The top quark is the heaviest of the six quarks, with a mass of about 175 GeV, the same as that of a gold nucleus. The top was discovered by the D0 and CDF collaborations in 1995. The existence of the top had been predicted almost 20 years earlier, but its mass was not known. The discovery nailed down the essential validity of the Standard Model. The discovery papers have now become the second most frequently cited papers in experimental high energy physics.

At the Tevatron the top quark is produced mostly as t-tbar pairs. The production rate is such that in a typical run we get about one t-tbar event per hour. The top and the antitop both immediately decay into a W boson and a b-quark. Each b-quark turns into jet, and each W decays either into two jets (2/3 of the time) or into a lepton and a neutrino (1/3 of the time). The cleanest experimental signature is the final state e + m + 2 jets + missing energy, but this decay mode is rare. The top was actually discovered in the final state e (or m) + 4 jets + missing energy, which occurs more frequently and is also relatively clean experimentally. The most common final state has six jets, but this mode has a large background and it is hard to study experimentally.

Why do particles have mass? It is thought that all fundamental particles acquire their masses through their interactions with the "Higgs field" -- a hypothetical field filling all of space. The mass of the associated Higgs particle depends most sensitively on the top (and the W) mass, and therefore it is important to pin down the top mass as accurately as possible. We hope to apply aPDE to this problem in a manner outlined under multivariate methods.