Top decay to lepton + jets: CDF B tags and cross section Page: 4 of 26
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purely hadronic and r channels which have large multijet backgrounds. The
lepton plus jets data are the main source for the top cross section measure-
ment. In addition, events which contain four energetic jets can be used to
fully reconstruct the top quarks in order to measure the top mass [4].
Top events in the lepton plus jets channels are distinguished by the pres-
ence of an energetic and frequently isolated lepton, missing transverse energy
(PT) from the W neutrino, and jets from the b's and the hadronic W decay.
By requiring at least three energetic jets, the signal-to-background ratio is
~1/6 for Mto0 = 180 GeV. Further background reduction is then afforded by
tagging b jets. In this note we discuss the two methods of b tagging used by
CDF. As a result of changes for the current run, the secondary vertex (SVX)
method dominates our b tagging in both purity and efficiency and hence will
be the main emphasis of this paper.
The data were taken in two separate runs of the Fermilab collider. Run
la took place in 1992-93 and resulted in a total integrated luminosity of 19
pb-lon tape. CDF observed a 2.8 a excess of top-like events over expected
backgrounds in this sample and measured a top mass of 174 1013 GeV and
cross section of 0tt = 13.9+1 pb. There have been a number of important
changes for run lb which began at the end of 1993 and is still in progress.
CDF has installed a new silicon detector with nearly identical geometry but
which is ac-coupled with radiation-hardened electronics. This device has sig-
nificantly higher signal and lower noise, resulting in an - 20% higher average
b tagging efficiency. Secondly, the SVX tagging algorithm has been modified,
resulting in an additional 25% efficiency increase with no significant change
in fake backgrounds. With regard to the SLT tagger, the acceptance has
been extended to pseudorapidity 7rdet 1< 1.0. Most importantly, the Fer-
milab accelerator division has enhanced the performance of the Tevatron to
achieve typical instantaneous luminosities of order 1031 cm-2sec-1 and peak
luminosities approaching three times this value. For the results presented
here, 48 pb-lof data from run 1b have been combined with the 19 pb-from
run la.
Event Selection
The event selection begins with the identification of an isolated e or lepton
with ET or PT > 20 GeV. Electrons from y -> e+e- conversions are vetoed2
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Incandela, J. Top decay to lepton + jets: CDF B tags and cross section, report, June 23, 1995; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc623450/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.