Naturally Light Hidden Photons in LARGE Volume String Compactifications (0909.0515v2)

Abstract: Extra "hidden" U(1) gauge factors are a generic feature of string theory that is of particular phenomenological interest. They can kinetically mix with the Standard Model photon and are thereby accessible to a wide variety of astrophysical and cosmological observations and laboratory experiments. In this paper we investigate the masses and the kinetic mixing of hidden U(1)s in LARGE volume compactifications of string theory. We find that in these scenarios the hidden photons can be naturally light and that their kinetic mixing with the ordinary electromagnetic photon can be of a size interesting for near future experiments and observations.

• The paper demonstrates that kinetic mixing in string compactifications can render hidden U(1) gauge bosons observable through interactions with visible photons.
• It models diverse scenarios including hidden CMB and dark matter candidates, predicting measurable effects consistent with astrophysical and laboratory constraints.
• The study identifies specific parameter spaces and emphasizes LARGE volume effects in determining kinetic mixing strengths for future experimental exploration.

Overview of Hidden U(1) in String Theory Compactifications

The paper explores the potential existence and implications of hidden U(1) gauge bosons, often termed hidden photons, theorized to emerge in large volume compactifications of string theory. These phenomena arise due to the presence of extra "hidden" gauge groups within the string theory framework that can kinetically mix with the Standard Model's electromagnetic U(1). Such kinetic mixings render these hidden photons observable in various cosmological, astrophysical, and laboratory settings, thus creating intriguing possibilities for new physics beyond the Standard Model.

Key Concepts and Results

1. Hidden U(1) and Kinetic Mixing: The investigation explores hidden U(1) gauge factors, focusing on their kinetic mixing with visible photons. This mixing is facilitated by higher-dimensional operators and influenced by vacuum expectation values, which could make hidden photons detectable through various physical phenomena, including astrophysical and experimental observations.
2. Phenomenological Implications: The paper presents detailed theoretical models that predict the mass and kinetic mixing scale of hidden photons across various regions of parameter space:
   • "Hidden CMB" scenario explores the implications of hidden photons as potential additional components of dark radiation, aligned with cosmic microwave background (CMB) observations.
   • "Lukewarm DM" scenario characterizes hidden photons as dark matter candidates within the MeV range.
   • "Unified DM" model considers hidden photons within the context of dark forces potentially explaining cosmic ray anomalies observed by experiments like PAMELA and DAMA.
3. String Theory Predictions: Through detailed computations, the authors extract the expected values for the kinetic mixing parameters from realistic string compactifications. The kinetic mixing parameter, crucial for phenomenology, depends on the bulk volume and hidden sectors described by different U(1) factors within type II string theories. The values are predicted to vary widely depending on compactification details, suggesting a range of possible experimental signatures.
4. Scenarios and Constraints: The authors discuss various string theory configurations (LARGE volume scenarios and hyperweak gauge interactions), some predicting very weak kinetic mixing, while others propose stronger visible-sector interactions. These predictions are shown to be consistent with current experimental constraints derived from CMB observations, Coulomb law deviation measurements, and laboratory-based "light-shining-through-walls" experiments.
5. Future Experimental Probes: The white region in the parameter space indicates areas not yet probed by current experiments, suggesting where future searches and technological advancements can further explore these hidden sectors.

Theoretical and Practical Implications

The research opens avenues for understanding the hidden sectors within string theory, which may inform us about darker components of the universe, potentially contributing to the composition of dark matter. Practically, the increasing sensitivity of experiments searching for weakly interacting sub-eV hidden photons could validate or constrain these predictions.

Speculation and Future Directions

Moving forward, the theoretical framework outlined presents a robust basis for predicting diverse physics implications from string theory, but continued cross-collaboration between experimental and theoretical physicists will be crucial. Rigorous experimental efforts, guided by the regions defined in the parameter space, may gradually unveil new particles predicted by these string scenarios, offering new insights into the fundamental laws governing our universe. The potential for these experiments to access previously unexplored regions presents significant opportunities for discoveries that could reshape our understanding of particle physics and cosmology within the context of string theory.