The type-I D-instanton and its M-theory origin

The tree-level amplitude for the scattering of two gauge particles constrai ned to move on the two distinct boundaries of eleven-dimensional space-time in the Horava-Witten formulation of M-theory is constructed. At low moment a this reproduces the corresponding tree-level scattering amplitude of the E-8 x E-8 heterotic string theory. After compactification to nine dimension s on a large circle with a suitable Wilson line to break the symmetry to SO (16) x SO(16) this amplitude is used to describe the scattering of two mass ive SO(16) spinor states - one from each factor of the unbroken symmetry gr oup. The amplitude contains a component that is associated with the exchang e of a Kaluza-Klein charge between the boundaries, which is interpreted as the exchange of a D-particle between orientifold planes in the type-IA theo ry. This is related by T-duality to the effect of a non-BPS D-instanton in the type-I theory which is only invariant under those elements of O(16) x S O(16) that are in SO(16) x SO(16).