Physics implications of flat directions in free fermionic superstring models. I. Mass spectrum and couplings - art. no. 055005

From the "top-down'' approach we investigate physics implications of the cl ass of D- and F-flat directions formed from non-Abelian singlets which are proved Aat to all orders in the nonrenormalizable superpotential, for a pro totype quasi-realistic free fermionic string model with the standard model gauge group and three families (CHL5). These flat directions have at least an additional U(1)' unbroken at the string scale. For each flat direction, the complete set of effective mass terms and effective trilinear superpoten tial terms in the observable sector are computed to all orders in the VEV's of the fields in the flat direction. The "string selection rules" disallow a large number of couplings allowed by gauge invariance, resulting in a ma ssless spectrum with a large number of exotics, in most cases excluded by e xperiment, thus signifying a generic flaw of these models. Nevertheless, th e resulting trilinear couplings of the massless spectrum possess a number o f interesting features which we analyze for two representative flat directi ons: for the fermion texture, baryon- and lepton-number violating couplings , R-parity breaking, non-canonical mu terms, and the possibility of electro weak and intermediate scale symmetry breaking scenarios for U(1)'. The gaug e coupling predictions are obtained in the electroweak scale case. Fermion masses possess t-b and t-mu universality, with the string scale Yukawa coup lings g and g/root 2, respectively. Fermion textures are present for certai n flat directions, but only in the down-quark sector. Baryon- and lepton-nu mber violating couplings can trigger proton decay, N-(N) over bar oscillati ons, leptoquark interactions and R-parity violation, leading to the absence of a stable LSP. [S0556-2821(99)00205-2].