QUANTUM STRING SCATTERING BY GRAVITATIONAL SHOCK-WAVES

The two-point amplitude, A2(k2, k1), describing the scattering of the lowest string excitation (the tachyon) by a gravitational shock wave (GSW) is computed. For this purpose, we use the appropriate vertex operator in the GSW background. We explicitly evaluate A2(k2, k1) for large impact parameters q. It is given by the Coulombian amplitude plus string corrections of order s q (s = (k1 + k2)2), for large q. These string corrections produce an infinite sequence of imaginary poles in s, the semi-infinite sequence of Coulomb poles noticed by 't Hooft always remaining present. © 1991.