Fayet-Iliopoulos D-Term in Non-Supersymmetric Heterotic String Orbifolds

Abstract

The Fayet-Illiopoulos D-term is a common feature in N = 1 string vacua that contain an anomalous U (1) gauge symmetry, and arises from a one-loop diagram in string perturbation theory. The same diagram is generated in string vacua in which supersymmetry is broken directly at the string scale, either via spontaneous Scherk-Schwarz breaking, in which case the gravitino mass is determined by the radius of the circle used in the Scherk-Schwarz mechanism, or via explicit supersymmetry breaking by the GSO projections. We analyse the resulting would-be Fayet-Illiopoulos D-term in the non-supersymmetric string vacua and its contribution to the vacuum energy. A numerical estimate in an explicit tachyon-free string-derived model suggests that the would-be D-term contribution may uplift the vacuum energy to a positive value.