We show that if the string scale is identifed with the intermediate scale, $M_s=\sqrt{ M_W M_{ Planck}} \sim 10^{ 11}$ GeV, then the notorious hierarchy, $M_W/M_{ Planck} \sim 10^{ -16}$, can be explained using only $M_c/M_s \sim 0.01 \sim \alpha_{ GUT} $ as small input parameters, where $M_c$ is the compactification scale. This is possible for weakly-coupled Type-I open-string vacua if the observed world is assumed to live in an N=1 supersymmetric 3-brane sector coupled to a separate, hidden, 3-brane world which breaks supersymmetry, because for such a model $M_W/M_{ Planck} = 1/2 \alpha_{ GUT}^2 (M_c/M_s)^6$. We discuss some of the phenomenological issues presented by such an intermediate-scale string, showing that its benefits include: (i) the possibility of logarithmic gauge-coupling unification of the SM couplings at $M_s$; (ii) a natural axionic solution to the strong-CP problem with a phenomenologically-acceptable Peccei-Quinn scale; (iii) experimentally-interesting neutrino masses, and more.
Categories: hep-ph astro-ph hep-th