Extended Technicolor Models with Two ETC Groups
  We construct extended technicolor (ETC) models that can produce the large splitting between the masses of the $t$ and $b$ quarks without necessarily excessive contributions to the $\rho$ parameter or to neutral flavor-changing processes. These models make use of two different ETC gauge groups, such that left- and right-handed components of charge $Q=2/3$ quarks transform under the same ETC group, while left- and right-handed components of charge -1/3 quarks and charged leptons transform under different ETC groups. The models thereby suppress the masses $m_b$ and $m_\tau$ relative to $m_t$, and $m_s$ and $m_\mu$ relative to $m_c$ because the masses of the $Q=-1/3$ quarks and charged leptons require mixing between the two ETC groups, while the masses of the $Q=2/3$ quarks do not. A related source of the differences between these mass splittings is the effect of the two hierarchies of breaking scales of the two ETC groups. We analyze a particular model of this type in some detail. Although we find that this model tends to suppress the masses of the first two generations of down-type quarks and charged leptons too much, it gives useful insights into the properties of theories with more than one ETC group. 
Neil D. Christensen and Robert Shrock