This story first appeared in Fermilab Today on September 24, 2009.
Fermilab is still the only place on Earth where physicists can produce top quarks in the laboratory. Physicists know the mass of the top quark quite well. However, they want to continue to improve this measurement since it is an important component in predicting the Higgs boson mass.
CDF experimenters have measured the top quark mass using a large data sample of events where the top quark decays into jets and electrons or muons. Some of the selected events are actually not from top quark decay but from other particles that mimic the process. Scientists at CDF use a neural network, software designed to mimic the thought process in the human brain to identify these events in order to compensate for them.
Because the top quark decays into jets of particles, the top quark mass measurement depends on the jet energy reconstruction. Physicists can calibrate this reconstruction by using the W boson, a particle of known mass. They derive the likelihood of the signal from theory calculations for each event for many values of the top mass and the jet energies. By using this technique, scientists can simultaneously calibrate the jet energy and extract the most likely top mass from the distribution of data.
A total of 630 selected top quark pairs in 4.3 inverse femtobarns of collected data yield the final result for the top mass: m_top = 172.64 +- 1.58 GeV/c2. The result of this analysis is the most precise single measurement of the top quark mass so far, truly making it a tip-top analysis.
View the public Web page for the top mass result.
By Craig Group for the CDF collaboration