Characterization of argon recoils at the keV scale with ReD and ReD+

The ReD experiment utilized a dual-phase Time Projection Chamber to measure the ionization yield of argon for nuclear recoils between 2 and 10 keV, revealing a higher yield at lower energies that is critical for optimizing the sensitivity of argon-based dark matter detectors to low-mass Weakly Interacting Massive Particles.

The Gist

The Big Picture: Hunting Ghosts in a Jar of Light

Imagine scientists are trying to catch "ghosts." In the world of physics, these ghosts are called Dark Matter. Specifically, they are looking for tiny, heavy particles called WIMPs (Weakly Interacting Massive Particles).

When these ghosts bump into normal matter, they don't leave a footprint or a sound. Instead, they give the atom a tiny, invisible "shove." This shove is called a nuclear recoil.

The problem? The shoves from the lightest ghosts are so tiny that they are harder to detect than a whisper in a hurricane. To hear them, scientists use giant jars filled with liquid argon (a noble gas, like neon, but colder). When a ghost hits an argon atom, the atom gets excited and does two things:

1. It flashes a tiny bit of light (like a firefly blinking).
2. It releases a few electrons (tiny negative charges), which act like a tiny electric spark.

The ReD experiment (Recoil Directionality) is a specialized detector designed to measure exactly how much "electric spark" (ionization) you get for a specific size of "shove" (energy).

The Problem: The "Missing Link"

For a long time, scientists knew how to measure these sparks for big shoves (high energy). But for the tiny shoves caused by light dark matter (low energy, specifically between 2 and 10 keV), they were flying blind.

It was like trying to bake a cake but having no recipe for the amount of sugar needed for the smallest cakes. They had to guess. If they guessed wrong, they might miss the ghosts entirely or think they found them when they didn't.

The Experiment: The "Pinball" Setup

The ReD team built a clever machine to solve this guessing game. Here is how they did it:

1. The Shooter: They used a radioactive source (Californium-252) that acts like a machine gun, shooting out neutrons. Think of these neutrons as tiny, invisible billiard balls.
2. The Target: They shot these neutrons into their small jar of liquid argon.
3. The Collision: Sometimes, a neutron hits an argon atom. The argon atom gets knocked back (recoils), creating the tiny electric spark the scientists want to measure.
4. The Witness: To know exactly how hard the argon was hit, they didn't just watch the argon. They also watched the neutron after it bounced off. They used a ring of detectors (like a camera array) to catch the scattered neutron.

The Analogy: Imagine you are in a dark room. You throw a ball at a glass vase. You can't see the ball hit the vase, but you can hear the ball bounce off and hit a wall. By measuring where and how fast the ball hit the wall, you can calculate exactly how hard it hit the vase. That's what ReD did: they measured the "bounce" to know the "hit."

The Results: The Spark is Brighter Than Expected

The team measured the "ionization yield" (how many electrons come out per unit of energy).

• The Old Guess: Scientists used a formula (the Thomas-Imel model) that predicted the spark would get very dim as the energy got lower.
• The New Discovery: ReD found that at the lowest energies (below 7 keV), the spark is actually brighter than the old formula predicted.

Why this matters: It's like realizing that when you whisper, the microphone picks up the sound better than you thought. This means future dark matter detectors (like the upcoming DarkSide-20k) might be much more sensitive to light dark matter than we previously believed. We might be able to catch ghosts we thought were too quiet to hear!

The Future: ReD+ (The Upgrade)

The paper also talks about ReD+, which is the "Pro" version of this experiment.

• Bigger Jar: They will use a larger detector to catch more events.
• Better Shielding: They will build better walls to stop stray neutrons from causing false alarms.
• New Gun: Eventually, they plan to swap the radioactive source for a Deuterium-Deuterium generator. This is like switching from a scatter-shot shotgun to a laser-guided rifle. It shoots neutrons with a very specific, consistent speed, allowing them to measure even tinier shoves (down to 0.2 keV).

The Bottom Line

The ReD experiment successfully filled a critical gap in our knowledge. They proved that liquid argon is very good at turning tiny nuclear shoves into electric signals, even at the lowest energy levels.

This is a huge win for the hunt for dark matter. It tells the builders of the next generation of giant detectors: "Don't worry, your sensors are sensitive enough. Keep building, and you might just find the ghosts."

Technical Summary

1. Problem Statement

Direct detection of low-mass Weakly Interacting Massive Particles (WIMPs) using liquid argon detectors relies on measuring nuclear recoils (NRs) with energies in the few-keV range (specifically 1–10 keV). In this low-energy regime, scintillation signals (S1) are often too faint to be detected, making the experiment dependent on ionization signals (S2).

• The Gap: Accurate sensitivity projections for future experiments (like DarkSide-20k) require a precise understanding of the ionization yield ($Q_y$)—the number of ionization electrons produced per unit of energy deposit.
• The Limitation: Prior to this work, direct experimental measurements of $Q_y$ in liquid argon were lacking below 7 keV. Existing models (e.g., the Thomas-Imel box formalism) relied on extrapolations from higher-energy data and Monte Carlo simulations, introducing significant uncertainty for low-mass WIMP searches.

2. Methodology

The ReD (Recoil Directionality) experiment was designed to perform a direct, model-independent measurement of $Q_y$ in the 2–10 keV range.

• Experimental Setup:

  • Detector: A compact dual-phase Time Projection Chamber (TPC) with an active volume of $5 \times 5 \times 6 \text{ cm}^3$, containing liquid argon topped by a thin gaseous layer.
  • Readout: Equipped with cryogenic Silicon Photomultipliers (SiPMs) at the top and bottom to detect S1 (scintillation) and S2 (electroluminescence) signals.
  • Source: A $^{252}\text{Cf}$ fission source (1 MBq activity) emitting neutrons with a Maxwellian distribution (mean energy $\approx 2.3$ MeV).
  • Kinematics: The recoil energy ($E_R$) is reconstructed event-by-event using two-body kinematics. This requires measuring the scattering angle ($\theta_S$) and the neutron kinetic energy ($E_n$).
    • Angle: Measured by a downstream neutron spectrometer consisting of 18 plastic scintillators (PScis) arranged in two $3\times3$ arrays, covering angles between $12^\circ$ and $17^\circ$.
    • Energy: Determined via Time-of-Flight (ToF) between the prompt $\gamma$-rays from fission (detected by BaF$_2$ scintillators) and the scattered neutron (detected by PScis). The system achieved a ToF resolution of $\approx 1$ ns, yielding a 5% precision on $E_n$.

• Data Analysis:

  • Trigger: Coincidences between BaF$_2$ and PScis within a 256 ns gate.
  • Event Selection: Candidate events were selected using ToF and Pulse Shape Discrimination (PSD) to suppress $\gamma$-backgrounds.
  • Signal Identification: Due to the low energy, many events appeared as "S2-only" (S1 signals were below the detection threshold). The analysis retained events with a single S2 signal within the central $4\times4 \text{ cm}^2$ of the TPC and compatible with the electron drift time.
  • Yield Extraction: The ionization yield ($Q_y$) was extracted by fitting the observed electron number ($N_e$) distributions in five energy bins using an unbinned likelihood method (Gaussian signal + flat background).

3. Key Contributions

• First Direct Measurement Below 7 keV: ReD successfully extended direct measurements of argon ionization yield down to 2 keV, filling a critical gap in experimental data.
• Model Independence: The measurement relies on kinematic reconstruction rather than theoretical extrapolation, providing a robust benchmark for detector response models.
• Validation of Background Rejection: The study demonstrated effective suppression of background events (primarily from multiple neutron scattering) using kinematic correlations and PSD, despite a sample purity of roughly 55% signal events.

4. Results

• Ionization Yield ($Q_y$) Trend:
  • Above 7 keV: The results are consistent with previous measurements from experiments ARIS, SCENE, and Joshi et al.
  • Below 7 keV: The data indicates a higher $Q_y$ at lower energies compared to previous extrapolations.
  • Magnitude: The measured yield increases as energy decreases, remaining qualitatively consistent with standard expectations based on the total work function ($W \approx 19.5$ eV) and nuclear-recoil quenching constraints.
• Precision:
  • Energy resolution ($E_R$) ranged from $\approx 9\%$ at 2 keV to 6% above 8 keV.
  • Electron number ($N_e$) resolution was 12% at $N_e=10$, improving to 7% for $N_e > 40$.
  • Systematic bias in reconstructed energy and electron count was verified to be better than 1% using Monte Carlo simulations.
• Sample Size: Approximately 800 candidate nuclear recoil events were identified in the 1–10 keV range.

5. Significance and Future Outlook

• Impact on Dark Matter Searches: The observed increase in $Q_y$ at low energies significantly improves the sensitivity projections for future argon-based experiments (e.g., DarkSide-20k) searching for low-mass WIMPs (1–10 GeV). Accurate modeling of the detector response in the few-keV regime is essential for distinguishing signal from background.
• ReD+ Upgrade: The paper outlines the ReD+ phase, designed to push sensitivity into the sub-keV regime (down to 0.2–0.5 keV).
  • Upgrades: A larger TPC, a repositioned spectrometer for smaller scattering angles ($6.5^\circ$–$10^\circ$), a higher activity source (3 MBq), and an expanded spectrometer array.
  • Future Source: A transition to a Deuterium-Deuterium (D-D) neutron generator (2.4 MeV quasi-monoenergetic neutrons) is planned to further reduce backgrounds and enable precise measurements down to 0.2 keV.
• Theoretical Refinement: These results provide essential input for refining theoretical models of ionization and recombination in liquid argon, moving beyond the Thomas-Imel formalism's limitations at very low energies.