Signals for Scalar Top Quark at $ep$ Collider HERA

In the framework of the minimal supersymmetric standard model (MSSM) and the R-parity breaking model (RBM), we investigate production processes of the scalar top (stop) at HERA energies. These models are characterized by the possible existence of the light stop whose mass is lighter than those of the top quark and the other squarks. It is shown that in the MSSM the stop pair production via boson-gluon fusion gives a sizeable cross section and the most serious background $ep\rightarrow ec\bar{c}X$ could be suppressed by appropriate kinematical cuts. We also show that in the RBM the stop is singly produced in the neutral current processes and we have a clear signal as a sharp peak in the Bjorken parameter $x$ distribution of the scattered electron.