Dark photon limits: a handbook

TL;DR

This paper provides a comprehensive guide on optimizing dark photon searches by accounting for their intrinsic polarisation and Earth's rotation, significantly improving detection sensitivity and clarifying reinterpretation of axion experiments.

Contribution

It introduces strategies to enhance dark photon detection sensitivity by considering polarisation effects and Earth's rotation, and clarifies limitations of reinterpreting axion search results.

Findings

Properly accounting for dark photon polarisation can increase discovery reach by over an order of magnitude.

Optimal experiment placement and timing can make searches insensitive to dark photon production mechanisms.

Many axion search techniques are unsuitable for dark photon exclusion limits.

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.