LARGE TOP-QUARK YUKAWA COUPLING AND HORIZONTAL SYMMETRIES

consider the maximal U(3) horizontal scheme as a handle on fermion mas ses and mixings. In particular, we attempt to explain the large top qu ark Yukawa coupling and the masses and mixing in the two heaviest gene rations. A simple model is constructed by enlarging the matter content of the standard model with that of a 10+<(10)over bar>pair of SU(5). The third generation particles get their masses when U(3) is broken to U(2). The top quark mass is naturally of order one. Bottom and tau ma sses are suppressed because of a hierarchy in the effective Yukawa cou plings and not from the hierarchy in the Higgs doubler vacuum expectat ion values. The hierarchy is a consequence of the fact that the partic le spectrum contains an incomplete vectorlike generation and can come from hierarchies between scales of breaking of different grand unified groups. Hierarchies and mixings between the second and third generati ons are obtained by introducing a single parameter epsilon' representi ng the breaking U(2)-->U(1). As a consequence, we show that the succes sful (and previously obtained) relations V(cb)approximate to m(s)/m(b) approximate to root m(c)/m(t) easily follow from our scheme. [S0556-2 821(98)01403-9].