Low-energy effective action for the superstring

We construct the low-energy D = 4, N = 1 supergravity that arises in superstring theories for an arbitrary number of generations. The couplings of all massless modes that carry low-energy gauge quantum numbers are calculated by truncating the heavy Kaluza-Klein modes of the ten-dimensional effective field theory. The resulting action is compared to the most general effective action compatible with the symmetries of the underlying ten-dimensional field (and string) theories. This comparison indicates which features of the truncation correctly approximate the exact low-energy action.