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Stringy Unification Helps See-Saw Mechanism

In this paper we explore the possibility of intermediate scale physics in the context of superstring models with {\it higher Kac-Moody levels}, by focusing on left-right and Pati-Salam symmetries. We find that the left-right scale may lie in the range $10^{10} - 10^{12}$ GeV which is favored by neutrino physics, while the Pati-Salam scale is at most two or three orders of magnitude below the unification scale $M_X$. We also show that the scale of $B-L$ breaking can be as low as 1 TeV or so, providing protection against too rapid proton decay in supersymmetry. Our results allow a natural value for the scale $M_X \sim 10^{18}$ GeV and the agreement with the experiment requires the value of $\sin^2 {θ_w}$ at $M_X$ to be in general very different from the usually assumed $3/8$.