Sensitivity of a closed dielectric haloscope to axion dark matter

Abstract

We present a method to determine the sensitivity of a closed dielectric haloscope to axion dark matter. Dielectric haloscopes aim to probe the theoretically well-motivated axion mass range of ~26 $\mathrmμ$eV to ~500 $\mathrmμ$eV by utilizing a stack of dielectric disks and a mirror to enhance the axion-photon conversion within an external magnetic field. Their conversion volume is nearly axion-mass independent, thereby favoring large-scale designs to increase sensitivity. The large volume causes simulations to be computationally expensive and time-consuming. This paper presents a simple model that can be used to determine the sensitivity of the experiment with minimal computational resources. The model is able to describe the electromagnetic response of a closed dielectric haloscope, accounting for realistic geometric imperfections, as well as the noise introduced by the receiver system. It is applied to data taken with a MAgnetized Disk and Mirror Axion Experiment (MADMAX) prototype within the 1.6 T Morpurgo magnet at CERN. This work underpins the first axion dark matter search using a dielectric haloscope and provides the foundation for future dark matter searches with MADMAX.

Figures (18)

• Figure 1: Exploded schematic view of the closed dielectric booster CB200 and the receiver chain considered in this paper. In addition to the disks and mirror, placed in an aluminum casing, it includes a taper and a dielectric lens designed to maximize coupling of the axion signal power to the low-noise receiver chain. The tuning rod allows to apply pressure to the mirror, therefore slightly offsetting its position. The shaded region indicates the components exposed to the magnetic field $\mathbf{B}_e$. The setup is identical to the latest axion search performed at CERN's Morpurgo magnet and the figure taken from its publication ary_dos_santos_garcia_first_2025.
• Figure 2: Transmission line configuration used to model the booster. It consists of four air and three disk regions of respective lenghts $d_i$ and $d_\text{disk}$, defined by their corresponding impedances $Z_{a}$ and $Z_{d}$ as well as propagation constants $\gamma_a$ and $\gamma_d$. The leftmost air region also includes the length of the taper $L$. It transitions from $Z_a$ to the coaxial line impedance $Z_l = 50Ω$. The mirror is modeled by its reflectivity $\Gamma_m$ given by its finite conductivity. The reflectivity measurement is modeled at the reference plane indicated by the dashed line, which corresponds to the connection of the VNA to the taper, the booster input.
• Figure 3: Measured (black) and modeled (green) reflectivity of CB200, run 1.1 (see \ref{['tab:booster_model']}), after parameter fit. Top curves: reflected power $|\Gamma|^2$. Bottom curves: group delay $\tau_\text{gd}$. The model curve shows the best fit and the band the $1\sigma$ fit uncertainty within the frequency range of the booster mode. The TE$_{11}$ resonance, that is the booster mode, is visible as a dip in reflected power and a peak in group delay. Fit parameters are listed in \ref{['tab:booster_model']} and fixed parameters in \ref{['tab:fixed-params']} in the \ref{['sec:params']} .
• Figure 4: Schematic of the circuit used to model LNA noise. It consists of a transmission line of impedance $Z_l = 50Ω$ terminated by the LNA impedance $Z_\text{LNA}$ on one side and a device-under-test (DUT) such as the booster on the other side. The LNA noise is introduced by a voltage and current noise source connected in parallel. The dashed line represents the LNA input port.
• Figure 5: Measured and simulated noise spectra $T_\text{sys}$ of the amplifier connected to open, short and matched load standards. The bands around the curves show their associated uncertainty, stemming from the power calibration of the data. The load (blue) is close to constant over the frequency range, while the open (green) and short (orange) measurements exhibit an oscillation of opposite phase. The simulation matches the measurements within uncertainties.