Two independent criteria are presented that together guarantee exponential suppression of the two-loop cosmological constant in nonsupersymmetric heterotic strings. They are derived by performing calculations in both the full string theory and in its effective field theory, and come respectively from contributions that involve only physical untwisted states, and contributions that include orbifold twisted states. The criteria depend purely on the spectrum and charges, so a model that satisfies them will do so with no fine-tuning. An additional consistency condition (emerging from the so-called separating degeneration limit of the two-loop diagram) is that the one-loop cosmological constant must also be suppressed, by Bose-Fermi degeneracy in the massless spectrum. We comment on the effects of the residual exponentially suppressed one-loop dilaton tadpole, with the conclusion that the remaining instability would be under perturbative control in a generic phenomenological construction. We remark that theories of this kind, that have continued exponential suppression to higher orders, can form the basis for a string implementation of the “naturalness without supersymmetry” idea.
Abel, S., & Stewart, R. J. (2017). Exponential suppression of the cosmological constant in nonsupersymmetric string vacua at two loops and beyond. Physical Review D, 96(10), Article 106013. https://doi.org/10.1103/physrevd.96.106013