YUKAWA COUPLINGS IN SUPERSTRING-DERIVED STANDARD-LIKE MODELS

I discuss Yukawa couplings in standardlike models which are derived from superstring theory in the free fermionic formulation. I introduce new notation for the construction of these models. I show how the choice of boundary conditions selects a trilevel Yukawa coupling either for +2/3 charged quarks or for -1/3 charged quarks. I prove this selection rule. I make the conjecture that in this class of standard-like models a possible connection may exist between the requirement of F and D flatness at the string level and the heavinesss of the top quark relative to the lighter quarks and leptons. I discuss how the choice of boundary conditions determines the nonvanishsing mass terms from quartic-order terms. I discuss the implication for the mass of the top quark.