RATIONAL UNIFICATION HELPS THE SEESAW MECHANISM

In this paper we explore the possibility of intermediate scale physics in the context of superstring models with higher Kac-Moody levels, by focusing on left-right and Pati-Salam symmetries. We find that the le ft-right scale may Lie in the range 10(10)-10(12) GeV, which is favore d by neutrino physics, while the Pati-Salam scale is at most two or th ree 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, providin g protection against too rapid proton decay in supersymmetry. Our resu lts allow a natural value for the scale M(x) similar to 10(18) GeV and the agreement with the experiment requires the value of sin(2) theta( w) at M(x) to be in general very different from the usually assumed 3/ 8.