  In the cubic string field theory, using the gauge invariant operators
corresponding to the on-shell closed string vertex operators, we have
explicitly evaluated the decay amplitudes of two open string tachyons or gauge
fields to one closed string tachyon or graviton up to level two. We then
evaluated the same amplitudes in the bosonic string theory, and shown that the
amplitudes in both theories have exactly the same pole structure. We have also
expanded the decay amplitudes in the bosonic string theory around the
Mandelstam variable s=0, and shown that their leading contact terms are fully
consistent with a tachyonic Dirac-Born-Infeld action which includes both open
string and closed string tachyon.
