Recovering the missed top quark events

Dominant production of top quark and lepton-plus-jets decay mode is shown. CDF physicists recover these events even though a charged lepton is missed.

The top quark is the most massive of the currently known elementary particles. Of the six quarks, the top quark has the largest mass, which is a whopping 40 times more massive than that of the next most-massive quark, the bottom quark. Because of the top’s very large mass, it may play a special role in our understanding of the fundamental theory that describes the electromagnetic and the weak interactions. Combined measurements of the W boson mass and the top quark mass provide an indirect determination of the mass of the recently observed Higgs particle. Because of the importance of the top quark mass, experimenters at the Tevatron have focused much of their efforts on improving the precision of its measurement.

When top quarks were created in Tevatron collisions, they almost immediately decayed into a bottom quark and a W boson and were most often produced in pairs. The top quark is best measured when one W decays into a neutrino and a charged lepton (electron or muon) and the other top quark decays into a pair of jets (light quarks). This so-called lepton-plus-jets decay mode, therefore, contains one well-reconstructed charged electron or muon. When making this standard lepton-plus-jets analysis, CDF scientists do not measure tau leptons that contain a strongly interacting particle (a hadron) in their decays. They also do not detect top-antitop events that contain electrons or muons that occur outside the detector coverage for these particles (in the central region of the detector). Recently, CDF scientists made a new measurement of the top quark mass recovering such missing events. This measurement adds unique information into the overall picture of the top quark mass and improves the precision of its mass.

The measurement under discussion is performed using a template method. Physicists first build several physical observables with samples whose top quark masses are already known. The shape of those samples is compared to the same observables built using the collected data set. Scientists extract a most-likely top quark mass for the data by deciding which known sample is most like the unknown sample.

Using the full Run II data set of this unique data sample, CDF physicists measured the top quark mass to be 173.93 ± 1.85 GeV/c2. The measurement is consistent with the recently published Tevatron average of 173.18 ± 0.94 GeV/c2 and contributes approximately 12 percent to the most recent preliminary Tevatron average.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. From left: Hyun Su Lee (Ewha Womans University, Seoul, Korea), Jian Tang (University of Chicago) and Fermilab Deputy Director Young-Kee Kim.