Tevatron Collider Events - A pair of top quarks reconstructed in the DZero experiment at Fermilab. This end view shows the final decay products: an electron (red towers), a neutrino (pink), and four jets of particles. The top quark, also known as the t quark (symbol: t) or truth quark, is the heaviest subatomic particle ever observed, with a mass that is about as heavy as an entire atom of gold. Top quarks are also among the most fleeting of particles, with a lifetime of about a trillionth of a trillionth of a second. Top quarks were first observed at Fermilab in 1995 and the two experiments at Fermilab's Tevatron collider each observed about 150,000 top quarks over their lifetimes. The Tevatron is a circular particle accelerator. It was completed in 1983 at a cost of $120 million and significant upgrade investments were made in 1983-2011. The main achievement of the Tevatron was the discovery in 1995 of the top quark - the last fundamental fermion predicted by the standard model of particle physics.