Fermilab

Is it a top quark? Physicists recognize the particles produced in proton-antiproton collisions by their electronic signatures, shown graphically by computers. In the "lego plot" below, the height of each lego tower shows the amount of energy detected in each cell of a detector's calorimeter after a particle collision like the one illustrated.

How can we recognize a top quark's electronic signature? Top quarks exist for such a short time that we don't actually find their signatures, but instead those of their known decay products. Here, the lego plot shows one characteristic signature pattern--called a "lepton plus jets event"--that we expect to see in a collision that produces a top-antitop quark pair. The top quarks instantly decay into two W particles and two b quarks. One W in turn decays into a muon and a neutrino, the other into up and down quarks. The up and down immediately decay into jets of particles. The b quarks travel a little way before they too decay into jets.

The results of this collision are a muon, a neutrino, and four jets of particles. The lego plot identifies the muon and the four jets. The neutrino leaves no tracks; we infer its existence from missing energy in the collision results.

Background Material on the Discovery of the Top Quark