D-branes as tachyon lumps in string field theory

It has been conjectured that the tachyonic lump solution of the open bosoni c string field theory describing a D-brane represents a D-brane of one lowe r dimension. We place the lump on a circle of finite radius and develop a v ariant of the level expansion scheme that allows systematic account of all higher derivative terms in the string field theory action, and gives a calc ulational scheme that can be carried to arbitrary accuracy. Using this appr oach we obtain lump masses that agree with expected D-brane masses to an ac curacy of about 1%. We find convincing evidence that in string field theory the lump representing a D-brane is an extended object with a definite prof ile. A gaussian fit to the lump gives a 6-sigma size of 9.3 root alpha'. Th e level truncation scheme developed here naturally gives rise to an infrare d and ultraviolet cut-off, and may be useful in the study of quantum string field theory.