Evaluating one-loop string amplitudes

We evaluate one-loop open-string amplitudes at finite α^′ for the first time. Our method involves a deformation of the integration contour over the modular parameter τ to a fractal contour introduced by Rademacher in the context of analytic number theory. This procedure leads to explicit and practical formulas for the one-loop four-point amplitudes in type-I superstring theory, amenable to numerical evaluation. We plot the amplitudes as a function of the Mandelstam invariants s and t and directly verify long-standing conjectures about their behaviour at high energies.