TROUBLES WITH THE MINIMAL SU(5) SUPERGRAVITY MODEL

We show that within the framework of the minimal SU(5) supergravity model, radiatively-induced electroweak symmetry breaking and presently available experimental lower bounds on nucleon decay, impose severe constraints on the available parameter space of the model, which correspond to fine-tuning of the model parameters of over two orders of magnitude. Furthermore, a straightforward calculation of the cosmic relic density of neutralinos (chi) gives OMEGA(chi)h2 much greater than 1 for most of the allowed parameter space in this model, although small regions may still be cosmologically acceptable. We finally discuss how the no-scale flipped SU(5) supergravity model avoids naturally the above troubles and thus constitutes a good candidate for the low-energy effective supergravity model.