The Fermilab accelerator collided a proton with an antiproton inside a detector that recorded the tracks of the various products of this collison.  A few hundred collisions among trillions gave physicists the evidence for top quark's existence.

The observed event itself appears to be a confusing clutter of various particle paths.  A computer analyzed this event to show clusters of mesons, a positron, and an inferred neutrino.  Looking at this, physicists were able to deduce that a top quark decayed into a W particle and a bottom quark, even though these decayed before we could observe them.

The links in the text on this page switch between different images for each of the event interpretations.