Top Quark Condensate Revisited
  In the light of recent discovery of a very heavy top quark, we reexamine the top quark condensate model proposed by Miransky, Tanabashi and Yamawaki (MTY) and by Nambu. We first review the original MTY formulation based on the ladder Schwinger-Dyson equation and the Pagels-Stokar formula. It is particularly emphasized that the critical phenomenon gives a simple reason why the top quark can have an extremely large mass compared with other quarks and leptons. Then we discuss the Bardeen-Hill-Lindner (BHL) formulation based on the renormalization-group equation and the compositeness condition, which successfully picks up 1/N_c-sub-leading effects disregarded by MTY. In fact BHL is equivalent to MTY at the 1/N_c-leading order. Such a simplest version of the model predicts the top quark mass, m_t \simeq 250 GeV (MTY) and m_t \simeq 220 GeV (BHL), for the cutoff on the Planck scale. In this version we cannot take the cutoff beyond the Landau pole of U(1)_Y gauge coupling, which yields a minimum value of the top mass prediction m_t \simeq 200 GeV. We then propose a ``top mode walking GUT'' : The standard gauge groups are unified into a (``walking'') GUT so that the cutoff can be taken to infinity thanks to the renormalizability of the four-fermion theory coupled to ``walking'' gauge theory. The top and Higgs mass prediction is then controlled by the Pendleton-Ross infrared fixed point at GUT scale and can naturally lead to m_t \simeq m_H \simeq 180 GeV. 
Koichi Yamawaki (Nagoya University)