Fermion Masses and Mixings in a String Inspired Model

In the context of Calabi-Yau string models we explore the origin ofcharacteristic pattern of quark-lepton masses and the CKM matrix. The discrete$R$-symmetry $Z_K \times Z_2$ is introduced and the $Z_2$ is assigned to the$R$-parity. The gauge symmetry at the string scale, $SU(6) \times SU(2)_R$, isbroken into the standard model gauge group at a very large intermediate energyscale. At energies below the intermediate scale down-type quarks and alsoleptons are mixed with unobserved heavy states, respectively. On the otherhand, there are no such mixings for up-type quarks. Due to the large mixingsbetween light states and heavy ones we can derive phenomenologically viablefermion mass hierarchies and the CKM matrix. Mass spectra forintermediate-scale matter beyond the MSSM are also determined. Within thisframework proton lifetime is long enough to be consistent with experimentaldata. As for the string scale unification of gauge couplings, however,consistent solutions are not yet found.Comment: 49 pages, 1 figure, Latex Revised version includes discussion on FCNC problems. Final version to appear in Prog. Theor. Phys. Vol.96 No.