Two-loop scalar diagrams from string theory

We show how to obtain correctly normalized expressions for the Feynman diagrams of Φ3 theory with an internal U(N) symmetry group, starting from tachyon amplitudes of the open bosonic string, and suitably performing the zero-slope limit by giving an arbitrary mass m to the tachyon. In particular we present explicit results for the two-loop amplitudes of Φ3 theory, in preparation for the more interesting case of multiloop amplitudes of non-abelian gauge theories.