Quasi Yukawa Fixed Point due to Decoupling of SUSY Particles

We study effects of SUSY particle decouplings on a quasi fixed point (QFP) of Yukawa coupling. From renormalization group analysis it is shown that if the SUSY breaking scale MS is large (> ∼ 1TeV), effects of decoupling of Higgsinos and squarks raise the top Yukawa QFP. This tendency is enhanced in most cases of non-universal SUSY breaking. For the case of MS < ∼ 1TeV, the decoupling of gluinos lowers m t . We checked some parameter dependencies for the top Yukawa QFP. The bottom-top Yukawa unified case is also studied. When top quark mass is measured more precisely, some patterns of soft mass spectra could be excluded if rather large initial top Yukawa coupling is realized by underlying theory. Alexander von Humboldt Fellow e-mail:[email protected] e-mail:[email protected] Supersymmetry (SUSY) [1] is now considered as a promising candidate for physics beyond the Standard Model (SM). The minimal supersymmetric extension of the Standard Model (MSSM) shows many successful results, i.e. the gauge coupling unification [2], the radiative symmetry breaking[3], etc. Recently a quasi fixed point (QFP) of the top Yukawa coupling [4] was reconsidered by Lanzagorta and Ross[5] and they pointed out that the QFP is also interesting in supersymmetric models. Such studies show that the QFP under the MSSM can predict the top quark mass closer to the experimental value[6] than the SM. Furthermore it is meaningful to study features of the QFP in the framework of supersymmetric models from various viewpoints, e.g. effects of threshold corrections due to non-universal SUSY particle masses. In general supergravity theories (SUGRAs) as well as superstring models lead to non-universal soft terms, i.e. non-universal soft scalar masses and gaugino masses [7, 8]. In Refs.[9, 10], it is shown that the effects of non-universal SUSY breaking on the gauge coupling unification are rather sizable. In addition much work has been devoted to phenomenological implications of non-universality of SUSY particles [11, 12]. In this paper we mainly study effects of such non-universal decoupling of SUSY particles on the QFP. Usually the MSSM has been considered in the framework of N=1 minimal SUGRA which leads universal soft SUSY breaking terms. This treatment is simple and has a powerful predictability so that much work has been done under this framework. However, in general, SUGRAs as well as superstring models often yield non-universal soft terms[7]. It seems that the minimal SUGRA is a special case from the viewpoint of superstring theory. The general forms of Kähler potential K and the superpotential W are written as follows, K = κK̂(Φ, Φ̄) +K(Φ, Φ̄)IJ̄Q Q̄ + ( 1 2 H(Φ, Φ̄)IJQ Q + h.c. )