Kinetic Mixing of the Photon with Hidden U(1)s in String Phenomenology (0803.1449v1)

Abstract: Embeddings of the standard model in type II string theory typically contain a variety of U(1) gauge factors arising from D-branes in the bulk. In general, there is no reason why only one of these - the one corresponding to weak hypercharge - should be massless. Observations require that standard model particles must be neutral (or have an extremely small charge) under additional massless U(1)s, i.e. the latter have to belong to a so called hidden sector. The exchange of heavy messengers, however, can lead to a kinetic mixing between the hypercharge and the hidden-sector U(1)s, that is testable with near future experiments. This provides a powerful probe of the hidden sectors and, as a consequence, of the string theory realisation itself. In the present paper, we show, using a variety of methods, how the kinetic mixing can be derived from the underlying type II string compactification, involving supersymmetric and nonsupersymmetric configurations of D-branes, both in large volumes and in warped backgrounds with fluxes. We first demonstrate by explicit example that kinetic mixing occurs in a completely supersymmetric set-up where we can use conformal field theory techniques. We then develop a supergravity approach which allows us to examine the phenomenon in more general backgrounds, where we find that kinetic mixing is natural in the context of flux compactifications. We discuss the phenomenological consequences for experiments at the low-energy frontier, searching for signatures of light, sub-electronvolt or even massless hidden-sector U(1) gauge bosons and minicharged particles.

• The paper demonstrates kinetic mixing between the photon and hidden U(1) sectors using detailed CFT and supergravity frameworks.
• It quantifies mixing effects in both supersymmetric and non-supersymmetric D-brane configurations, impacting gauge coupling unification.
• It explores experimental implications such as photon oscillations and potential detection of minicharged particles.

Analysis of Kinetic Mixing of Photons with Hidden U(1)s in String Phenomenology

The paper entitled "Kinetic Mixing of the Photon with Hidden U(1)s in String Phenomenology" offers an in-depth analysis of the kinetic mixing phenomenon between visible sector gauge bosons, such as the photon, and additional U(1) gauge bosons that arise naturally in the context of string theory. The embedding of the Standard Model (SM) in type II string theories typically incorporates a plethora of U(1) factors, some of which reside in what are termed hidden sectors. These hidden U(1)s, due to their coupling through kinetic mixing with the visible sector's U(1) hypercharge, present observable consequences even if they remain massless or have extremely small masses.

Key Insights and Methodological Advances

The authors adopt various methodologies to investigate the emergence of kinetic mixing from type II string compactifications. They address both supersymmetric and non-supersymmetric configurations of D-branes, examining large volume scenarios and warped backgrounds with fluxes.

1. Supersymmetric Framework and CFT Techniques: The paper begins by using conformal field theory (CFT) techniques to demonstrate kinetic mixing in a completely supersymmetric setting. This methodological choice allows precise computations of kinetic mixing terms and explicitly shows that kinetic mixing can be non-trivial even between anomaly-free and massless U(1) gauge groups.
2. Supergravity Approach: Extending beyond CFT, the paper develops a supergravity framework to evaluate kinetic mixing within more general backgrounds. This approach is crucial not only for its broader applicability but also for incorporating flux compactifications, which play a significant role in phenomenological string models.
3. Phenomenological Implications: A significant aspect of this paper is the exploration of its experimental consequences at the low-energy frontier. The paper predicts potential signatures that experiments, such as those testing Coulomb's law or probing photon oscillations, might detect. Specifically, the investigation highlights the possibility of observing light, sub-electronvolt, or even massless hidden-sector U(1) gauge bosons and minicharged particles.

Practical and Theoretical Implications

The implications of this research are twofold:

• Practical Implications:

The presence of hidden sector U(1)s that kinetically mix with the SM hypercharge U(1) provides a unique window through which high-scale physics might be probed, potentially extending our understanding to energy scales inaccessible by current collider experiments. This kinetic mixing, devoid of suppression by the messenger mass scales, allows experimental searches to constrain or reveal new physics embedded within the string framework.

• Theoretical Implications:

At a theoretical level, the paper enriches the understanding of gauge coupling unification and anomaly cancellation within string landscapes. The analysis of kinetic mixing within different compactification scenarios lays groundwork for predicting and constraining string-derived modifications to SM physics. The results also prompt reconsideration of hidden sectors' roles in cosmological and astrophysical phenomena, further motivating interdisciplinary research.

Directions for Future Research

The paper sets the stage for several avenues of future inquiry:

• Further exploration of non-trivial compactifications and their impact on kinetic mixing could elucidate additional phenomenological phenomena.
• Expanding the paper to investigate the implications of kinetic mixing on dark matter models, especially given the potential for weakly interacting light particles arising from the hidden sector.
• Cross-comparisons with other frameworks, such as those involving heterotic or type I string theories, to assess universality and distinctions in kinetic mixing predictions.

In conclusion, the paper presents comprehensive and technically robust work illuminating the interplay between visible and hidden sectors in string theory, demonstrating significant potential for experimental verification. It solidifies the role of kinetic mixing as a valuable indicator for bridging string theory with observable high-energy physics.