Searching for axion and dark photon dark matter using dielectric haloscopes

Abstract

The axion and dark photon are two popular dark matter candidates with expected masses below 1 eV. The axion was originally predicted by a solution to the strong CP problem, that is, the unexpected absence of charge-parity (CP) violating effects in quantum chromodynamics (QCD). Its small coupling to the Standard Model and simple production mechanism sparked a strong interest in the axion as a dark matter candidate and various experiments started searching for it. An example are haloscope experiments that aim to detect axions from the local dark matter halo. They employ highly resonant microwave cavities that enhance the conversion of the axion to the photon in external magnetic fields. These experiments are also sensitive to the dark photon, which mixes with the Standard Model (SM) photon with the practical advantage of not requiring a magnet. However, their sensitivity decreases significantly at masses above 40 µeV.

Therefore, the MAgnetized Disk and Mirror Axion eXperiment (MADMAX) was designed to search for axions in the mass range of 40 µeV to 400 µeV. It uses the dielectric haloscope concept that, instead of a cavity, employs a booster consisting of a stack of dielectric disks and a mirror to enhance the axion-photon conversion. By moving the disks, the axion mass to which the booster is sensitive can be tuned. MADMAX has developed several prototypes categorized as open boosters, which have open boundaries around the disks and mirror, and closed boosters, which are encased in a metal cylinder. To enable free movement of the disks, MADMAX aims at employing an open design for the full-scale setup. The closed booster design allows for easier modeling.

Determining the sensitivity of a dielectric haloscope presents a major challenge. No axion-like calibration source exists and the large detector size compared to the photon wavelength λ of O(104λ3) renders full-wave simulations difficult. Therefore, a single-mode booster model is presented that is capable of replacing time-consuming and computationally expensive full-wave simulations of closed booster systems. Additionally, connecting a receiver system to the dielectric haloscope modifies its electromagnetic response by reflecting part of the emitted signal power. The change is quantified using a model of the noise emitted by the receiver, which is shown to reproduce power spectra of the dielectric haloscope. The presented methods enabled the first dark matter searches by the MADMAX collaboration.

An axion search with a closed booster prototype, encased in an aluminum cylinder, placed limits on the axion in the mass range 76.56 µeV to 76.82 µeV and 79.31 µeV to 79.53 µeV down to an axion-photon coupling of gaγ ≃2×10−11 GeV−1. The receiver system caused a decrease of the expected axion signal power by up to 20 %. The full procedure to determine the experiment’s sensitivity, using the booster and receiver noise model, is presented. The limit is world-leading within its mass range, demonstrating the viability and competitive sensitivity of dielectric haloscopes.

A dark photon search using an open booster prototype, with free-standing disks with open boundaries, placed limits on the dark photon mass range of 78.62 µeV to 83.95 µeV. The peak sensitivity to the kinetic mixing of the dark photon with the SM photon of χ ≃2 × 10−13 surpasses previous limits by up to three orders of magnitude. Its sensitivity in absence of the receiver system was determined as part of the PhD thesis by J. Egge. Using the receiver noise model, the receiver system was found to increase the expected dark photon signal power by up to 25 %, demonstrating that the noise model is applicable to a broad range of configurations.

These dark matter searches provide a solid foundation for future MADMAX experiments. With the necessary tools in place, efforts can now focus on enhancing the experiment’s sensitivity and extending its reach to a broader mass range.