Dynamics of (n, 1) Strings

An $(n, 1)$ string is a bound state of a D-string and $n$ fundamental strings. It may be described by a D-string with a world volume electric field turned on. As the electric field approaches its critical value, $n$ becomes large. We calculate the 4-point function for transverse oscillations of an $(n, 1)$ string, and the two-point function for massless closed strings scattering off an $(n, 1)$ string. In both cases we find a set of poles that becomes dense in the large $n$ limit. The effective tension that governs the spacing of these poles is the fundamental string tension divided by $1+(n\lambda)^2$, where $\lambda$ is the closed string coupling. We associate this effective tension with the open strings attached to the $(n, 1)$ string, thereby governing its dynamics. We also argue that the effective coupling strenth of these open strings is reduced by the electric field and approaches zero in the large $n$ limit.