The Top Quark Must Exist

I compare the anticipated properties of the top quark with what is known experimentally about the heaviest fermion. I review the scienti c opportunities that will be presented in the near future by copious samples of top. These include re ned tests of electroweak radiative corrections, studies of the nature of top itself, and exploration of electroweak symmetry breaking. I conclude by illustrating top's in uence in the everyday world. 1 The Top Quark Must Exist Ever since the existence of the b-quark was inferred from the discovery of the family of resonances in 1977 [1], we have been on the lookout for its weak-isospin partner. Once the charge of the b-quark was established to be eb = 1=3, it was natural to expect that the missing partner should be the upper member of a doublet, with charge +2=3. Completing the weak-isospin doublet by nding the top quark is the most natural way to cancel the anomaly of the ( ; )L doublet and ensure an anomaly-free electroweak theory. The absence of avor-changing neutral currents in b decays, which would lead to signi cant branching fractions for dilepton channels such as b! s`` , added phenomenological support to the idea that b is a member of a left-handed weak doublet. More recently, the accumulation of results on the neutral-current interactions of the b-quark has made it possible to characterize the Zb b vertex and measure the weak isospin of the b-quark. Consider a generalization of the SU (2)L U (1)Y theory in which the b-quark may carry both left-handed and right-handed weak isospin. The chiral neutral-current couplings can be written as Lb = IL3 eb sin W Rb = IR3 eb sin W ; (1) which di er from the standard-model chiral couplings by the presence of IR3. Characteristics of the reaction ee ! b b permit us to determine the values of Lecture given at LISHEP95, LAFEX International School of High Energy Physics, Workshop on Heavy Flavors cbt, Rio de Janeiro, February 1995. I have revised the text to incorporate information from the CDF and D discovery papers. hep{ph/9507257 FERMILAB{CONF{95/139{T IL3 and IR3 directly from experiment. The partial width (Z 0 ! b b) measures the combinationLb+R 2 b . On the Z 0 resonance, the forward-backward asymmetry AFB(Z ! b b) measures the combination (Lb Rb)=(Lb + Rb). Far below the resonance, where the forward-backward asymmetry is dominated by -Z interference, AFB(ee ! b b) measures the combination Lb Rb. The unique overlap of the allowed regions is for IL3 = 1=2, IR3 = 0, the standard-model solution.