Signatures of Scalar Top with R-parity Breaking Coupling at Hera

In the framework of the minimal supersymmetric standard model with an R-parity breaking coupling of the scalar top quark (stop) we investigate production processes and decay properties of the stop at HERA energies. The model is characterized by a light stop possibly lighter than the other squarks. We show that the stop could be singly produced not only in the neutral current processes but also in associated processes whose final states contain some heavy flavor quarks, bottom and top quarks. These signatures would be useful to discriminate the stop from leptoquarks. The search for the supersymmetric particles (sparticles) with masses below an order of the magnitude of 1TeV is one of the most important purpose of the present collider experiments. For this purpose theoretical physicists should give answers to following questions ; i) ”which sparticle will be discovered first?” and ii) ”what signature will be expected in such sparticle production?” Answers are obviously model dependent. The simplest model is the minimal supersymmetric standard model (MSSM) with conserved R-parity [1] ; R ≡ (−), (1) where S, B and L denote the spin, baryon and lepton numbers, respectively. In the model the sparticle expected to be discovered first, i.e., the lightest charged sparticle, is the slepton l̃, the lighter chargino W̃1 or the stop t̃1. Irrespective of kinds of the lightest ∗After 1 April, 1994, Fukui University of Engineering, Fukui, 910, Japan