A Review on Tachyon Condensation in Open String Field Theories

We review the recent studies of tachyon condensation in string field theory. After introducing the open string field theory both for bosonic string and for superstring, we use them to examine the conjecture that the unstable configurations of the D-brane will decay into the `closed string vacuum' through the tachyon condensation. And we describe the attemps to construct a lower dimensional bosonic D-brane as an unstable lump solution of the string field equation. We obtain exact results from another formulation, background independent open string field theory. We also discuss some other topics which are related to tachyon condensation in string theory, such as the construction of a D-brane as a noncommutative soliton and some field theory models. This paper is based on my master's thesis submitted to Department of Physics, Faculty of Science, University of Tokyo on January 2001.