On string loops in Calabi-Yau orientifolds in large volume

Abstract: We explain and illustrate how to compute string-loop amplitudes in Calabi-Yau orientifold compactification in the large volume limit with the help of the patch-by-patch description of string field theory. We compute the one-loop partition function of the D1-instanton in type IIB string theory compactified on an O9 orientifold of a Calabi-Yau threefold to the first order in the large volume expansion. We show that the unphysical divergence arising from a naive choice of PCOs is canceled by vertical integration. The corollaries of this result, including the universal part of the normalization of the D-instanton superpotential and the one-loop renormalization of Kähler moduli, will be presented elsewhere.