Separating wheat from chaff

The distribution of the top mass data is compared to the signal and to the background noise.

A peculiar characteristic of the top quark is its surprisingly heavy mass, allowing it to assume a special role in the Standard Model. The top quark mass is connected to the mass of the actively sought for Higgs boson – making a precise measurement of the top quark mass all that more important.

Tevatron collisions produce mainly quark or gluons, detected as sprays of particles called jets. Among these collisions, only about one in 10 billion produces a pair of top quarks. Each top quark decays almost immediately into a W boson and a b quark. This b quark produces a spray of particles called a b-jet. Along with the b-jet, it is the decay of the W bosons that defines the event signature, according to the number of charged leptons produced. As top quarks are produced in pairs, the signature can involve two leptons, one lepton or no leptons. The channel with no leptons at all is called all-hadronic, with six well-defined energetic jets. Unfortunately, a multijet background swamps this signature. The signal of selected jets to the background noise is approximately one in 2000.

Two years after its discovery with the more favorable leptonic signatures, CDF observed the top quark production in the all-hadronic channel and measured its mass.

Now, 15 years later, we have increased our collected data by a factor of 60. We’ve also applied a more sophisticated computer algorithm for the selection of events. The signal to background ratio is now one to 2.4. With these improvements, CDF has the most precise measurement of the top quark mass in this all-hadronic channel – 172.5 +/- 2.0 GeV/c2. The current CDF all-hadronic measurement contributes 14 percent to the determination of the top quark mass, and this new measurement will increase its contribution.

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—Edited by Andy Beretvas

Andrea Castro and Lucas Brigliadori, both of INFN in Bologna, Italy, were responsible for this analysis.