Abstract:
We study a deformed $su(m|n)$ algebra on a quantum superspace. Some interesting aspects of the deformed algebra are shown. As an application of the deformed algebra we construct a deformed superconformal algebra. {}From the deformed $su(1|4)$ algebra, we derive deformed Lorentz, translation of Minkowski space, $iso(2,2)$ and its supersymmetric algebras as closed subalgebras with consistent automorphisms.

Abstract:
We study differential calculus on h-deformed bosonic and fermionic quantum space. It is shown that the fermionic quantum space involves a parafermionic variable as well as a classical fermionic one. Further we construct the classical $su(2)$ algebra on the fermionic quantum space and discuss a mapping between the classical $su(2)$ and the h-deformed $su(2)$ algebras.

Abstract:
We study soft SUSY breaking terms in heterotic M-theory. We show that both weakly and strongly coupled heterotic string models lead to the same relations of soft SUSY breaking terms, $A=-M$ and $m^2 = M^2/3$, up to $O((\alpha T/S)^2)$.

Abstract:
We investigate non-commutative differential calculus on the supersymmetric version of quantum space where the non-commuting super-coordinates consist of bosonic as well as fermionic (Grassmann) coordinates. Multi-parametric quantum deformation of the general linear supergroup, $GL_q(m|n)$, is studied and the explicit form for the ${\hat R}$-matrix, which is the solution of the Yang-Baxter equation, is presented. We derive the quantum-matrix commutation relation of $GL_q(m|n)$ and the quantum superdeterminant. We apply these results for the $GL_q(m|n)$ to the deformed phase-space of supercoordinates and their momenta, from which we construct the ${\hat R}$-matrix of q-deformed orthosymplectic group $OSp_q(2n|2m)$ and calculate its ${\hat R}$-matrix. Some detailed argument for quantum super-Clifford algebras and the explict expression of the ${\hat R}$-matrix will be presented for the case of $OSp_q(2|2)$.

Abstract:
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 $M_S$ is large ($\simg 1$TeV), 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 $M_S\siml 1$TeV, the decoupling of gluinos lowers $m_t^{\rm QFP}$. 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.

Abstract:
We obtain the low-energy effective theory from string models with anomalous $U(1)$ symmetry. The feature of soft supersymmetry breaking scalar masses and some phenomenological implications are discussed. We show that it is, in general, difficult to keep the degeneracy and the positivity of squared soft scalar masses at the Planck scale.

Abstract:
In the framework of $Z_N$ orbifolds, we discuss effects of heterotic string backgrounds including discrete Wilson lines on the Yukawa matrices and their connection to CP violation.

Abstract:
We consider the model with the dilaton and twisted moduli fields, which is inspired by type I string models. Stabilization of their vacuum expectation values is studied. We find the stabilization of the twisted moduli field has different aspects from the dilaton stabilization.

Abstract:
We derive formula of soft supersymmetry breaking scalar masses from 4-dimensional string models within a more generic framework. We consider effects of extra gauge symmetry breakings including an anomalous $U(1)$ breaking through flat directions, that is, $D$-term and $F$-term contributions, particle mixing effects and heavy-light mass mixing effects. Some phenomenological implications are discussed based on our mass formula.

Abstract:
``Anomalous'' U(1) gauge symmetry with Green-Schwarz anomaly cancellation mechanism is discussed in the orbifold construction of four-dimensional heterotic string models. Some conditions are given as criteria to have ``anomalous'' U(1) in orbifold string models. In particular, ``anomalous'' U(1) is absent if the massless twisted matter has no mixing between visible and hidden sectors or if a certain type of discrete symmetries are found. We then give a general procedure for classifying orbifold models with ``anomalous'' U(1) and for identifying the ``anomalous'' U(1) basis. We illustrate our procedure in Z_3 and Z_4 orbifold models. According to our procedure, the classification of ``anomalous'' U(1) can be reduced to the classification in the absence of a Wilson line. We also discuss discrete symmetries left unbroken after the ``anomalous'' U(1) breaking. This includes a possible relation between ``anomalous'' U(1) and discrete R-symmetries.