fVA 0.210.420.25 0.21
• Expect 0.1stat 0.1syst with 2 fb-1

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Run II Outlook

• Run II with 2 fb1 (2005)
• 14000 top events produced per expt
• O(500) l jets with b-tagging.
• Improvements to Mt
• Increased statistics 20
• New method (2.5)
• Improved b-tagging (CDF increased acceptance, DØ
  lifetime tag)
• Better understanding of jet energy scale (W?jj
  helps)
• Better knowledge of ISR/FSR
• DMt 2-3 GeV / expt
• Rare decays cg ,cg, cZ, Hb
• Vtb (single top x-section)
• Is top playing a special role in the breaking of
  electroweak symmetry?

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Summary

• Mass from Run II (CDF)
• 171.2 13.4 9.9 GeV
• Improved mass from Run I (DØ)
• 179.9 3.6 6.0 GeV
• W helicity from Run I (CDF)
• fVA 0.210.420.25 0.21

SV
m -
Jet 2
IP
Jet 2
Jet 1
IP
Jet 1
SV
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Top production and decay

• 6 jets 45
• e/m 4 jets 15 each
• 2l 2 jets 1 each
• 35 with t
• 2 b jets

• 5 pb cross section (7 pb _at_ 1.96 TeV)

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Fermilab Tevatron

• Main Injector (150 GeV) has replaced Main Ring
• pbar recycler??
• vs 1.8?1.96 TeV
• increases 35
• Run IIa
• goal 8.61031 cm-2s-1 (5)
• 2 fb-1 by 2004-05 (20)

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DØ Run I Mass Reanalysis (l jets)

• New method
• Use of full set of event observables through
  likelihood
• integration over n incident parton momenta (5
  dim)
• Real life issues
• Acceptance
• Final state parton momenta not really observed

allows unnormalized P
parameters (Mt, ...)
i-th event final state
x e and jets
transfer function from partons to jets
x e and partons
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Technicalities

• For each event, probability is an integral over 5
  variables
• 20 variables (2 in 63 final)
• 4 constraints (E, p conservation)
• 11 measures (3 pe 8 ?partons)
• optimized choice Mt, Mtbar, mW, mW, E1(1 jet)
• 12 undistiguishable parton configurations sum 12
  probabilities
• W(x,y) probability of measuring x when y was
  produced in the collision
• Monte Carlo used to compute Wjet (different for q
  and bs)
• Parametrized as the sum of 2 gaussians
  (E-dependent parameters)

x e and jets
transfer function from partons to jets
x e and partons