Abstract
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.
Highlights
Most approaches to string phenomenology have historically proceeded under the assumption that the Standard Model (SM) becomes supersymmetric at a higher energy scale parametrically near the electroweak symmetry-breaking scale
While bottom-up factors included a strong belief in the existence of weak-scale supersymmetry, a critical top-down factor was the fact that nonsupersymmetric strings are generally unstable, with large one-loop dilaton tadpoles
Because our construction necessarily involves largevolume compactifications, one pressing issue concerns the behavior of the gauge couplings—especially if we require perturbativity both at the electroweak scale as well as in the UV limit
Summary
Most approaches to string phenomenology have historically proceeded under the assumption that the Standard Model (SM) becomes supersymmetric at a higher energy scale parametrically near the electroweak symmetry-breaking scale. Our aim is to present a concrete example and existence proof of models with these features within the context of a fully selfconsistent string construction, and to demonstrate that these features can coexist with other fundamental phenomenological building blocks of realistic string models such as the Standard-Model gauge group, complete chiral generations, and Higgs fields—all in a stable, nonsupersymmetric setting While this model is not intended to serve as a fully realistic description of nature, it serves as an illustrative example of what can be accomplished within this framework. V, we briefly discuss several other phenomenological aspects of metastable string models that result from their GUT-precursor and e-SUSY structures These include cancellations in radiative corrections to scalar masses, the possible existence of a corresponding approximate moduli space, and the prevention of rapid proton decay.
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