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 back Ziegler, Veronique Testing grand unified theories using gauge coupling unification and proton decay Order No. 9633805 We test the validity of Grand Unified Theories by the simultaneous constraints from proton decay and gauge coupling unification. For minimal supersymmetric SU(5), we obtain a strict bound $\alpha\sb3 > 0.117,$ which is above measurements of $\alpha\sb3$ from deep inelastic scattering and the charmonium spectrum by more than 3$\sigma.$ This signals the failure of the minimal SU(5) model. The dismissal of ordinary SU(5) gives a strong incentive to investigate flipped supersymmetric SU(5) $\times$ U(1). A careful analysis, including all threshold effects, indicates that the parameter space of flipped supersymmetric SU(5) $\times$ U(1) is compatible with experimental range. We obtain an upper bound on the splitting between the heavy mass-scales $M\sb{D\sp{c}}$ and $M\sb{X}$, of the order of 10$\sp4$. We find gravitational perturbations affecting predictions from the level of the Planck scale to be negligible. An attempt to increase the superunification scale, $M\sb{SU}$, via the mass of the right handed neutrino, gives us a value for $M\sb{SU}$ still short of the string scale. Source: DAI, 57, no. 06B (1996): p.3814 back