The Disk Partition Function in String Theory

We investigate the disk partition function for the open string. This is a subtle problem because of the presence of a residual gauge group $\mathrm{P}\mathrm{S}\mathrm{L}(2,\mathbb{R})$ on the worldsheet even after fixing the conformal gauge. It naively has infinite volume and leads to a vanishing answer. We use different methods that all demonstrate that $\mathrm{P}\mathrm{S}\mathrm{L}(2,\mathbb{R})$ effectively behaves like a group with finite negative volume in the path integral, which leads to a simple prescription for the computation of the disk partition function. We apply our findings to give a simple rederivation of the D-brane tensions.