FINE-TUNING PROBLEM IN MINIMAL SO(10) SUPERSYMMETRIC GRAND UNIFIED THEORIES

In grand unified theories (GUT's) based on SO(10) all fermions of one generation are embedded in a single representation. As a result, the t op quark, the bottom quark, and the tau lepton have a universal Yukawa coupling at the GUT scale. This implies a very large ratio of Higgs v acuum expectation values, tan beta similar or equal to m(t)/m(b). We a nalyze the naturalness of such a scenario quantitatively including all the relevant radiative corrections and find, that in minimal unified supergravity models with universal soft supersymmetry-breaking paramet ers at the GUT scale, the necessary amount of fine-tuning is excessive . The GUT threshold correction to the universal Higgs boson mass param eter can significantly reduce the fine-tuning required for such large values of tan beta. We also point out that the top quark mass can play a crucial role in explaining the hierarchy between the SUSY-breaking scale and the electroweak scale and, hence, the naturalness of large v alues of tan beta.