Listening for ultra-heavy dark matter with underwater acoustic detectors

TL;DR

This work proposes large underwater acoustic detectors as a complementary method to search for ultra-heavy dark matter by detecting thermo-acoustic pulses generated as DM traverses seawater. It develops a first-principles model of energy deposition along a DM track, solves the corresponding acoustic wave equation, and incorporates frequency-dependent attenuation in both pure water and seawater, revealing significant amplitude loss and pulse broadening. Through a detailed sensitivity analysis for a ~100 km^3 hydrophone array, the paper shows potential reach in the $m_\chi\sim10^{-3}$ g and $\sigma_\chi\sim10^{-8}$ cm^2 regime, with both spin-independent and spin-dependent couplings accessible, and emphasizes the feasibility of repurposing existing hydrophone data. The results underscore the method’s complementarity to traditional searches and its potential to constrain previously unexplored ultra-heavy DM parameter space.

Abstract

Ultra-heavy dark matter candidates evade traditional direct detection experiments due to their low particle flux. We explore the potential of large underwater acoustic arrays, originally developed for ultra-high energy neutrino detection, to detect ultra-heavy dark matter interactions. These particles deposit energy via nuclear scattering while traversing seawater, generating thermo-acoustic waves detectable by hydrophones. We present the first robust first-principles calculation of dark matter-induced acoustic waves, establishing a theoretical framework for signal modelling and sensitivity estimates. Our framework incorporates frequency-dependent attenuation effects, including viscous and chemical relaxation, not considered in previous calculations. A sensitivity analysis for a hypothetical 100 cubic kilometre hydrophone array in the Mediterranean Sea demonstrates that such an array could extend sensitivity to the previously unexplored mass range of $0.1$-$10\,μ\mathrm{g}$ ($\sim10^{20}$-$10^{23}\,\mathrm{GeV}$), with sensitivity to both spin-independent and spin-dependent interactions. Our results establish acoustic detection as a complementary dark matter search method, enabling searches in existing hydrophone data and informing future detector designs.

Figures (12)

• Figure 1: Acoustic detection of thermo-acoustic waves generated when ultra-heavy dark matter scatters with oxygen and hydrogen nuclei in water. The red region indicates the primary energy deposition from dark matter, which create acoustic waves that propagate outwards as cylindrical waves to an array of hydrophones (underwater pressure wave detectors) spanning a volume on the order of $100 \,\text{km}^3$. Here, $\rho$ is the radial distance from the dark matter track to a hydrophone, $z$ is aligned along the centre of the energy deposition
• Figure 2: Expected acoustic pressure without attenuation from ultra-heavy dark matter with $m_\chi = 10^{-2}\,\mathrm{g}$ and $\sigma_\chi = 10^{-10}\,\mathrm{cm}^2$ at a radial distance $\rho = 300\,\mathrm{m}$. We show contributions from oxygen (blue) and hydrogen (red) separately. The full solution is the sum of these signals. The oxygen signal has a greater amplitude and is broader because $dE_A/dz$ and $\sigma_A$ are both larger, relative to hydrogen.
• Figure 3: Absolute magnitude of frequency-domain acoustic pressure solutions without attenuation from ultra-heavy dark matter with $m_\chi = 10^{-2}\,\mathrm{g}$ and $\sigma_\chi = 10^{-10} \, \mathrm{cm}^2$ at a radial distance $\rho = 300\,\mathrm{m}$. We show contributions from oxygen (blue) and hydrogen (red) separately. Dotted lines labelled 'Hankel' show an analytic approximation, which provides a good match for $\omega\ll c_s/\sigma_A$.
• Figure 4: Absolute magnitude of frequency-domain acoustic pressure in seawater (solid lines) and pure water (dotted lines) from ultra-heavy dark matter with $m_\chi = 10^{-2}\,\mathrm{g}$ and $\sigma_\chi = 10^{-10} \, \mathrm{cm}^2$ at radial distances of $\rho = 300\,\mathrm{m}$ (blue) and $\rho = 1\,\mathrm{km}$ (pink). The absorption coefficient introduces distance-dependent frequency cut-offs, most prominently in seawater, leading to significant suppression at high frequencies compared to the unattenuated case in fig. \ref{['fig:PressureSourceFreq']}.
• Figure 5: Expected acoustic pressure in seawater from ultra-heavy dark matter with $m_\chi = 10^{-2}\,\mathrm{g}$ and $\sigma_\chi = 10^{-10}\,\mathrm{cm}^2$ at radial distances of $\rho = 300\,\mathrm{m}$ (blue) and $\rho = 1\,\mathrm{km}$ (pink, multiplied by $10$ for clarity). Compared to the unattenuated pulse in fig. \ref{['fig:PressureSourceTime']}, attenuation causes significant amplitude reduction and temporal broadening. The signal experiences a small delay in arrival time, which is most pronounced for the $\rho = 1\,\mathrm{km}$ pulse, due to dispersion from the absorption coefficient's imaginary component.