Search for Low-Mass Axion Dark Matter with ABRACADABRA-10 cm

Two of the most pressing questions in physics are the microscopic nature of the dark matter that comprises 84% of the mass in the Universe and the absence of a neutron electric dipole moment. These questions would be resolved by the existence of a hypothetical particle known as the quantum chromodynamics (QCD) axion. In this work, we probe the hypothesis that axions constitute dark matter, using the ABRACADABRA-10 cm experiment in a broadband configuration, with world-leading sensitivity. We find no significant evidence for axions, and we present 95% upper limits on the axion-photon coupling down to the world-leading level $g_{a\gamma \gamma }<3.2\times {10}^{-11}{\mathrm{GeV}}^{-1}$, representing one of the most sensitive searches for axions in the 0.41–8.27 neV mass range. Our work paves a direct path for future experiments capable of confirming or excluding the hypothesis that dark matter is a QCD axion in the mass range motivated by string theory and grand unified theories.

Figure 1

Top: schematic of ABRACADABRA-10 cm showing the effective axion-induced current (blue), sourced by the toroidal magnetic field, generating a magnetic flux (magenta) through the pickup cylinder (green) in the toroid bore. Bottom: simplified schematic of the ABRACADABRA-10 cm readout (full circuit diagram in Supplemental Fig. S1 [30]). The pickup cylinder $L_p$ is inductively coupled to the axion effective current ${\mathbf{J}}_{\mathrm{eff}}$. The power spectrum of the induced current is read out through a dc SQUID inductively coupled to the circuit through $L_{\mathrm{in}}$. An axion signal would appear as excess power above the noise floor at a frequency corresponding to the axion mass.

Figure 2

The gain shown here is defined as the change in amplifier output voltage over a corresponding change in input flux amplitude on the pickup cylinder ($\partial V_{\mathrm{out}}/\partial {\mathrm{\Phi }}_a$). Both transfer functions roll off at high frequencies due to the amplifier bandwidth, which we estimate to have a cutoff frequency of approximately 1 MHz. We believe the difference in calculated and measured gain is due to inconsistency in the total inductance of the pickup circuit.

Figure 3

The survival function of TS values from the likelihood analysis of the run 3 results. The $y$ axis indicates the fraction of mass points tested with a discovery TS at or above the value on the $x$ axis. Under the null hypothesis, the distribution should follow the survival function of the one-sided ${\chi }^2$ distribution with one degree of freedom (“expected,” dotted gray line). This is indeed the case after data cleaning for, e.g., single-channel excesses in time slices, magnet-off vetoes, and the inclusion of a systematic nuisance parameter, which is tuned in a sliding window at $4\sigma$ local significance to give the correct number of excesses at or above that significance, masking the signal of interest. No excesses are found beyond our indicated $5\sigma$ look-elsewhere effect-corrected discovery threshold.

Figure 4

Left: The one-sided 95% upper limit (U.L.) on $g_{a\gamma \gamma }$ from this work excludes previously unexplored regions of ADM parameter space. The $1\sigma$ and $2\sigma$ containment regions are constructed by taking the appropriate percentiles of the distributions of the limits over narrow mass ranges; note that this means that $\sim 16\%$ of the upper limits lie at the bottom of the green band. Around 11.1 million mass points are analyzed, so the plotted data are smoothed for clarity. Our limits surpass those from a number of indicated astrophysical and laboratory searches in this mass range (see the text for details). Right: the unsmoothed limit in a narrow mass range between 2.997 90 and 2.997 98 neV. This provides a detailed view of variations in the limit at each axion mass that arise from statistical fluctuations across the collected data that are not visible in the smoothed data shown in the left plot. This range also depicts the location where our maximum sensitivity is achieved, with our strongest limit at $g_{a\gamma \gamma }\lesssim 3.2\times {10}^{-11}{\mathrm{GeV}}^{-1}$.