  Recent work has established a method of constructing non-supersymmetric
string models that are stable, with near-vanishing one-loop dilaton tadpoles
and cosmological constants. This opens up the tantalizing possibility of
realizing stable string models whose low-energy limits directly resemble the
Standard Model rather than one of its supersymmetric extensions. In this paper
we consider the general structure of such strings and find that they share two
important phenomenological properties. The first is a so-called "GUT-precursor"
structure in which new GUT-like states appear with masses that can be many
orders of magnitude lighter than the scale of gauge coupling unification. These
states allow a parametrically large compactification volume, even in weakly
coupled heterotic strings, and in certain regions of parameter space can give
rise to dramatic collider signatures which serve as "smoking guns" for this
overall string framework. The second is a residual "entwined-SUSY" (or e-SUSY)
structure for the matter multiplets in which different multiplet components
carry different horizontal $U(1)$ charges. As a concrete example and existence
proof of these features, we present a heterotic string model that contains the
fundamental building blocks of the Standard Model such as the Standard-Model
gauge group, complete chiral generations, and Higgs fields --- all without
supersymmetry. Even though massless gravitinos and gauginos are absent from the
spectrum, we confirm that this model has an exponentially suppressed one-loop
dilaton tadpole and displays both the GUT-precursor and e-SUSY structures. We
also discuss some general phenomenological properties of e-SUSY, such as
cancellations in radiative corrections to scalar masses, the possible existence
of a corresponding approximate moduli space, and the prevention of rapid proton
decay.
