Modulation sensitive search for nonvirialized dark-matter axions

Nonvirialized dark-matter axions may be present in the Milky Way halo in the form of low-velocity-dispersion flows. The Axion Dark-Matter eXperiment performed a search for the conversion of these axions into microwave photons using a resonant cavity immersed in a strong, static magnetic field. The spread of photon energy in these measurements was measured at spectral resolutions of the order of 1 Hz and below. If the energy variation were this small, the frequency modulation of any real axion signal due to the orbital and rotational motion of Earth would become non-negligible. Conservative estimates of the expected signal modulation were made and used as a guide for the search procedure. The photon frequencies covered by this search are 812–852 and 858–892 MHz, which correspond to an axion mass of 3.36–3.52 and $3.55–3.69\mu \mathrm{eV}$. No axion signal was found, and limits were placed on the maximum local density of nonvirialized axions of these masses.

Figure 1

A schematic of the experimental apparatus for ADMX, showing the cavity, receiver chain, and ancillary electronics. During each measurement, switches A and B are arranged as shown. However between measurements and when performing diagnostics, power may be sent to either the major or minor port of the cavity via adjustment of switch A, while power may be read out by a network analyzer prior to mixing via adjustment of switch B.

Figure 2

Power distribution for $n=1$ using a subset of the HR data consisting of 200 spectra.

Figure 3

Power distribution for $n=2$ using the same subset of data shown in Fig. 2.

Figure 4

Power distribution for $n=4$ using the same subset of data shown in Fig. 2.

Figure 5

Power distribution for $n=13$ using the same subset of data shown in Fig. 2.

Figure 6

Power distribution for $n=26$ using the same subset of data shown in Fig. 2.

Figure 7

A typical example of a power spectrum from the HR channel. The roll-off at the edges is due to the bandwidth of the HR spectrum being larger than that of the crystal filter. The waviness in the central portion of the spectrum stems directly from the spectral shape of the crystal filter. A reference spectrum showing this shape can be seen in Fig. 8.

Figure 8

A reference spectrum showing the spectral shape of the crystal filter. This reference was created by averaging about 10 000 individual spectra which cover a range of frequencies.

Figure 9

A polynomial of degree 9 is fit to a cropped coarse resolution spectrum following the removal of the crystal filter shape. This fit is the receiver response function which is used to remove any residual shape from the truncated spectrum.

Figure 10

A strong external radio signal can be seen in the latter half of this spectrum. Such a signal would dwarf any axion signal rendering the spectrum useless.

Figure 11

The radio signal shown in Fig. 10 is identified during processing by its $n=1$ power distribution. At high powers, this distribution exhibits large deviations from the exponential behavior.

Figure 12

Exclusion limits (90% C.L.) placed on the local density of nonvirialized axions at each resolution. Densities for KSVZ axions are shown on the left axis while those for DFSZ axions are shown on the right. The density limits shown from 853 through 857 MHz are the limits published in [28] (shown in gray), which have a resolution of 10.8 Hz in each plot.