Precision measurement of positron decay modes of Xe-125 in the LUX-ZEPLIN experiment

The LUX-ZEPLIN experiment reports the first direct constraint on the individual positron emission branching levels of the activation product $^{125}\text{Xe}$, measuring a total branching ratio of $0.29\pm0.08_{\text{stat.}}\pm0.04_{\text{sys.}}$ % with a statistical significance of 5.5$\sigma$.

The Gist

The Big Picture: A Dark Matter Detective's Side Quest

Imagine the LUX-ZEPLIN (LZ) experiment as a giant, ultra-sensitive underwater camera sitting deep underground in a mine in South Dakota. Its main job is to take pictures of "ghosts" (Dark Matter particles) that rarely interact with anything. To do this, it uses a massive tank of liquid xenon (a heavy, noble gas) that acts like a super-clear pool.

Usually, scientists want to keep this pool perfectly quiet. But sometimes, to make sure their camera is working correctly, they have to "poke" the pool with a known force. In this case, they used a neutron cannon (a device that shoots neutrons) to hit the xenon atoms.

The Analogy: Think of the liquid xenon as a calm pond. The scientists threw a few rocks (neutrons) into the pond to create ripples. They wanted to see if their cameras could detect those ripples.

The Surprise Guest: The "Positron" Party

When the neutrons hit the xenon, they didn't just make ripples; they accidentally created a new, short-lived character in the pool: an isotope called Xenon-125 ($^{125}\text{Xe}$).

This new character is a bit of a troublemaker. It has a very short lifespan (about 17 hours) and usually calms down by swallowing an electron (a process called Electron Capture). However, the scientists suspected that occasionally, this character throws a wilder party: it spits out a positron (the antimatter twin of an electron).

The Mystery:

• Scientists knew this "positron party" might happen, but they had never seen it clearly before.
• They knew it happened at a high energy level (243 keV), but they weren't sure if it happened at a lower energy level (188 keV).
• It was like knowing a magician might pull a rabbit out of a hat, but you've never actually seen the rabbit, and you don't know if he has a second, smaller rabbit hiding in his pocket.

The Investigation: Catching the Ghosts in the Act

Detecting this positron is incredibly hard because the "pool" is full of background noise (other radioactive atoms, cosmic rays, etc.). It's like trying to hear a single pin drop in a rock concert.

To solve this, the LZ team used a clever trick: The "Multi-Scatter" Detective Work.

1. The Positron's Signature: When a positron is born, it doesn't just sit there. It zips around, loses energy, and then crashes into an electron. When matter (electron) meets antimatter (positron), they annihilate each other in a flash of light, creating two gamma-ray photons that fly off in opposite directions.
2. The Clue: This creates a specific pattern of energy deposits in the detector. Instead of one single "blip," the detector sees a complex dance: a flash here, a bounce there, and two flashes from the annihilation.
3. The Filter: The scientists programmed their computer to ignore single "blips" (which are usually background noise) and only look for these complex, multi-part dances. They also compared the data before they shot the neutrons (the quiet pond) with the data after (the ruffled pond).

The Result: A Confirmed Discovery

After analyzing the data, the scientists found something amazing:

• The Rabbit is Real: They confirmed that the positron party happens at the 243 keV level.
• The Second Rabbit: They found strong evidence (though not a 100% direct confirmation yet) that the party also happens at the 188 keV level.
• The Score: The statistical certainty of this discovery is 5.5 sigma. In the world of science, 5 sigma is the "gold standard" for a discovery. It means there is less than a 1 in 3.5 million chance that this result was just a fluke or a glitch.

The Analogy: Imagine you are listening to a radio station. You hear a faint, strange sound. You think, "Is that a ghost, or just static?" You listen for a long time, filter out the static, and realize the sound is a clear, distinct voice singing a song you've never heard before. You are now 99.9999% sure it's a real voice, not static.

Why Does This Matter?

1. It's a New Tool: This experiment proved that the LZ detector is so sensitive it can act as a "positron factory." By shooting neutrons at xenon, they can create a known source of positrons right inside the detector. This is like having a built-in calibration tool for future experiments.
2. Understanding the Rules: They measured exactly how often this happens (about 0.29% of the time). This helps physicists refine their understanding of how atomic nuclei behave.
3. Future Searches: This technique could help scientists look for other rare, weird decays in the future, essentially turning the dark matter detector into a multi-purpose physics lab.

Summary

The LUX-ZEPLIN experiment, famous for hunting Dark Matter, took a break to study a tiny, short-lived atom it created by accident. By using advanced math to filter out noise, they successfully "saw" this atom spitting out antimatter (positrons) for the first time with high confidence. It's a bit like a burglar alarm system that, while waiting for a burglar, accidentally discovered a new type of bird that only sings in the dark, and proved it exists with absolute certainty.

Technical Summary

1. Problem and Motivation

The LUX-ZEPLIN (LZ) experiment is a leading dark matter search utilizing a dual-phase liquid xenon (LXe) Time Projection Chamber (TPC). During calibration campaigns involving neutron sources (specifically Deuterium-Deuterium fusion), the liquid xenon target becomes activated, producing short-lived radioactive isotopes.

• The Specific Isotope: The neutron capture on stable $^{124}\text{Xe}$ produces $^{125}\text{Xe}$ ($T_{1/2} \approx 16.9$ hours).
• The Physics Gap: While $^{125}\text{Xe}$ is known to decay primarily via Electron Capture (EC, >99%), a positron emission ($\beta^+$) branch has been theoretically predicted and previously inferred but not precisely measured in this specific context.
• The Challenge: Detecting the rare $\beta^+$ decay of $^{125}\text{Xe}$ amidst a high background of EC decays and other activation products (like $^{135}\text{Xe}$ and $^{137}\text{Xe}$) requires distinguishing complex event topologies (multiple scatters) from single-site interactions and accurately modeling detector response for high-energy events.

2. Methodology

The analysis leverages the unique capabilities of the LZ detector to identify and reconstruct multi-site events resulting from the specific decay chain of $^{125}\text{Xe}$.

• Data Acquisition:

  • Data was collected immediately following an 11-day neutron calibration exposure.
  • The analysis utilized a cumulative 1.5 live-days of post-activation data.
  • A pre-activation background model was constructed using 85.2 live-days of WIMP search data (WS2024) to constrain non-activation backgrounds.

• Event Reconstruction & Selection:

  • Multi-Scatter Adaptation: Standard LZ reconstruction assumes single-site interactions. For this analysis, the algorithm was modified to handle multiple simultaneous energy depositions (scatters).
  • Signal Signature: The $\beta^+$ decay of $^{125}\text{Xe}$ produces a distinct signature:
    1. Kinetic energy of the positron ($\beta^+$).
    2. De-excitation $\gamma$-rays (or internal conversion electrons) from the excited $^{125}\text{I}$ nucleus.
    3. Two 511 keV annihilation photons from the positron-electron annihilation.
  • Energy Window: The total reconstructed energy is expected between 1000 keV and 1650 keV (centered around 1210–1645 keV for full containment).
  • Cuts:
    • S2 Threshold: All interaction sites must have $S2 > 645$ phd (14.5 extracted electrons) to remove noise.
    • Fiducial Volume: A ~1-tonne volume ($r < 45$ cm, $35 < z < 95$ cm) was selected to reduce external $\gamma$ backgrounds by 99.6%.
    • Spatial Cuts: Physical radius $R_i < 72.8$ cm and depth $0 < z_i < 145.6$ cm to ensure valid reconstruction.

• Statistical Analysis:

  • A binned likelihood fit was performed on the reconstructed energy spectrum.
  • The fit simultaneously maximized two terms: pre-activation (background only) and post-activation (background + signal).
  • Background components included: Pre-activation $\gamma$-rays, $^{125}\text{Xe}$ EC decays, and other activation products ($^{135}\text{Xe}$, $^{137}\text{Xe}$).
  • The $\beta^+$ branching ratios for the 188 keV and 243 keV levels of $^{125}\text{I}$ were treated as free parameters in the fit.

3. Key Contributions

• First Direct Constraint on Individual Branching Levels: This work provides the first simultaneous measurement and constraint on the individual branching ratios for the decay of $^{125}\text{Xe}$ to the 188 keV and 243 keV excited states of $^{125}\text{I}$.
• Multi-Scatter Event Analysis: The paper demonstrates the successful adaptation of LZ's reconstruction algorithms to handle complex, multi-site topologies, proving the detector's capability to resolve individual interaction sites within a single event.
• In-situ Calibration Source: It validates the use of neutron-activated xenon as a source of $\beta^+$ decays for calibrating detectors for future rare $\beta^+$ decay searches.

4. Results

• Discovery Significance: The analysis observed the $\beta^+$ signal with a statistical significance of 5.5 $\sigma$ against the null hypothesis (EC-only decay).
• Branching Ratio:
  • Total $\beta^+$ branching ratio: $0.29 \pm 0.08 \text{ (stat)} \pm 0.04 \text{ (sys)} \%$.
  • This result is consistent with the existing literature value of $0.3 \pm 0.1 \%$.
• Event Counts:
  • Best-fit $\beta^+$ signal: $96^{+29}_{-27}$ counts.
  • The fit to the 243 keV level yielded a significant signal, while the 188 keV level was consistent with zero ($0^{+17}_{-0}$ counts), though the total branching ratio constraint includes both.
• Systematics: The dominant systematic uncertainties arose from the constraints on the measured EC modes and uncertainties in the detector gain factors ($g_1$ and $g_2$).

5. Significance

• Validation of Detector Performance: The ability to precisely measure a rare decay mode with a complex topology (multiple scatters, annihilation photons, and nuclear de-excitation) confirms the LZ detector's high fidelity in energy reconstruction and event topology discrimination.
• Nuclear Physics Data: This provides the first direct experimental constraint on the specific branching fractions of $^{125}\text{Xe}$ decay to the 188 keV and 243 keV levels, refining nuclear data sheets used in various physics applications.
• Future Rare Event Searches: The demonstration that LXe TPCs can effectively identify and characterize $\beta^+$ decays via neutron activation suggests a new methodology for in-situ calibration. This is crucial for future experiments searching for rare $\beta^+$ processes (e.g., neutrinoless double beta decay or solar neutrino interactions) where understanding the detector response to positrons is vital.

In summary, the LZ collaboration successfully utilized its dark matter detector to perform a precision nuclear physics measurement, confirming the positron decay of $^{125}\text{Xe}$ with high statistical significance and providing new constraints on its decay scheme.