D-particles in the space-time of the shock wave

We study the interaction of two D-particles in the space-time of the shock wave. We first write the amplitude in string theory and find that, at large distances from the shock-wave source, the O(v(4)) term in the relative vel ocity v is an alpha'-independent function of the interbrane separation b. T he amplitude is therefore that of supergravity-for large b, only closed-str ing massless modes contribute. We then show how the same result is obtained in the matrix model (at small b) by setting up the formulation of the dime nsionally reduced super Yang-Mills theory in the curved background of the s hock wave.