The Infrared Fixed Point of the Top Quark Mass and its Implications within the MSSM

We analyse the general features of the Higgs and supersymmetric particle spectrum associated with the infrared fixed point solution of the top quark mass in the Minimal Supersymmetric Standard Model. We consider the constraints on the mass parameters, which are derived from the condition of a proper radiative electroweak symmetry breaking in the low and moderate tanβ regime. In the case of universal soft supersymmetry breaking parameters at the high energy scale, the radiative SU(2)L ×U(1)Y breaking, together with the top quark Yukawa fixed point structure imply that, for any given value of the top quark mass, the Higgs and supersymmetric particle spectrum is fully determined as a function of only two supersymmetry breaking parameters. This result is of great interest since the infrared fixed point solution appears as a prediction in many different theoretical frameworks. In particular, in the context of the MSSM with unification of the gauge and bottom–tau Yukawa couplings, for small and moderate values of tanβ the value of the top quark mass is very close to its infrared fixed point value. We show that, for the interesting range of top quark mass values Mt ≃ 175± 10 GeV, both a light chargino and a light stop may be present in the spectrum. In addition, for a given top quark mass, the infrared fixed point solution of the top quark Yukawa coupling minimizes the value of the lightest CP-even Higgs mass mh. The resulting upper bounds on mh read mh ≤ 90 (105) (120) GeV for Mt ≤ 165 (175) (185) GeV. CERN-TH.7320/94 June 1994 ∗ Talk presented by M. Carena at the 2nd IFT Workshop on Yukawa Couplings and the Origins of Mass, Gainesville, Florida, Feb. 1994. To appear in the Proceedings.