Is String Phenomenology an Oxymoron?

TL;DR

Is String Phenomenology an Oxymoron? evaluates the achievements and challenges of linking string theory to observable physics. It argues that, while only limited low-energy predictions are generic and many remain experimentally inaccessible, string theory provides a coherent UV framework with gravity, gauge interactions, and moduli that shape cosmology; the field has driven significant mathematical insights (e.g., mirror symmetry, dualities) and concrete mechanisms for moduli stabilization (e.g., KKLT, LVS). The author advocates focusing on model-independent questions, exploring flux compactifications and landscape dynamics, and seeking correlations with forthcoming experiments, rather than claiming immediate empirical vindication. The work emphasizes patience and breadth, recognizing that progress may unfold over decades while underscoring the potential long-term impact of string-inspired scenarios on particle physics and cosmology.

Abstract

A brief discussion is presented assessing the achievements and challenges of string phenomenology: the subfield dedicated to study the potential for string theory to make contact with particle physics and cosmology. Building from the well understood case of the standard model as a very particular example within quantum field theory we highlight the very few generic observable implications of string theory, most of them inaccessible to low-energy experiments, and indicate the need to extract concrete scenarios and classes of models that could eventually be contrasted with searches in collider physics and other particle experiments as well as in cosmological observations. The impact that this subfield has had in mathematics and in a better understanding of string theory is emphasised as spin-offs of string phenomenology. Moduli fields, measuring the size and shape of extra dimensions, are highlighted as generic low-energy remnants of string theory that can play a key role for supersymmetry breaking as well as for inflationary and post-inflationary early universe cosmology. It is argued that the answer to the question in the title should be, as usual, {\it No}. Future challenges for this field are briefly mentioned. This essay is a contribution to the conference: "Why Trust a Theory?", Munich, December 2015.

Figures (3)

• Figure 1: The standard model on a mug. The first row has the Einstein-Hilbert term for gravity ($\lambda =2$) and the kinetic and topological terms for the gauge fields ($\lambda =1$) describing the electromagnetic, weak and strong interactions. The second line has the kinetic energy for the matter fields: quarks and leptons $\lambda =1/2$ as well as their (Yukawa) couplings to the Higgs field $H$ ($\lambda =0$). The third line is the kinetic and potential energy for the Higgs field.
• Figure 2: A sketch of the running of the SM gauge couplings with energy through the renormalisation group equations. At the TeV scale the three gauge couplings are very different but with increasing energy they tend towards unification. We plot the quantities $\alpha_i^{-1}=\frac{4\pi}{g_i^2}$ against (log of) energy. Dashed lines correspond to the SM and solid lines to the supersymmetric extension of the SM. Two points are usually emphasised: supersymmetry remarkably unifies the three interactions at an energy close to the Planck scale and even without supersymmetry the running is towards weaker couplings, raising expectations for a weakly coupled ultra violet completion. Gravity is not included in the plot.
• Figure 3: Table of representatitive models of string inflation (from bcq).