Worldsheet Description of Tachyon Condensation in Open String Theory

In this thesis we review the fundamental framework of boundary string field theory (BSFT) and apply it to the tachyon condensation on non-BPS systems in the superstring theory. The boundary string field theory can be regarded as a natural extension of the nonlinear sigma model. By using this theory we can describe the tachyon condensation exactly and also obtain the effective actions on non-BPS systems consisting of the Dirac-Born-Infeld type action and the Wess-Zumino type action. Especially the Wess-Zumino action is written by superconnection and coincides with the mathematical argument by K-theory. Moreover we also discuss the tachyon condensation keeping the conformal invariance (on-shell). The exact argument by using the boundary state formalism gives a good support to the conjecture of the tachyon condensation and it is also consistent with boundary string field theory.