Tachyon condensation in string field theory

It has been conjectured that at a stationary point of the tachyon potential for the D-brane of bosonic string theory, the negative energy density exac tly cancels the D-brane tension. We evaluate this tachyon potential by off- shell calculations in open string field theory. Surprisingly, the condensat ion of the tachyon mode alone into the stationary point of its cubic potent ial is found to cancel about 70% of the D-brane tension. Keeping relevant s calars up to four mass levels above the tachyon, the energy density at the shifted stationary point cancels 99% of the D-brane tension.