Top Quark

The top quark, also known as the t quark (symbol: t) or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-1⁄₂, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of +2⁄₃ e, and is the most massive of all observed elementary particles. It has a mass of 172.9±1.5 GeV/c2, which is about the same mass as an atom of tungsten. The antiparticle of the top quark is the top antiquark (symbol: t, sometimes called antitop quark or simply antitop), which differs from it only in that some of its properties have equal magnitude but opposite sign.

The top quark interacts primarily by the strong interaction but can only decay through the weak force. It decays almost exclusively to a W boson and a bottom quark, but it can decay also into a strange quark, and on the rarest of occasions, into a down quark. The Standard Model predicts its mean lifetime to be roughly 5×10−25 s. This is about 20 times shorter than the timescale for strong interactions, and therefore it does not form hadrons, giving physicists a unique opportunity to study a "bare" quark. (All other quarks hadronize, meaning they can only be found in hadrons.) Because it is so massive, the properties of the top quark allow predictions to be made of the mass of the Higgs boson under certain extensions of the Standard Model (see Mass and coupling to the Higgs boson below). As such, it is extensively studied as a means to discriminate between competing theories.

Its existence (and that of the bottom quark) was postulated in 1973 by Makoto Kobayashi and Toshihide Maskawa to explain the observed CP violations in kaon decay, and was discovered in 1995 by the CDF and DØ experiments at Fermilab. Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for the prediction of the top and bottom quark, which together form the third generation of quarks.