Dark photon limits: a handbook (2105.04565v3)

Abstract: The dark photon is a massive hypothetical particle that interacts with the Standard Model by kinetically mixing with the visible photon. For small values of the mixing parameter, dark photons can evade cosmological bounds to be a viable dark matter candidate. Due to the similarities with the electromagnetic signals generated by axions, several bounds on dark photon signals are simply reinterpretations of historical bounds set by axion haloscopes. However, the dark photon has a property that the axion does not: an intrinsic polarisation. Due to the rotation of the Earth, accurately accounting for this polarisation is nontrivial, highly experiment-dependent, and depends upon assumptions about the dark photon's production mechanism. We show that if one does account for the DP polarisation, and the rotation of the Earth, an experiment's discovery reach can be enhanced by over an order of magnitude. We detail the strategies that would need to be taken to properly optimise a dark photon search. These include judiciously choosing the location and orientation of the experiment, as well as strategically timing any repeated measurements. Experiments located at $\pm$35$^\circ$ or $\pm$55$^\circ$ latitude, making three observations at different times of the sidereal day, can achieve a sensitivity that is fully optimised and insensitive to the dark photon's polarisation state, and hence its production mechanism. We also point out that several well-known searches for axions employ techniques for testing signals that preclude their ability to set exclusion limits on dark photons, and hence should not be reinterpreted as such.

• The paper presents a comprehensive analysis of dark photon constraints by accounting for their intrinsic polarization and kinetic mixing with standard model photons.
• The paper outlines optimal experimental strategies, showing that detector placements at specific latitudes can boost sensitivity by over an order of magnitude.
• The paper reinterprets existing axion limits for dark photons, highlighting unique interaction challenges and pathways to enhance dark matter detection.

Essay on "Dark Photon Limits: A Handbook"

The exploration of dark components of the universe, particularly dark matter (DM), has driven considerable interest towards hypothetical particles such as dark photons (DPs). These are postulated to kinetically mix with standard model (SM) photons, providing a mechanism through which they may be detected. The paper "Dark photon limits: a handbook" delivers a comprehensive examination of the experimental, cosmological, and theoretical limits imposed on DPs and their role as potential DM candidates.

In this work, Caputo et al. scrutinize the characteristics of DPs as massive particles with intrinsic polarization properties that interact weakly with visible photons. The focus is on small kinetic mixing, which allows DPs to evade stringent cosmological bounds, making them viable as a DM constituent. Notably, this paper confronts the challenge that arises due to the polarization of DPs and the Earth's rotation, which affects detection probabilities depending on the experimental setup, location, and DP production mechanisms. Accounting correctly for these factors, notably polarization, could enhance the sensitivity of experiments by over an order of magnitude.

The manuscript meticulously outlines the theoretical framework underpinning DP models, highlighting that many of the observational constraints on DPs are reinterpretations of axion limits, as these particles often produce similar electromagnetic signals. However, this reinterpretation is non-trivial due to the considerable differences in how DPs and axions interact with experimental setups. DPs have an intrinsic polarization, unlike axions, whose electromagnetic signals depend on an applied magnetic field.

An array of experimental constraints has been meticulously compiled in the paper, offering a detailed map of existing DP limits. These constraints stem from diverse sources, including Coulomb law tests, light-shining-through-wall (LSW) experiments, and direct detection experiments like the Axion Dark Matter eXperiment (ADMX) that have also been used to constrain DPs.

A significant contribution of this work is the detailed analysis of optimization strategies for DP detection experiments. The authors suggest that experimental setups at certain latitudes (±35° and ±55°) yield optimal sensitivity. This is critical as it harnesses the rotation of the Earth to maximize the orientation of detector sensitivities relative to the DP polarization.

The outlined strategies for optimizing DP search experiments aim to address constraints in scenarios of unknown physical parameters, offering a structured tactic to ensure robustness against the DP polarization state. This approach seeks to exploit intrinsic polarization to determine properties of particle DM and suggests that careful measurement scheduling could improve effective sensitivity.

In terms of implications, the research offers both theoretical and practical advancement. Theoretically, it delineates the complex interactions and characteristics of DPs, establishing a framework for addressing the considerable variability in experimental DP limits. Practically, it provides concrete recommendations for optimizing experimental setups, potentially bridging gaps between detection capabilities and theoretical predictions.

Future prospects in AI and experimentations could leverage the insights from this paper to devise automated systems that dynamically adjust experiment configurations, improving the real-time analysis and interpretation of DP signals. Additionally, integrating AI can enhance the process of sifting through large datasets to identify signatures consistent with intermittent or weak DP signals influenced by Earth's rotation.

The paper encapsulates a thorough evaluation of DP characteristics, constraints, and detection strategies, making it a pivotal resource for furthering the understanding and exploration of dark photons as DM constituents. While comprehensive, the findings underscore the necessity of continued refinement in both our experimental approaches and theoretical models to unravel the mysteries surrounding DPs and their place in the cosmic tapestry.