Measurements of top quark decay properties Page: 2 of 9
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The top quark was discovered during Run I of the Tevatron with about 60pb-1 of data [1, 2].
With that sample it was clear that the top quark had a mass near 175 GeV, making it the only fermion
with a Yukawa coupling near unity. This leads to speculation that non-standard model electroweak
symmetry breaking mechanisms may be revealed through a detailed study of the properties of the
top quark. In addition, such detailed studies would reveal the presence of an exotic component to
the events being interpreted as tt.
While the Run I sample was insufficient for the detailed measurement of the top quark charge,
branching ratios, or decay vertex structure, we are in a much better position with Run II. Not only
has the Tevatron's luminosity increased by over an order of magnitude, the center of mass energy
has also been raised from 1800 to 1960 GeV, which increases the tt production cross section by
about 30%. In addition, both the CDF and DO detectors have upgraded their capablities for top
quark detection and measurement. Therefore we are now entering the era of precision measure-
ments of the top quark's properties, the first set of which are reported here.
2. Common features
The analyses presented hear are based upon pair-produced top quarks, since single top produc-
tion has yet to be observed. All use events in which one or both top quarks decay semileptonically,
since these are easier to observe above background than the all-hadronic decay mode. In general
the analyses are built upon event selection criteria used to measure the tt cross section, and may
take advantage of the knowledge of the background levels gained in those analyses. Events in
which one top quark decays semileptonically are called "+ jets" events, while those in which both
top quarks decay semileptonically are called "dilepton" events. Backgrounds in the +jets channel
arise predominantly from W+jets production or from QCD multijet production where one jet is
misidentified as a lepton and missing ET arises due to mismeasurement of jet and lepton energies.
In the dilepton channel backgrounds arise from several sources, notably Z+ jet production, with
Z -> Li or Z -> zz -> Li, and WW or WZ+jet production. The dilepton channel benefits from the
small cross-section for background processes, but tt decays to the +jets channel are about six
times more common, making the f+ jets channel the most powerful for measurements of top quark
Systematic uncertainties arise from limited knowledge of the jet energy calibration and un-
certainties in the modeling of signal and background events. In the event that the top quark mass
is used as an input to the analysis, the uncertainty on that mass is also a systematic. Although
most of the analyses presented here are statistically limited, it is important to note that many of
the systematic uncertainties will not scale simply as 1/v/N. This means that acheiving the ultimate
precision on top quark properties measurements (i.e., during the LHC era) will require creativity
and dedicated study to reduce the systematic uncertainties.
3. Top quark branching ratio
In the standard model the top decays to Wb with a branching fraction > 0.998 at 90% C.L.,
constrained by the unitarity of the three-generation CKM matrix. However, if there are additional
Measurements of top quark decay properties
Erich W. Varnes
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Varnes, Erich W. & U., /Arizona. Measurements of top quark decay properties, article, April 1, 2006; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc883225/m1/2/: accessed July 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.