The second string (phenomenology) revolution

TL;DR

The article argues that the landscape of string phenomenology has expanded beyond the heterotic paradigm, with Type I/II corners and D-brane constructions enabling viable 4D $D=4$, $N=1$ vacua in which the string scale $M_s$ can be decoupled from the Planck scale and take intermediate or even TeV values. It highlights Type IIB orientifolds as a fertile framework, where multiple anomalous $U(1)$ factors and twisted moduli can induce mirage or precocious gauge coupling unification, and where full semi-realistic, three-generation models have been built, including non-supersymmetric examples with explicit SUSY breaking. A concrete $Z_3$ orientifold with 7-branes demonstrates a three-generation, left-right symmetric Standard-Like Model that naturally exhibits intermediate-scale unification tendencies, albeit with stability questions in non-SUSY vacua. Overall, the work marks the second string phenomenology revolution, expanding the scope of viable embeddings of the SM into string theory and motivating further development of D-brane model-building and moduli stabilization strategies.

Abstract

In the past three years our views on how the standard model of particle physics could be embedded into string theory have dramatically changed. The heterotic string is no longer the only possibility for such an embedding and other perturbative (or non-perturbative) corners of M-theory, like Type I or Type II strings seem now possible. It has also been realized that the string scale $M_s$ is not necessary close to the Planck scale and could be much smaller, of order the intermediate scale $\sqrt{M_WM_p}$ or even close to the weak scale. In addition, semi-realistic three generation models have recently been constructed starting with Type IIB compact orientifolds. I briefly discuss some of these developements which represent a revolution in our understanding of string phenomenology.