Characterization of the ionization response of argon to nuclear recoils at the keV scale with the ReD experiment

The ReD experiment utilized a compact dual-phase argon Time Projection Chamber irradiated by neutrons to model-independently measure the ionization yield of argon for nuclear recoils between 2 and 10 keV, revealing higher yields at lower energies than previously observed and extending experimental coverage crucial for detecting low-mass Dark Matter.

The Gist

Imagine you are trying to find a ghost. But this isn't a spooky ghost; it's a Dark Matter particle, a mysterious substance that makes up most of the universe but refuses to interact with normal matter. Scientists have been hunting for these particles for decades, usually looking for heavy ones (like boulders). But recently, they've started wondering: What if the ghosts are actually tiny, like dust motes?

If these "low-mass" Dark Matter particles exist, they would be so light that when they bump into an atom, the atom barely shivers. It's like a ping-pong ball hitting a bowling ball; the bowling ball moves, but only a tiny, tiny bit.

This is where the ReD experiment comes in. It's a high-tech detective story designed to catch these tiny shivers in a tank of liquid argon (a noble gas, like the stuff in neon signs, but frozen cold).

Here is the story of how they did it, explained simply:

1. The Challenge: The "Invisible" Shiver

When a Dark Matter particle hits an argon atom, the atom recoils (shivers). This shiver creates two things:

1. A flash of light (Scintillation).
2. A few electrons (Ionization).

For heavy Dark Matter, the shiver is big, and you see a bright flash. But for the tiny, low-mass Dark Matter, the shiver is so weak that the flash of light is almost invisible. It's too dim to see.

However, the electrons are still there. If you can count those electrons, you can prove the shiver happened. The problem? Scientists didn't have a perfect "ruler" to measure exactly how many electrons come out of an argon atom when it gets a tiny, keV-scale shiver. They were guessing.

2. The Solution: The "Neutron Billiards" Game

To build a better ruler, the ReD team needed to create artificial shivers that they could measure perfectly. They couldn't use real Dark Matter (because they haven't found it yet), so they used neutrons.

Think of the experiment like a game of billiards:

• The Source: They used a radioactive source (Californium-252) that spits out neutrons like a machine gun firing tiny, invisible bullets.
• The Table: In the middle of the room sits the Liquid Argon Tank (the TPC). This is the "table" where the action happens.
• The Shot: A neutron flies in and hits an argon atom. The argon atom recoils (the shiver we want to study).
• The Catch: The neutron doesn't stop; it bounces off and flies away. The team built a wall of 18 plastic detectors behind the tank to catch the bouncing neutron.

The Magic Trick:
By measuring where the neutron bounced off and how fast it was going, the scientists could use simple physics (like calculating the speed of a car after a crash) to know exactly how much energy the argon atom received.

• If they know the energy of the shiver, and they count the electrons coming out, they finally have their ruler. They can say, "For every 1 unit of energy, we get X electrons."

3. The Setup: A High-Tech Ice Box

The experiment took place in a specialized lab in Italy.

• The Tank: It's a small, cube-shaped box filled with liquid argon, kept at -186°C. It's like a super-cold, super-clear aquarium.
• The Eyes: The tank is lined with ultra-sensitive cameras (called SiPMs) that can see single photons of light.
• The Trigger: Because the shivers are so small, the team had to be clever. They waited for a "Start" signal (a gamma ray from the source) and a "Stop" signal (the neutron hitting the back wall). If those two happened at the right time, they knew a neutron had passed through the tank. Then, they looked inside the tank to see if an argon atom had shivered.

4. The Discovery: The Ruler Gets Sharper

The team successfully measured the "ionization yield" (the number of electrons per unit of energy) for shivers between 2 and 10 keV.

Why is this a big deal?

• Breaking the Barrier: Previous experiments could only measure down to about 7 keV. ReD pushed the limit down to 2 keV. It's like going from seeing a mountain to seeing a pebble.
• The Surprise: They found that at these very low energies, argon produces more electrons than scientists previously thought. It's like the argon is "shouting" louder than expected when it gets a tiny tap.
• The Implication: This changes the rules for future Dark Matter hunters. If argon is more sensitive to these tiny shivers than we thought, then the next generation of giant detectors (like DarkSide-20k) might be able to find those low-mass Dark Matter particles much easier than we predicted.

5. The Future: Going Even Smaller

The paper ends with a promise: "We aren't done."
The team is planning a new campaign (ReD+) to push the limit even further, down to 0.5 keV and eventually 0.2 keV. They plan to use a different neutron generator that acts like a laser beam of neutrons, giving them even more precision.

The Bottom Line

This paper is about calibrating the sensitivity of our universe's best Dark Matter detectors. By using neutrons as a proxy for Dark Matter, the ReD team built a precise ruler for the smallest shivers in liquid argon. They discovered that argon is more sensitive to these tiny hits than we thought, giving hope that we are finally close to catching the elusive, low-mass ghosts of the universe.

Technical Summary

Here is a detailed technical summary of the paper "Characterization of the ionization response of argon to nuclear recoils at the keV scale with the ReD experiment."

1. Problem Statement

The search for Dark Matter, specifically Weakly Interacting Massive Particles (WIMPs), has increasingly focused on the "low-mass" regime (1–10 GeV). In this mass range, WIMP interactions with atomic nuclei produce nuclear recoils (NRs) with energies below 10 keV.

• Detection Challenge: In liquid argon (LAr) detectors, low-energy NRs produce negligible scintillation light (S1), making them nearly undetectable via traditional dual-phase Time Projection Chamber (TPC) methods. Detection relies almost exclusively on ionization signals (S2).
• Knowledge Gap: The ionization yield ($Q_y$), defined as the number of electrons produced per unit of recoil energy, is poorly understood for argon below 7 keV. Existing direct measurements only extend down to $\sim$7 keV. Below this threshold, experiments like DarkSide-50 have relied on phenomenological models (e.g., Thomas-Imel box formalism) and Monte Carlo simulations, which yield conflicting predictions depending on the nuclear stopping power model used (e.g., Ziegler vs. Lenz-Jensen).
• Goal: To provide a direct, model-independent measurement of the argon ionization yield in the 2–10 keV energy range to reduce systematic uncertainties for future multi-tonne experiments like DarkSide-20k.

2. Methodology

The ReD (Recoil Directionality) experiment utilized a compact, dual-phase LAr TPC irradiated by neutrons to generate controlled nuclear recoils.

Experimental Setup

• Neutron Source: A $^{252}\text{Cf}$ spontaneous fission source ($\sim$25 kBq SF activity) provided a neutron spectrum with a mean energy of 2.3 MeV.
• Target: A cubic dual-phase LAr TPC with an active volume of $5 \times 5 \times 6 \text{ cm}^3$. It featured:
  • Transparent ITO-coated windows acting as anode and cathode.
  • Silicon Photomultipliers (SiPMs) for detecting S1 (scintillation) and S2 (electroluminescence) signals.
  • Electric fields: 200 V/cm (drift), 3.8 kV/cm (extraction), and 5.7 kV/cm (gas phase).
• Kinematic Reconstruction:
  • Tagging: Two BaF$_2$ scintillators near the source detected prompt $\gamma$-rays from fission, providing the "START" signal for Time-of-Flight (ToF) measurements.
  • Spectrometer: A downstream array of 18 plastic scintillators (EJ-276) detected neutrons scattered off argon nuclei, providing the "STOP" signal.
  • Geometry: The spectrometer was positioned $\sim$1 m downstream, covering a scattering angle ($\theta_S$) of $12^\circ$–$17^\circ$.
• Recoil Energy Calculation: The recoil energy ($E_r$) was reconstructed event-by-event using two-body kinematics:
  $$E_r = \frac{2 K_n m_n m_{Ar}}{(m_n + m_{Ar})^2} (1 - \cos \theta_S)$$
  where $K_n$ is derived from the neutron ToF, and $\theta_S$ is determined by the fixed geometry of the detector and spectrometer.

Data Analysis & Selection

• Event Selection: Events were selected based on a coincidence between a BaF$_2$ tagger and a plastic scintillator (PSci) within a specific ToF window ($40\text{--}160$ns). Pulse Shape Discrimination (PSD) in the PSci was used to reject$\gamma$-ray backgrounds.
• TPC Signal: The analysis focused on events with a valid S2 signal (ionization) in the TPC. S1 signals were often too small to detect for low-energy recoils.
• Calibration: The ionization gain ($g_2$, PE per electron) was determined using $^{241}\text{Am}$ $\gamma$-ray calibrations ($60$ keV).
• Statistical Approach: The data was binned into five energy intervals (1–10 keV). An unbinned maximum likelihood fit was used to determine the mean number of electrons ($N_e$) for each bin, from which $Q_y = N_e / E_r$ was calculated.

3. Key Contributions

1. First Direct Measurement Below 7 keV: The ReD experiment successfully extended direct experimental coverage of argon ionization yield down to 2 keV, a region previously inaccessible to direct measurement.
2. Model-Independent Approach: Unlike previous studies that relied on extrapolations or specific stopping power models, ReD used kinematic reconstruction of scattered neutrons to determine recoil energy independently of the ionization yield model.
3. Validation of Analysis Chain: The authors developed a comprehensive Monte Carlo framework (GEANT4 based) that validated the reconstruction algorithms, confirming that the analysis procedure is unbiased and accurately recovers the input $Q_y$ model within $<0.5\%$.
4. Precision Characterization: The experiment achieved a total uncertainty on $Q_y$ between 4.3% and 5.7%, dominated by systematic uncertainties in the ionization gain ($g_2$).

4. Key Results

The measured ionization yield ($Q_y$) for argon nuclear recoils shows a distinct energy dependence:

• Trend: $Q_y$ increases significantly as recoil energy decreases.
  • At 7.63 keV: $Q_y = 5.08 \pm 0.22$ e$^-$/keV.
  • At 2.40 keV: $Q_y = 7.42 \pm 0.42$ e$^-$/keV.
• Comparison with Models:
  • The results are consistent with existing data above 7 keV (Joshi et al., ARIS, SCENE).
  • The rising trend at lower energies is qualitatively consistent with the Lenz-Jensen nuclear stopping power model.
  • The results contradict the Ziegler model, which predicts a nearly constant $Q_y$ below 5 keV.
• Data Consistency: The results support the use of the Lenz-Jensen model for extrapolating detector sensitivity to the sub-keV regime in future Dark Matter searches.

5. Significance

• Impact on Dark Matter Searches: These measurements are critical for the DarkSide-20k experiment and other next-generation LAr detectors. Accurate knowledge of $Q_y$ at low energies is essential to correctly estimate the detection efficiency and background rejection for low-mass WIMPs (1–10 GeV).
• Reducing Systematic Uncertainties: By providing a direct measurement, ReD removes the reliance on theoretical extrapolations for the 2–7 keV range, significantly reducing the systematic uncertainties in Dark Matter exclusion limits.
• Future Directions: The paper outlines the ReD+ initiative, which plans to use a Deuterium-Deuterium (DD) generator to push measurements down to 0.2 keV, further refining the understanding of argon's response to the lowest energy nuclear recoils.

In summary, the ReD experiment has successfully characterized the ionization response of liquid argon to nuclear recoils in the critical 2–10 keV range, providing essential empirical data that validates the Lenz-Jensen stopping power model and paves the way for more sensitive searches for low-mass Dark Matter.