Prospects for searching axionlike particle dark matter with dipole, toroidal, and wiggler magnets

In this work, we consider searches for dark matter made of axions or axionlike particles using resonant radio frequency cavities inserted into dipole magnets from particle accelerators, wiggler magnets developed for accelerator based advanced light sources, and toroidal magnets similar to those used in particle-physics detectors. We investigate the expected sensitivity of such axionlike-particle dark-matter detectors and discuss the engineering aspects of building and tuning them. Brief mention is also made of even stronger field magnets which are becoming available due to improvements in magnetic technology. It is concluded that new experiments utilizing already-existing magnets could greatly enlarge the mass region in searches for axionlike dark matter particles.

Figure 1

Existing bounds for axionlike particles [15, 16, 17, 18, 19, 39, 40]. Compilation adapted from Ref. 41. The true QCD-axion region is marked as the hatched band. In the orange region, axions are a natural candidate for dark matter. In the lighter shaded orange area, axions can still be dark matter, but with decreasing mass, this requires an increasing amount of fine-tuning. Axionlike particles can be dark matter in a large part of the so-far untested parameter space (cf. Refs. 12, 13). The right green (red) regions are the conservatively (realistically) expected sensitivity of a dipole or wiggler search for axionlike particle dark matter. The more solidly shaded part in the middle of this region corresponds to the region probed by a single cavity with parameters as discussed in Sec. 2a. The left green/red region shows the masses and couplings that could be probed in a setup with a toroidal magnet.

Figure 2

Electric field distribution of the ${\mathrm{TE}}_{105}$ mode in a rectangular waveguide.

Figure 3

Left: geometric form factor according to Eq. (2.6) as a function of $\Delta w$. Right: snapshot of the electric field in the cavity, showing mode localization for $\Delta w=0.14\mathrm{mm}$. The dimensions of the cavity are $w=30$, $h=20$, $l=1500\mathrm{mm}$. Note that the field plots are not to scale.

Figure 4

Cross section of the waveguide with dielectric tuning rods on each side. The relative electric field intensity in $y$ direction (parallel to the static magnetic field of the external magnet) is shown in color code ($\mathrm{\text{blue}}=\mathrm{\text{negative}}$, $\mathrm{\text{green}}=\mathrm{\text{neutral}}$, $\mathrm{\text{red}}=\mathrm{\text{positive}}$).

Figure 5

Result of a numerical simulation of the tunable waveguide structure. From left to right: resonant frequency, unloaded $Q$ factor, and the geometric $C$ factor are plotted vs the insertion depth of the dielectric rods.

Figure 6

Twisted waveguide section (90°). A structure consisting of several straight sections connected by 180° twists is predicted to have a high geometric factor close to $C\approx 0.6$ in a dipole magnet, while avoiding many engineering challenges of other setups.