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. M1/2, m0, μ, . . . ) compatible with a correct electroweak breaking and a realistic top-quark mass. To do so we have included all the one-loop corrections to the effective potential V1 and checked their importance in order to obtain consistent results. We also consider the fine-tuning problem due to the enormous dependence of MZ on ht (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. CERN-TH.7024/93 IEM-FT-78/93 September 1993 Talk given at the XVI Kazimierz Meeting on Elementary Particle Physics: New Physics with New Experiments, 24-28 May 1993, Kazimierz (Poland). Supported by a Comunidad de Madrid grant. On leave from Instituto de Estructura de la Materia (CSIC), Serrano 123, 28006-Madrid, Spain.