Light scalars and SU(5) predictions for sin2thetasub(w) and the proton lifetime

We investigate the dependence of the weak angle THETAsub(W) and the proton lifetime tausub(p) on the structure of the Higgs sector in the SU(5) model of grand unification. It is remarkable that only a small number of low dimensional scalar multiplets are sufficient to stabilize the proton considerably while accommodating experimental information on sin 2THETAsub(W). We present some modifications of the minimal model which predict a proton lifetime of the order of its present experimental lower bound as well as a richer particle spectrum in the 100 GeV range. (orig.)