Extended technicolor models with two extended technicolor 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.