TopQuarks

Top quark, symbol t, is the heaviest known elementary particle in the Standard Model. It is an up-type quark of the third generation, carrying electric charge +2/3e and a color charge. Its large Yukawa coupling to the Higgs field gives it a mass of about 173 GeV/c^2, making it unusually heavy among fundamental fermions.

Top quarks are produced predominantly in pairs in high-energy hadron collisions (ttbar production) through strong interactions, and also singly via electroweak processes. At the Large Hadron Collider, top quark pairs are produced far more copiously than single tops. The top quark almost always decays to a W boson and a bottom quark (t → W b) with a lifetime of about 5×10^-25 seconds, too short for hadronization. The information about the top's spin is carried by its decay products, enabling studies of spin correlations and the Wtb vertex.

Experimentally, the top quark was discovered in 1995 by the CDF and DØ collaborations at the Fermilab Tevatron. Its mass is a key parameter in precision tests of the Standard Model, and its production cross sections, decay properties, and kinematic distributions are measured to test quantum chromodynamics and electroweak theory. The magnitude of Vtb, the relevant CKM matrix element, is constrained by top decays.

Beyond establishing the third generation quark, the top quark serves as a sensitive probe for new physics, including anomalous couplings, new heavy resonances that couple to top quarks, and contributions to loop-level corrections affecting Higgs physics and electroweak fits.