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

This paper proposes that next-generation liquid noble gas neutrino experiments could detect axions produced by axion antiquark nuggets ($\rm{A\bar{Q}Ns}$) captured in the Earth, offering a novel method to simultaneously verify these macroscopic dark matter candidates and explain the universe's matter-antimatter asymmetry.

The Gist

The Big Idea: Hunting for "Cosmic Snowballs" that Glow with Invisible Light

Imagine the universe is filled with a mysterious, invisible substance called Dark Matter. For decades, scientists have been looking for it like detectives searching for a ghost. Most theories suggest it's made of tiny, ghostly particles (like WIMPs) that rarely bump into anything.

But this paper proposes a different theory: What if Dark Matter isn't made of tiny ghosts, but of giant, heavy snowballs?

These "snowballs" are called Axion Quark Nuggets (AQNs). They are massive clumps of matter, roughly the size of a grain of sand but weighing as much as a small car. They are so dense that a single one could contain more mass than a mountain, packed into a space smaller than a human hair.

The Story: The Earth as a Cosmic Trap

1. The Capture
Imagine the Earth is a giant magnet moving through a blizzard of these heavy snowballs. Because they are so heavy, the Earth's gravity pulls them in. Over billions of years, these snowballs have been sinking through the Earth's crust and mantle, eventually settling deep in the core, like heavy stones sinking to the bottom of a pond.

2. The Melting (The Problem)
Here is the twist: Some of these snowballs are made of "anti-matter" (let's call them Anti-Snowballs). When an Anti-Snowball hits the regular matter in the Earth's core, they don't just bounce off; they annihilate. It's like mixing a drop of acid with a drop of base—they destroy each other instantly.

3. The Glow (The Solution)
When these Anti-Snowballs crash and burn in the Earth's core, they don't just disappear. The paper suggests this violent crash causes the "skin" of the snowball to vibrate. As it vibrates, it sheds a special kind of invisible light called axions.

Think of it like a drum. If you hit a drum hard (the collision), it vibrates and makes sound waves (the axions). These axions are like invisible sound waves that can travel right through the solid rock of the Earth without getting stuck.

The Detective Work: Listening for the Signal

So, we have invisible axions streaming out of the Earth's core. How do we catch them?

The Problem with Current Detectors
Most current experiments are like trying to hear a whisper in a hurricane. They are looking for axions from space, but the ones coming from the Earth are a different, broader signal that current tools might miss.

The New Idea: The Giant Liquid Bathtub
The authors suggest using the next generation of massive neutrino detectors (like DUNE or XENON). These are huge tanks filled with liquid noble gases (like liquid Argon or Xenon).

• The Analogy: Imagine a giant, super-clear swimming pool filled with liquid Argon.
• The Event: When an axion from the Earth's core hits an atom in this liquid, it gives the atom a tiny "kick."
• The Flash: This kick makes the atom jump excitedly and then settle down, releasing a tiny flash of light (a scintillation).

Because these tanks are so massive (holding thousands of tons of liquid), they act like a giant net. Even though the axions are shy and rarely interact, the sheer size of the net means we might finally catch a few flashes.

Why This Matters

If this works, it solves two massive mysteries at once:

1. What is Dark Matter? It's these giant nuggets.
2. Where is the missing Anti-Matter? The universe was supposed to have equal amounts of matter and anti-matter, but we only see matter. These nuggets might be the "missing" anti-matter, hiding in plain sight inside the Earth.

The Challenge: Finding a Needle in a Haystack

The paper admits this is hard. The flashes of light are incredibly faint. It's like trying to see a single firefly blinking in a stadium full of bright lights (background radiation).

To solve this, the scientists propose using super-sensitive cameras (like SiPMs) and special filters to ignore the "noise" and only look for light coming from below (since the axions are coming from the Earth's core, they would enter the detector from the bottom and move up).

Summary in One Sentence

This paper suggests that if giant, heavy clumps of dark matter are trapped inside the Earth and melting, they are shooting out invisible axions that we might finally catch by watching for tiny flashes of light in massive tanks of liquid gas.

Technical Summary

1. Problem Statement

The paper addresses two major unresolved issues in modern physics:

1. The Nature of Dark Matter (DM): While Weakly Interacting Massive Particles (WIMPs) are the standard candidate, they have not been detected. The authors propose Macroscopic Dark Matter, specifically Axion Quark Nuggets (AQN) and Axion Anti-Quark Nuggets ($\bar{A}QN$), as viable alternatives. These are composite objects with strong interactions, high mass (gram-scale), and microscopic radii (micrometers).
2. Matter-Antimatter Asymmetry: The $\bar{A}QN$ model offers a mechanism to restore matter-antimatter symmetry in the Universe by positing that the observed dark matter density is composed of antimatter nuggets ($\bar{A}QN$) that balance the visible matter baryon asymmetry.

The Specific Challenge: If $\bar{A}QNs$ are captured by the Earth's gravitational field and accumulate in the core, their interaction with terrestrial matter causes baryon annihilation. This process is predicted to emit axions (or Axion-Like Particles, ALPs). The problem is detecting these specific axions, which differ from standard galactic axion searches due to their broad energy spectrum and production mechanism.

2. Methodology

The authors employ a theoretical framework combining particle physics, astrophysics, and detector physics to propose a detection strategy:

• Theoretical Modeling of $\bar{A}QNs$:

  • Structure: Modeled as a core of quarks/antiquarks in a color-superconducting phase (CFL or 2CS phase), surrounded by an electrosphere (positrons for $\bar{A}QN$) and stabilized by an axion domain wall.
  • Earth Capture: The authors estimate the accumulation of $\bar{A}QNs$ in the Earth's core over 4.5 billion years. They assume a baryon number $B \approx 10^{24} - 10^{25}$, leading to masses in the gram range ($\sim 1.6 - 16$ g).
  • Axion Production Mechanism: When $\bar{A}QNs$ interact with Earth's matter, baryon annihilation occurs. This reduces the nugget's mass, causing the stabilizing axion domain wall to oscillate. These oscillations radiate relativistic axions ($v \approx 0.6c$) with a broad energy spectrum.

• Detection Strategy:

  • Target Medium: The authors propose using Liquid Noble Gas detectors (specifically Liquid Argon (LAr) and Xenon-doped LAr), which are the next generation of neutrino experiments (e.g., DUNE, ProtoDUNE).
  • Interaction Channels: The study focuses on the interaction of axions with electrons in the detector via:
    1. Axio-electric effect: $a + (Z, e) \to e + (Z, e)^*$ (dominant for high-Z elements).
    2. Inverse Compton scattering: $a + e \to \gamma + e$.
  • Signal Identification: The detection relies on the scintillation light produced when the axion transfers energy to an electron. The authors analyze the kinematics to ensure the energy transfer is sufficient to produce detectable scintillation in LAr/Xe.

3. Key Contributions

• Novel Detection Channel: The paper proposes that liquid noble gas neutrino detectors (designed for massive active volumes) are uniquely suited to detect axions from Earth-captured $\bar{A}QNs$, a scenario previously unexplored by these specific experiments.
• Mass and Coupling Constraints: The authors identify a specific mass window for ALPs ($m_a \gtrsim 2.15$ keV) where the kinematic energy transfer to electrons in liquid argon is sufficient to trigger scintillation. They argue that for these masses, the axio-electric cross-section is significant enough to be detectable.
• Directionality and Background Rejection: The study suggests a unique signature: upward-moving particles. Since $\bar{A}QNs$ accumulate in the Earth's core, the emitted axions would travel upward through the detector. This allows for background rejection against downward-going cosmic rays and standard radioactive backgrounds.
• Quantitative Event Rate Estimates: The authors provide rough estimates for event rates in a DUNE-like detector (Xe-doped LAr), suggesting that for a single $\bar{A}QN$ with $B=10^{25}$, the number of produced axions could be $\sim 1.4 \times 10^{30}$, potentially yielding $10^3$ to $10^6$ scintillation events depending on the cross-section and detector efficiency.

4. Results and Analysis

• Feasibility of Scintillation: Calculations confirm that for ALP masses $m_a \ge 2.15$ keV, the maximum energy transfer to an electron in a binary collision is sufficient to excite the liquid noble gas atoms, leading to scintillation.
• Cross-Section Analysis:
  • The axio-electric cross-section ($\sigma_{ae}$) scales with $Z^5$, making high-Z elements (like Xenon) or Xenon-doped Argon highly favorable.
  • For $m_a \approx 2.9$ keV, the estimated cross-section in Argon is $\sigma_{ae} \sim 4 \times 10^2$ cm$^2$ (normalized to specific coupling assumptions), which is orders of magnitude higher than standard WIMP interactions, though the flux is low.
• Background Considerations:
  • Cosmogenic Backgrounds: Muons, neutrons, and neutrinos are the primary concerns.
  • Intrinsic Radioactivity: $^{39}$Ar in natural argon is a significant background source ($\beta$-decay).
  • Mitigation: The authors propose using directionality (selecting only upward-moving tracks) and pulse shape discrimination (distinguishing between singlet and triplet states in noble gases) to separate the signal from background. They also highlight the potential of ARAPUCA technology (photon trapping) and digital SiPMs to detect the low number of photons expected from these rare events.

5. Significance

• Dual Solution: If detected, this method would simultaneously confirm the existence of macroscopic dark matter ($\bar{A}QNs$) and provide evidence for the Peccei-Quinn mechanism (axions), solving the Strong CP problem and the Dark Matter mystery in one stroke.
• Repurposing Existing Infrastructure: The study demonstrates that next-generation neutrino experiments (like DUNE), which are already being built with massive active volumes, have the potential to detect axions without requiring new, dedicated hardware, provided the data analysis is adapted to look for upward-moving, low-energy scintillation events.
• New Parameter Space: It opens a new window for exploring ALP parameter space, specifically in the keV-MeV mass range with electron couplings, which is currently less constrained than the photon coupling sector.

In conclusion, the paper presents a compelling theoretical case that the Earth acts as a natural trap for macroscopic dark matter, which subsequently acts as a source of detectable axions. It argues that the next generation of liquid noble gas detectors is the ideal instrument to test this hypothesis.