Explicit evaluation of covariant one-loop four-point amplitude for open fermionic string

The authors carry out the explicit evaluation of the covariant one-loop amplitude with four massless external bosons for open fermionic string by the operator formalism. The resulting expression of the amplitude completely coincides with that of the light-cone new formalism for type-I superstring theory in both the non-regularized and regularized schemes, giving an explicit demonstration for the one-loop equivalence of the old and new formalisms for the open superstring theory at the four-point interacting level