A Light Scalar Top Quark in MSGUT
  We reexamine a possibility for the existence of a light supersymmetric partner of the top quark (stop) with mass 15$\sim$16GeV in the framework of the minimal supergravity GUT model (MSGUT). Such a stop could explain the slight excess of the high $p_{T}$ cross section of the $D^{*\pm}$-meson production in the two-photon process at TRISTAN. We find two types of solution for the RGEs in the MSGUT allowing the existence of the light stop. The type I [type II] solution is characterized by the heavy [light] top quark, $m_t\simeq$ 150GeV [100GeV], and the light [heavy] squarks . It is found that the type II solution is more favorable because these parameter sets seem to satisfy all constraints settled by the recent collider experiments as well as by the cosmological considerations. The existence of such a stop could change the dominant decay mode of sparticles, the top quark and the Higgs bosons. Consequently,the present experimental bounds on the supersymmetric parameters as well as on masses of the top and the Higgs could be weakened substantially. However, the allowed parameter region is rather restricted and in turn masses and mixing parameters of the other SUSY partners as well as masses of the Higgs and the top are severely constrained. 
Tadashi Kon and Toshihiko Nonaka