Top quark mass measurements at the Tevatron Page: 4 of 11
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3.3 Lepton + Jets Channel
The DO lepton + jets analysis 7) requires an isolated central lepton, four or more
jets and at least 20 GeV missing transverse energy. Since the neutrino's longitudinal
momentum is not measured, the kinematic fit to the tt hypothesis is now a 2C fit.
DO also computes a discriminant D, based on 4 variables weakly correlated with the
top-quark mass. This discriminant is computed through a comparison of signal and
background using classical methods, as well as a neural network. The likelihood fit
to determine the top mass is then performed on a two-dimensional grid of D versus
the top mass. Both discriminants yield results that are in good agreement with each
other, which when combined give m(top) = 173.3 t 5.6 Statt) t 5.5 (syst) GeV/c2.
The background contamination is estimated to be approximatively 50 out of the 77
events that pass all selection criteria.
CDF 8) uses a similar procedure, but finds that optimal results can be
obtained by dividing the sample into 4 subsamples: events with two SVX tags
(5 events), events with a single SVX tag (15 events), events without SVX tags
but where one jet is identified as a b-quark jet through a soft lepton tag ("SLT
tag") (14 events), and finally events without any tags (42 events). The samples are
fitted separately and the results, shown in Fig. 1, when combined yield m(top) =
175.9 5.1 Statt) t 5.3 (syst) GeV/c2.
3.4 Dilepton Channel
As indicated in Section 2, the dilepton channel has fairly low statistics. On the other
hand, the background is small, dominated by Z boson decays to charged leptons and
particle misidentification. The difficulty arises from the presence of two neutrinos,
for which only the combined transverse momentum can be measured. The system is
thus kinematically underconstrained, and both experiments use the so-called "neu-
trino weighting" technique to measure the top quark mass: for every chosen top
quark mass, there is a sampling of allowed neutrino rapidities. Given the neutrino
rapidities, the assumed top mass, and the charged-lepton and b-quark momenta,
the system can now be solved for the transverse and longitudinal momentum com-
ponents of the neutrinos (with a two-fold ambiguity for each neutrino). A weight is
then assigned to each solution based on the agreement between the calculated and
measured missing transverse momentum. All weights are summed for all possible
combinations of neutrino rapidities. This is done as a function of top mass value for
each event.
DO 9) has observed 6 events in a sample corresponding to an integrated
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Brooijmans, Gustaaf. Top quark mass measurements at the Tevatron, article, May 16, 2000; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc707986/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.