Suppressing the cosmological constant in non-supersymmetric type I strings

We construct non-supersymmetric type I string models which correspond to consistent flat-space solutions of all classical equations of motion. Moreover, the one-loop vacuum energy is naturally fixed by the size of compact extra dimensions which, in the two-dimensional case, can be lowered to a fraction of a millimetre. This class of models has interesting non-Abelian gauge groups and can accommodate chiral fermions. In the large-radius limit, supersymmetry is recovered in the bulk, while D-brane excitations, although non-supersymmetric, exhibit Fermi-Bose degeneracy at all mass levels. We also give some evidence for a suppression of higher-loop corrections to the vacuum energy. (C) 2003 Elsevier B.V. All fights reserved.