Top-ology Top Search and Discovery

Top is a most remarkable particle, even for a quark. A single top quark weighs 175 GeV/c 2 , about as much as an atom of gold. But unlike the gold atom, which can be disassembled into 79 protons, 79 electrons, and 118 neutrons, top seems indivisible, for we discern no structure at a resolution approaching 10 −18 m. Top's expected lifetime of about 0.4 yoctosecond (0.4× 10 −24 s) makes it by far the most ephemeral of the quarks. The compensation for this exceedingly brief life is a measure of freedom: top decays before it experiences the confining influence of the strong interaction. In spite of its fleeting existence, the top quark helps shape the character of the everyday world. Ever since the existence of the b-quark was inferred from the discovery of the Υ (Upsilon) family of resonances at Fermilab in 1977, particle physicists have been on the lookout for its partner, called top. The long search, which occupied experimenters at laboratories around the world, came to a successful conclusion in 1995 with the announcement that the top quark had been observed in the CDF and DØ experiments at Fermilab [1]. Top is the last of the fundamental constituents of subnuclear matter that gauge theories of the strong, weak, and electromagnetic interactions and a wealth of experimental information have led particle physicists to expect. Top's existence was required lest quantum corrections clash with the symmetries of the electroweak theory, leaving it internally inconsistent. It was signalled too by the pattern of disintegrations of the b-quark and by the characteristics of the b-quark's neutral–weak-current interactions measured in e + e − annihilations into b ¯ b pairs. Higher-order processes involving virtual top quarks are an important element in quantum corrections to the predictions the electroweak theory makes for many observables. A case in point is the total decay rate, or width, of the Z 0 boson, which has been measured to exquisite precision at the CERN and SLAC Z factories. The comparison of experiment and theory shown in the inset to Figure 1 favors a top mass in the neighborhood of 180 GeV/c 2. The top mass favored by simultaneous fits to many electroweak observables Top-ology for Physics Today 3 is shown as a function of time in Figure 1. Figure 1: m t (t) It is worth mentioning another hint that I have to confess seems more suggestive …