Scattering meets absorption in dark matter detection

This paper investigates the interplay between dark matter scattering and light mediator absorption in direct detection experiments for Dirac and atomic dark matter models, identifying viable parameter spaces constrained by cosmology and astrophysics while highlighting the importance of distinguishing these signals to uncover the underlying physics.

The Gist

Imagine the universe is filled with a mysterious, invisible fog called Dark Matter. For decades, scientists have been trying to catch this fog by waiting for it to bump into the atoms in their detectors, like a ghost bumping into a wall.

But this new paper suggests that catching the ghost isn't the only way to find it. The authors, a team of physicists from the Netherlands, propose a new strategy: Don't just wait for the ghost to bump into you; look for the "ghostly wind" blowing through your detector.

Here is the story of their discovery, broken down into simple concepts.

1. The Two Ways to Catch the Invisible

The paper explores two different theories about what Dark Matter is made of. Think of these as two different types of "ghosts."

• The "Dirac" Ghost (The Lone Wolf): In this model, Dark Matter is just a single, heavy particle (like a dark electron). It's lonely and floats around by itself.
• The "Atomic" Ghost (The Family Unit): In this model, Dark Matter is made of two particles stuck together, like a dark proton and a dark electron forming a "dark atom." It's a tiny, invisible family.

2. The Two Signals: The Bump vs. The Wind

The authors realized that if these dark particles interact with our world through a "light mediator" (a force carrier called a Dark Photon), we might see two very different things in our detectors:

• Signal A: The Bump (Scattering)
  Imagine a dark particle flying through your detector and physically hitting an atom, knocking it loose. This is like a billiard ball hitting another ball. This is what most experiments have been looking for.
• Signal B: The Wind (Absorption)
  The Sun is a giant factory that constantly spits out these "Dark Photons." They fly to Earth and get absorbed by the detector, giving it a tiny jolt of energy. This is like feeling a warm breeze on your face. You didn't get hit by a ball; you just felt the wind.

The Big Discovery: The authors show that for the first time, we might be able to see both the "Bump" and the "Wind" at the same time in the same experiment. It's like hearing a knock on the door and feeling a draft at the same time, proving someone is outside.

3. The Rules of the Game (Why it's hard)

Detecting these ghosts is tricky because of a cosmic "traffic rule" called Self-Interaction.

• The Analogy: Imagine a crowd of people (Dark Matter) in a room. If they are too friendly and keep bumping into each other (self-interacting), they clump together and ruin the structure of the room (the universe).
• The Constraint: The universe we see is very orderly. This means Dark Matter particles can't be too friendly. They have to be mostly shy.
• The Result: This shyness limits how strong the "Bump" signal can be. For the "Lone Wolf" model, the Bump might be too faint to see unless we build super-sensitive detectors. However, the "Wind" (Absorption) doesn't care about this shyness. The Sun can still blow that wind even if the particles are shy.

4. The Atomic Twist

The "Atomic" model is even more interesting. Because these dark atoms are made of two parts, they can get "broken up" (ionized) by the heat of the universe.

• This means our detectors might see three things at once:
  1. The whole dark atom bumping into things.
  2. The loose dark electron bumping into things.
  3. The loose dark proton bumping into things.
• It's like a family car crashing into a wall. You might see the whole car, or you might see the driver and the passenger flying out separately. Distinguishing between these "crash patterns" tells us exactly what the dark family looks like.

5. Why This Matters

The authors are telling future scientists: "Don't just look for one thing. Look for the whole package."

• If you see a Bump: You know there is a particle out there.
• If you see a Wind: You know the Sun is making these particles.
• If you see BOTH: You have a smoking gun. You can prove that the particle hitting your detector is the same one being blown by the Sun, and you can measure exactly how heavy it is and how strong its "dark force" is.

The Bottom Line

This paper is a roadmap for the next generation of experiments. It tells us that the universe might be whispering to us in two different languages at once. By learning to listen to both the "Bump" and the "Wind," and by understanding the complex family dynamics of "Atomic Dark Matter," we are much closer to finally solving the mystery of what the dark universe is made of.

In short: We are moving from trying to catch a ghost in a dark room to feeling the draft it leaves behind, and realizing that the ghost might actually be a whole family.

Technical Summary

1. Problem Statement

Direct detection experiments are currently exploring two distinct signatures of dark matter (DM):

1. Scattering: DM particles (fermions or atoms) scattering off electrons or nucleons in the detector.
2. Absorption: The absorption of bosonic mediators (specifically dark photons) produced in the Sun, which deposit energy by ionizing atoms in the detector.

While these signals have been studied independently, it remains unclear if they can occur simultaneously within a single, consistent theoretical model. Furthermore, existing models often ignore the stringent constraints imposed by astrophysics (e.g., stellar cooling, the Bullet Cluster) on the dark sector parameters required to produce observable signals.

The paper addresses the interplay between DM scattering and mediator absorption for two specific models: Dirac Dark Matter and Atomic Dark Matter, focusing on scenarios with light vector mediators (dark photons) in the sub-keV mass range.

2. Methodology

The authors employ a multi-faceted approach combining theoretical modeling, astrophysical constraints, and experimental sensitivity analysis:

• Theoretical Framework:

  • They consider a dark sector with a $U(1)_d$ gauge symmetry and Dirac fermions ($\chi$).
  • The dark photon ($A_d$) acquires mass via the Stueckelberg mechanism or a dark Higgs.
  • Interaction with the Standard Model (SM) occurs via kinetic mixing ($\epsilon$) between the dark photon and the SM hypercharge field.
  • Two models are analyzed:
    1. Dirac DM: The DM candidate is the lightest dark fermion ($\chi = e'$).
    2. Atomic DM: The DM candidate is a bound state ("dark hydrogen," $H'$) of a dark electron ($e'$) and a dark proton ($p'$).

• Constraint Analysis:

  • Astrophysics/Cosmology: The authors apply bounds from:
    • Stellar Cooling: Limits on energy loss in the Sun, red giants, and supernovae (constraining $\epsilon$ and millicharged particles).
    • Bullet Cluster & Small-Scale Structure: Limits on DM self-interaction cross-sections ($\sigma_T/m_{DM}$) at high ($1000$ km/s) and low ($10$ km/s) velocities.
    • CMB/BBN: Constraints on dark radiation and dark acoustic oscillations (DAO).
  • Relic Density: They consider both thermal freeze-out (requiring asymmetries for Atomic DM) and freeze-in mechanisms.

• Direct Detection Formalism:

  • They calculate scattering cross-sections for:
    • Dark fermions off nucleons and electrons (mediated by light dark photons, long-range).
    • Dark atoms off nucleons and electrons (mediated by short-range contact interactions due to charge neutrality).
    • Inelastic scattering (hyperfine transitions) for degenerate mass ratios.
  • They calculate absorption rates for solar dark photons, which depend on the solar flux and the kinetic mixing parameter.
  • They compare these predictions against current experimental limits (XENON1T, PandaX, SENSEI, DarkSide) and forecast future sensitivities (DARWIN, PandaX-30T).

3. Key Contributions

• Simultaneous Signal Prediction: The paper demonstrates that for specific parameter spaces, both scattering and absorption signals are naturally predicted and can be observed simultaneously in the same detector.
• Atomic DM Phenomenology: It provides a comprehensive analysis of Atomic Dark Matter, distinguishing signals from:
  • Neutral dark atoms ($H'$).
  • Ionized dark constituents ($e'$ and $p'$).
  • Solar dark photon absorption.
• Astrophysical Viability: The authors map out the "viable" parameter space where models satisfy all cosmological and astrophysical bounds (particularly self-interaction limits) while remaining accessible to direct detection.
• Discrimination Strategy: They propose methods to distinguish between scattering and absorption signals (e.g., using energy spectra or S1+S2 vs. S2-only signals in xenon detectors).

4. Results

A. Dirac Dark Matter

• Constraints: The dark gauge coupling ($\alpha_d$) is severely constrained by the Bullet Cluster and small-scale structure observations. For the DM to be a sub-dominant component or for specific mass ranges, these constraints relax.
• Scattering vs. Absorption:
  • For DM masses $m_{DM} \gtrsim 6$ GeV, scattering off nucleons and dark photon absorption can be distinguished via the S1+S2 signal (scattering) vs. S2-only (absorption).
  • For lighter masses ($1-6$ GeV), distinguishing the two requires analyzing the energy spectrum.
  • Freeze-in: The paper identifies regions where the correct relic abundance is achieved via freeze-in. In these scenarios, if small-scale structure bounds are strict, scattering rates may be too low for current detection, but absorption remains a viable probe as it is independent of $\alpha_d$.
• Sensitivity: Future experiments could probe the "neutrino fog" for electron scattering, but for nucleon scattering with light mediators, the fog is not yet fully defined.

B. Atomic Dark Matter

• Complexity: The phenomenology is richer due to the coexistence of neutral atoms and ionized constituents in the halo.
• Ionization Fraction ($f_i$): The fraction of ionized dark atoms is constrained to be small ($f_i \lesssim 0.01$) to avoid excessive self-interactions. This suppresses the flux of constituents ($e', p'$) relative to atoms ($H'$).
• Dominant Signals:
  • Heavy DM ($m_{H'} \gtrsim \text{few GeV}$): Scattering of dark atoms off nucleons often dominates the signal rate due to the higher abundance of atoms compared to constituents, despite the constituents having larger cross-sections.
  • Light DM: If constituents are light enough, dark electron/proton scattering can dominate, particularly in low-threshold experiments (SENSEI).
  • Absorption: Solar dark photon absorption is observable across most of the viable parameter space.
• Simultaneous Observation:
  • In specific regions (small mass ratios $R$, specific masses), experiments could observe all four signals simultaneously: dark atom scattering, dark proton scattering, dark electron scattering, and dark photon absorption.
  • Differentiation:
    • Dark Atom Scattering: WIMP-like signal (S1+S2).
    • Constituent Scattering: Light-mediator signal (S2-only for electrons, S1+S2 for protons).
    • Absorption: S2-only signal with a distinct energy spectrum (monochromatic or thermal-like depending on $m_d$).

5. Significance

• Model Discrimination: The ability to observe multiple signal types simultaneously allows for the reconstruction of the underlying dark sector model. For instance, observing both scattering and absorption confirms the mediator nature of the interaction.
• Experimental Strategy: The paper argues that future experiments should not just look for a single "smoking gun" but should analyze the energy distributions and signal topologies (S1 vs. S2) to disentangle overlapping signals.
• Astrophysical Consistency: It highlights that viable DM models must satisfy self-interaction bounds, which often pushes the parameter space toward regions where absorption is the most robust detection channel, even if scattering is suppressed.
• Future Projections: The work sets concrete targets for next-generation experiments (DARWIN, PandaX-30T), suggesting that the "neutrino fog" may be bypassed for absorption searches or that specific mass/coupling regions will be probed where both scattering and absorption are visible.

In conclusion, the paper establishes that scattering and absorption are not mutually exclusive but are complementary probes of light-mediator dark matter models. Distinguishing between them is crucial for identifying the correct dark matter candidate (Dirac vs. Atomic) and understanding the dark sector's structure.