Top Turns Ten Symposium, Fermilab, October 21st 2005
Jessica Levêque, University of Arizona
The fingerprints of the Top Quark
Ten years after its discovery, the top quark is now being studied with improved accelerator and detectors at Fermilab.
A precise determination of its characteristics, through the measurement of its mass, production modes
and decay properties allows us to test the Standard model very deeply, constraint the Higgs boson mass
and look for signs of new physics.
Top Quark Mass Measurement
Projected uncertainty on the top quark mass
as a function of integrated luminosity
The top quark mass is a fundamental parameter of the Standard Model, but its value can
not be predicted. DØ has measured the top quark mass using different techniques :
- In the lepton+jets channels, the Template method uses a kinematic fit to reconstruct the
top quark’s invariant mass. The mass spectrum is then fitted using template distributions
for signal and background. In a different approach used in the Ideogram and Matrix
Element methods an analytical likelihood including a description of the differential crosssections and full event kinematics is taken to calculate a probability per event to be signal
or background.
- In the dilepton channel, the Matrix Weighting method is used. The production of 2
neutrinos leads to an underconstrained event kinematics. As proposed by Dalitz, Goldstein
and Kondo, a hypothesized value of the top quark mass is used to determine the top quark
pair momenta. A weight distribution is derived for each event as a function of the top quark
mass. Using its peak as an estimator of the mass for each event, and comparing the
resulting distribution in data to signal and background templates provides an estimate of
the top quark mass.
The goal is to measure the top quark mass
with 1% precision during Run II.
Summary of the most recent top quark mass measurements
W Helicity Measurement
In the Standard Model, the W boson coming from a top
quark decay is either left-handed (f- = 70%) or longitudinal
(f0 = 30%). A measured fraction of right-handed W bosons
f+  0 would be an unambiguous signature of new physics.
The W boson helicity is measured through the angle q
between the charged lepton and the top quark direction in
the W boson rest frame. The figure on the left shows the
Standard Model prediction.
In the lepton+jets final states, the top quark pairs are
selected with 2 analyses, topological and b-tagged.
The four vectors of all final state particles are
reconstructed using a kinematic fit. f+ is then
extracted from a likelihood fit to the angular
distribution of the charged lepton cos q*, using
template distributions for various right-handed
fractions. In the dilepton channel, f+ is extracted from
a fit of the charged lepton momentum distribution.
L=230 pb-1
f   0.04  0.11(stat)  0.06 (syst)
f   0.25 @ 95% C. L.
Combined result :
Search for resonances
Top Quark Branching Fraction
The very large mass of the top quark
could be an indication that this particle
plays a special role in the electroweak
symmetry breaking mechanism.
Topcolor-assisted-technicolor models
predict the existence of a new gauge
boson Z’ which is strongly coupled to the
third generation of quarks. An evidence
for this new boson would be a
significantly higher top quark production
cross-section than predicted by the
No evidence for a resonance was found. Upper limits on
Standard Model, or a peak in the top
quark’s pair invariant mass distribution. a leptophobic Z’ boson mass can be derived :
Mz'  680 GeV/c 2 with Γz'  0.0012Mz' @ 95% C. L.
Assuming the CKM matrix unitarity with
exactly 3 quark generations, the Standard
Model predicts the following branching
ratio fraction :
B (t  Wb)
R
1
B (t  Wq)
Any deviation would be a hint for new physics.
A measurement of R was performed in the
lepton+jets channel. The excess of events with 1
and 2 or more b-jets after background estimate is
used to simultaneously measure the top quark
cross-section and the branching ratio fraction.
R  1.03
 0.19
 0.17
σ tt  7.9
1.7
1.5
(stat  syst)
(stat  syst)  0.5 (lumi) pb