A New Way to Detect Axions from $\rm{A\bar{Q}Ns}$ Captured in the Earth

Abstract: Macroscopic dark matter with dominating strong interactions, supposed to be composites, represents an alternative to the most popular WIMP particles. Predicted in various models as strangelets, nuclearites, nuggets, having different internal structures and properties, but not yet observed experimentally, these forms of dark matter are associated with the existence of a large number of still unexplained observations. Nuggets, initially predicted by Witten, were reconsidered from the point of view of their internal structure and further theorized in 2003 by Zhitnitsky as axion quark nuggets and axion antiquark nuggets, as being made of quarks in a superconducting colour state, in the core, an electrosphere of electrons or positrons and a domain wall that maintain the stability of the macros with an incredible density, mass in the gram range and radius on the order of micrometers. If the existence of $\rm{AQNs}$ and $\rm{A\bar{Q}Ns}$ is demonstrated, two major open problems in physics could be addressed simultaneously: they would constitute viable dark matter candidates and, at the same time, provide a natural mechanism for restoring matter-antimatter symmetry in the Universe. The experimental evidence of the $\rm{AQNs}$ and $\rm{A\bar{Q}Ns}$ is a challenge for current and future experiments. The present study demonstrates that if these macroscopic systems exist, axions produced by $\rm{A\bar{Q}N}$s could be detected by the next generation of neutrino physics experiments using liquid noble gases, due to their huge active volumes.

• The paper introduces a novel model where axion emissions from AQNs captured by Earth can provide detectable signals through liquid noble gas experiments.
• It details the capture dynamics and domain wall oscillation mechanisms that drive axion production, presenting mass-dependent yield predictions.
• The study proposes experimental strategies using the axio-electric effect and pulse shape discrimination to isolate rare axion events from background noise.

Axion Emission from Captured Axion Quark Nuggets in the Earth: Detection Prospects in Liquid Noble Gas Experiments

Introduction

This paper presents an in-depth theoretical investigation into the phenomenology of axion quark nuggets (AQNs) and their antimatter counterparts ($\rm{A\bar{Q}Ns}$), examining their potential as macroscopic dark matter (DM) candidates and the prospects for detecting axions emitted from $\rm{A\bar{Q}Ns}$ captured in the Earth. The study revisits the internal structure, capture dynamics, and axion emission mechanisms of these objects, with particular attention to their detectability in current and next-generation large-volume liquid noble gas detectors.

Axion Quark Nuggets and Dark Matter

Macroscopic DM Candidates

The work situates AQNs and $\rm{A\bar{Q}Ns}$ within the broader context of macroscopic DM candidates (so-called “macros”), which are poised as alternatives to weakly interacting massive particles (WIMPs). These objects are hypothesized to possess baryonic (or anti-baryonic) content bound in color superconducting phases, stabilized by axion domain walls. Key properties include:

• Masses in the gram-scale range and radii on the order of micrometers
• Ultra-high densities ($\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$)
• An internal structure featuring a color superconducting core surrounded by an electrosphere of electrons or positrons
• Stability conferred by axion domain walls, which shield against rapid baryon decay

AQNs offer interest not only as DM candidates but also as engines for matter-antimatter asymmetry restoration, positing that the observed baryon-to-photon ratio could arise from an initial zero baryon net charge universe, partitioned between visible matter and macroscopically sequestered anti-baryons in AQ$\bar{\text{N}}$s.

Capture in the Earth

The capture of AQ$\bar{\text{Q}}$Ns in the Earth is considered under the premise that repeated interactions and Earth's gravity over 4.5 Gyr could concentrate a significant population in the terrestrial core. The analysis incorporates:

• Total captured DM estimate: $$M_{\rm capt}^{\oplus} \sim 4 \times 10^{18}~\mathrm{g}$$
• Typical AQN characteristics assumed: $B=10^{24}$ (corresponding to $\sim 1.6$ g per object)
• Number of nugget objects in the core: $\sim 2.5 \times 10^{18}$

Contrary to some prior literature suggesting only light AQ$\rm{A\bar{Q}Ns}$0Ns can be efficiently captured and stopped, this work postulates that repeated scatterings and cumulative gravitational focusing ensure a plausible accumulation of even heavier $\rm{A\bar{Q}Ns}$1 in Earth's deep interior.

Axion Production from AQ$\rm{A\bar{Q}Ns}$2N Annihilation

Domain Wall Dynamics and Axion Emission

AQ$\rm{A\bar{Q}Ns}$3Ns are stabilized by axion domain walls. Annihilation of baryon charge (due to interactions with ambient matter) perturbs the equilibrium state, leading to:

• Oscillations in the domain wall, driving emission of relativistic axions
• Domain wall thickness: $\rm{A\bar{Q}Ns}$4 (with $\rm{A\bar{Q}Ns}$5 the axion mass)
• Emission durations: For representative $\rm{A\bar{Q}Ns}$6 and $\rm{A\bar{Q}Ns}$7, emission spans $\rm{A\bar{Q}Ns}$8 s

The model predicts substantial axion yields per AQ$\rm{A\bar{Q}Ns}$9N, strongly dependent on assumptions for the axion mass range.

Numerical Predictions for Axion Yield

Depending on $\rm{A\bar{Q}Ns}$0, the emitted axion number per AQ$\rm{A\bar{Q}Ns}$1N (for $\rm{A\bar{Q}Ns}$2) is:

• $\rm{A\bar{Q}Ns}$3: $\rm{A\bar{Q}Ns}$4–$\rm{A\bar{Q}Ns}$5
• $\rm{A\bar{Q}Ns}$6: $\rm{A\bar{Q}Ns}$7–$\rm{A\bar{Q}Ns}$8

These are substantial fluxes, highlighting the feasibility of indirect detection if appropriate interactions and detection strategies are available.

ALP Interactions and Detection

The analysis focuses on axion-like particles (ALPs) primarily coupled to electrons ($\rm{A\bar{Q}Ns}$9), reviewing existing laboratory and astrophysical bounds:

• Direct detection threshold: Current experiments are sensitive if $\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$0 for $\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$1 in the keV–MeV range

Detection channels considered are:

• Axio-electric effect: $\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$2
• Inverse Compton scattering: $\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$3

For high-$\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$4 elements like Xe (and to some extent Ar), the axio-electric effect benefits from sizable cross-sections ($\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$5).

Detection Strategy Using Liquid Noble Gas Detectors

Detector Suitability

The study proposes that large-scale liquid argon (LAr), xenon (LXe), or Xe-doped LAr TPCs (e.g., DUNE, LUX-ZEPLIN, ProtoDUNE) represent optimal targets for ALP-induced rare event searches. The arguments include:

• Huge target mass and high electron density
• Efficient scintillation (with $\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$6 visible photons/MeV and fast timing)
• Good pulse shape discrimination, allowing separation of heavy-particle and electron/gamma interactions
• Wavelength-shifting and photon-trapping solutions (e.g., ARAPUCA) to enhance detection of low-light events

Signal Characterization and Event Rates

For a reference scenario ($\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$7 AQ$\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$8N, $\sim 3.5 \times 10^{14}~\mathrm{g/cm}^3$9 keV, DUNE module, $\bar{\text{N}}$0):

• Cross-section (axio-electric): $\bar{\text{N}}$1 in Ar
• Number of scintillation events: $\bar{\text{N}}$2–$\bar{\text{N}}$3 per AQ$\bar{\text{N}}$4N

The directionality of events (originating from the core and traversing the detector “upwards”) and high timing resolution provide potential triggers for background rejection.

Backgrounds and Experimental Realism

Detailed attention is given to:

• Radiogenic backgrounds: e.g., $\bar{\text{N}}$5Ar in LAr, radon progeny, cavern walls
• Cosmogenic backgrounds: muons, atmospheric neutrino fluxes
• Detector-specific noise: natural radioactivity, photodetector dark counts

Modern LAr and LXe experiments employ extensive radiopurity and shielding protocols, but the paper highlights:

• Challenges associated with rare-event searches at low energies
• Requirements for high-sensitivity single-photon detection, e.g., with CMOS-integrated SiPMs
• Pulse shape discrimination and the possibility of incorporating Cherenkov-based directionality (in hybrid or water-based detectors)

Advanced and Future Directions

The DUNE, EOS, and THEIA projects are noted as platforms for cross-examination of scintillation versus Cherenkov emission, allowing improved background rejection, and the development of low-threshold, high-mass detectors with enhanced radiopurity as crucial next steps for testing the proposed scenario.

Theoretical and Experimental Implications

The paper provides an explicit pathway connecting non-WIMP DM models with imminent experimental observable signatures, underlining:

• The importance of exploring non-standard DM candidates – macroscopic, strongly interacting, and composite objects – beyond the usual WIMP and axion cold DM paradigms
• A direct connection to observable physics in liquid noble TPCs, provided the axion mass and coupling lie in experimentally accessible regions
• Sensitivity to the properties of axion–electron couplings, positioning these rare-event searches at the intersection of DM and axion physics

Potential future developments include:

• Systematic searches for upgoing, low-energy, low-photon-count events in large LAr/LXe TPCs
• Dedicated low-background modules or subdetectors for enhanced sensitivity in the keV–MeV mass range
• Cross-discipline studies connecting axion phenomenology, matter–antimatter asymmetry resolution, and direct/indirect DM detection

Conclusion

This study rigorously demonstrates that the annihilation of axion anti-quark nuggets captured in the Earth can lead to the emission of relativistic axions with fluxes and energies potentially accessible to large-volume liquid noble gas detectors via the axio-electric effect, especially for $\bar{\text{N}}$6 keV. The scenarios outlined establish a clear experimental target and motivate ongoing and future searches for macroscopic DM candidates using liquid argon and xenon experiments. This avenue offers a viable detection channel for strongly interacting composite dark matter that complements, and is distinct from, traditional WIMP- and galactic halo-axion searches.