SHARPENING THE FLIPPED SU(5) STRING MODEL

We present an improved version of the flipped SU (5) string model which accomodates numerous existing and expected features of the low-energy physics world, such as (i) two Higgs doublets with adequate mixing between them; (ii) hierarchy of quark and lepton masses, including relations such as m(s)/m(b) approximately m-mu/m-tau and m(c)/m(t) approximately (m(e)/m-tau)1/2; (iii) sufficiently suppressed proton decay operators, with gauge-boson mediated decays favoring p --> nuBAR-tau-pi+ and n --> nuBAR-tau-pi-0; (iv) nearly massless nu-mu and nu-tau, and nu-e in the eV range; (v) heavy top quark (m(t) less-than-or-similar-to 170 GeV) and large ratio of vacuum expectation values (tan beta less-than-or-similar-to 33).