The Fine-Tuning Problem of the Electroweak Symmetry Breaking Mechanism in Minimal SUSY Models

We calculate the region of the MSSM parameter space (i.e. $M_{1\mathrm{/}2}$, $m_0$, $\mu$, \ldots) compatible with a correct electroweak breaking and a realistic top-quark mass. To do so we have included {\em all} the one-loop corrections to the effective potential $V_1$ and checked their importance in order to obtain consistent results. We also consider the fine-tuning problem due to the enormous dependence of $M_Z$ on $h_t$ (the top Yukawa coupling), which is substantially reduced when the one-loop effects are taken into account. We also explore the reliability of the so-called “standard” criterion to estimate the degree of fine-tuning. As a consequence, we obtain a new set of upper bounds on the MSSM parameters or, equivalently, on the supersymmetric masses perfectly consistent with the present experimental bounds.