Cyclic products of higher-genus Szeg\"o kernels, modular tensors and polylogarithms

A wealth of information on multiloop string amplitudes is encoded in fermionic two-point functions known as Szeg\"o kernels. In this paper we show that cyclic products of any number of Szeg\"o kernels on a Riemann surface of arbitrary genus may be decomposed into linear combinations of modular tensors on moduli space that carry all the dependence on the spin structure $\delta$. The $\delta$-independent coefficients in these combinations carry all the dependence on the marked points and are composed of the integration kernels of higher-genus polylogarithms. We determine the antiholomorphic moduli derivatives of the $\delta$-dependent modular tensors.