FULL ELECTROWEAK SUPERSYMMETRIC QUANTUM EFFECTS ON R(B) IN THE MSSM

We address the computation of Gamma(z) and of the intriguing quantity R(b) in the MSSM including full treatment of the Higgs sector. Contrary to previous partial approaches, and due to the possible relevance of the result to the fate of the MSSM, we perform a complete calculation, without approximations. For a pseudoscalar Higgs mass m(A0) > 70GeV and CDF limits on m(t), the bounds on R(b) at 1 sigma level leave no room to the MSSM to solve the ''R(b) crisis'' for any combination of the parameters, not even admitting the possibility of a light chargino and a light stop of O(50) GeV; however, for m(t) not restricted by CDF, a ''tangential'' solution exists in the window 2 < tan beta < 10 with a light chargino and stop. In contrast, for a pseudoscalar mass 40 GeV less than or similar to < 60 GeV and CDF limits on m(t), the ''R(b) crisis'' can be solved in a comfortable way, for any SUSY spectrum above the phenomenological bounds, provided tan beta greater than or similar to m(t)/m(b). Our general conclusion is that, if there is a ''R(b) crisis'' at all, its solution within the MSSM has to do more with the peculiar structure of the SUSY Higgs sector rather than with the spectrum of genuine supersymmetric particles. In view of the range predicted for m(A0), LEP 200 should be able to definitely settle this question.