New Observables for Direct Detection of Axion Dark Matter

TL;DR

The paper addresses direct detection of ultralight axion/ALP dark matter by exploiting time-varying observables generated by their classical field, rather than single-particle scattering. It introduces several couplings—most notably the axion-EDM coupling, as well as axion-nucleon and axion-electron couplings—that produce oscillating moments at frequency $m_a$ and generate spin-precession signals detectable by resonant NMR-like techniques. It provides quantitative expressions for the oscillating nucleon EDM $d_n = g_d a$ and axial-spin interactions, analyzes current astrophysical and laboratory bounds, and outlines practical detection strategies with realistic magnetometer sensitivities, showing potential to probe parameter space beyond existing limits and to reach high $f_a$ regions for the QCD axion. The work also emphasizes the coherent, directional nature of these signals, offering the possibility of mapping the local dark-matter wind and velocity distribution. Overall, it proposes a new class of laboratory observables for axion/ALP DM that could significantly expand the accessible parameter space and complement photon-coupling searches like ADMX.

Abstract

We propose new signals for the direct detection of ultralight dark matter such as the axion. Axion or axion like particle (ALP) dark matter may be thought of as a background, classical field. We consider couplings for this field which give rise to observable effects including a nuclear electric dipole moment, and axial nucleon and electron moments. These moments oscillate rapidly with frequencies accessible in the laboratory, ~ kHz to GHz, given by the dark matter mass. Thus, in contrast to WIMP detection, instead of searching for the hard scattering of a single dark matter particle, we are searching for the coherent effects of the entire classical dark matter field. We calculate current bounds on such time varying moments and consider a technique utilizing NMR methods to search for the induced spin precession. The parameter space probed by these techniques is well beyond current astrophysical limits and significantly extends laboratory probes. Spin precession is one way to search for these ultralight particles, but there may well be many new types of experiments that can search for dark matter using such time-varying moments.

Figures (5)

• Figure 1: Reproduced with permission from Fig. 2 of A. Ringwald Ringwald:2012hr, this figure is adapted from Hewett:2012nsArias:2012azCadamuro:2011fd (see also Ringwald:2012cu). ALP parameter space in axion-photon coupling (as in Eq. \ref{['eqn: axion photon coupling']}) vs mass of ALP. The QCD axion is the yellow band. The width of the yellow band gives an indication of the model-dependence in this coupling, though the coupling can even be tuned to zero.
• Figure 2: ALP parameter space in EDM coupling Eq. \ref{['eqn: axion EDM coupling']} vs mass of ALP. The green region is excluded by excess cooling in SN1987A. The blue regions are excluded by the best static nucleon EDM experiments. The purple band is the QCD axion region, with dark purple showing the most theoretically-motivated region for QCD axion dark matter. The width of the band shows the uncertainty in the calculation of the axion-induced EDM and the axion mass. The ADMX region shows the part of QCD axion parameter space which has been covered (darker blue) Asztalos:2009yp or will be covered in the near future (lighter blue) ADMXwebpagesnowdarktalk by ADMX. For the static EDM and ADMX bounds we assume that the ALP makes up all of the dark matter. See also Figure 2 of NMR paper for sensitivity of the proposed NMR experiment.
• Figure 3: Geometry of the experiment, adapted from NMR paper. The applied magnetic field $\vec{B}_\text{ext}$ is collinear with the sample magnetization $\vec{M}$. The relative velocity $\vec{v}$ between the sample and the dark matter ALP field is in any direction that is not collinear with $\vec{M}$. The SQUID pickup loop is arranged to measure the transverse magnetization of the sample.
• Figure 4: ALP parameter space in pseudoscalar coupling of axion to nucleons Eqn. \ref{['eqn:gaNN']} vs mass of ALP. The purple line is the region in which the QCD axion may lie. The width of the purple band gives an approximation to the axion model-dependence in this coupling. The darker purple portion of the line shows the region in which the QCD axion could be all of the dark matter and have $f_a < M_\text{pl}$ as in Figure \ref{['Fig:EDM']}. The green region is excluded by SN1987A from Raffelt:2006cw. The blue region is excluded by searches for new spin dependent forces between nuclei Vasilakis:2008yn. The red line is the preliminary sensitivity of an NMR style experiment using Xe, the blue line is the sensitivity using $^3\text{He}$. The dashed lines show the limit from magnetization noise for each sample. These lines assume the parameters in Table \ref{['Tab: experiments']}. The ADMX region shows the part of QCD axion parameter space which has been covered (darker blue) Asztalos:2009yp or will be covered in the near future (lighter blue) ADMXwebpagesnowdarktalk by ADMX.
• Figure 5: ALP parameter space in pseudoscalar coupling of axion to electrons Eqn. \ref{['eqn:gaee']} vs mass of ALP. The green region is excluded by White Dwarf cooling rates from Raffelt:2006cw. The blue region is excluded by searches for new spin dependent forces between electrons Dobrescu:2006auelectronspin. The region below the solid purple line shows the possible parameter space for a QCD axion, with the region bounded by darker purple lines being the region where the QCD axion could be all of dark matter and have $f_a < M_\text{pl}$. The frequency range of the QCD axion covered by ADMX is identical to the range plotted in Figure \ref{['Fig:Nucleon']}.