Some exact results on tachyon condensation in string field theory

The study of open string tachyon condensation in string field theory can be drastically simplified by making an appropriate choice of coordinates on t he space of string fields. We show that a very natural coordinate system is suggested by the connection between the worldsheet renormalization group a nd spacetime physics. In this system only one field, the tachyon, condenses while all other fields have vanishing expectation values. These coordinate s are also well-suited to the study of D-branes as solitons. We use them to show that the tension of the D25-brane is cancelled by tachyon condensatio n and compute exactly the profiles and tensions of lower dimensional D-bran es.