Characterizing the energy resolution of the MicroBooNE LArTPC at the MeV scale using monoenergetic features of $^{208}$Tl decays

This paper presents the first-ever measurement of energy resolution in a Liquid Argon Time Projection Chamber (LArTPC) at the MeV scale, utilizing monoenergetic signals from $^{208}$Tl decays in the MicroBooNE detector to determine a resolution of approximately 7.52% and validate simulation predictions.

The Gist

Imagine the MicroBooNE detector as a giant, ultra-sensitive 3D camera filled with liquid argon (essentially super-cold, liquid air). Its job is to take pictures of tiny particles zipping through it. Usually, this camera is designed to catch high-energy particles, like those from a particle accelerator, which leave long, bright trails across the sensor.

However, scientists wanted to know: Can this camera also see very faint, tiny blips of energy? Specifically, can it measure the energy of particles with the precision needed to detect low-energy neutrinos from the sun or exploding stars?

To answer this, the MicroBooNE team performed a "calibration test" using a natural source of radiation already present inside the detector. Here is the story of how they did it, explained simply.

1. The "Invisible Ink" in the Detector

The detector is built with strong fiberglass struts (think of them as the metal beams holding up a bridge). Unfortunately, these struts contain tiny, natural traces of radioactive material, specifically an isotope called Thallium-208.

Every time a Thallium-208 atom decays, it shoots out a high-energy "bullet" of light called a gamma ray. This bullet has a very specific, known energy: 2.614 MeV. It's like a factory stamping out coins that all weigh exactly the same amount.

2. The "Magic Trick" of Pair Production

When these gamma rays hit the liquid argon, they usually just bounce off (Compton scattering). But about 5% of the time, they perform a magic trick called pair production.

Imagine the gamma ray hits the liquid and instantly splits into two new particles: an electron and a "positron" (the electron's antimatter twin).

• The positron immediately stops and crashes into an electron, vanishing in a flash of two new photons.
• These new photons bounce off other atoms, creating tiny, isolated sparks of energy.

Because the original gamma ray had a fixed energy, the total energy of these new sparks is also fixed and predictable. It's like a magician pulling a rabbit out of a hat, but the rabbit always weighs exactly 1.592 MeV.

3. The "Blip" Problem

The MicroBooNE camera is great at seeing long trails (tracks), but these tiny sparks are very small. They only touch a few wires on the sensor. The scientists call these tiny, isolated sparks "blips."

The challenge was: Can the camera measure the energy of these tiny blips accurately? If the camera is blurry, it might think a 1.592 MeV blip is 1.4 MeV or 1.8 MeV. If it's sharp, it will see exactly 1.592 MeV.

4. The Detective Work

To test the camera's sharpness (resolution), the team had to find these specific "magic trick" blips among millions of other random sparks caused by noise or other radiation.

They acted like detectives looking for a specific pattern:

• The Clue: The two sparks created by the positron's crash should be on opposite sides of the original split, forming a nearly straight line (180 degrees).
• The Filter: They used computer algorithms to scan through hundreds of thousands of events, throwing away anything that didn't look like this specific "straight line" pattern.

They also had to be careful to ignore "cosmic noise" (random particles from space) and other background radiation that could fake the signal. They compared the "signal area" (where the fiberglass struts are) against a "background area" (where there are no struts) to subtract the noise.

5. The Result: How Sharp is the Camera?

After cleaning up the data, they looked at the energy of the 640 "magic trick" blips they found.

• The Prediction: Their computer simulations predicted the camera would be about 9.7% "blurry" at this energy level.
• The Reality: The actual data showed the camera was even sharper, with a blur of only 7.5%.

What does 7.5% mean?
Imagine you have a scale that weighs a 1.6 kg bag of sugar. If the scale is 7.5% off, it might say the bag weighs anywhere between 1.48 kg and 1.72 kg. While that's not perfect, it is a very good measurement for such a tiny, faint signal.

The Bottom Line

This paper is the first time anyone has successfully measured how well a Liquid Argon detector can see and measure these tiny, low-energy "blips."

• They proved that MicroBooNE can see these faint signals.
• They proved that the detector's measurements are consistent with their computer models (the data and simulation agreed within a small margin of error).
• They established a new method to "calibrate" these detectors using natural radioactive decay, which is crucial for future experiments that hope to catch neutrinos from the sun or supernovas.

In short, they took a giant, complex camera, found a natural "test coin" hidden inside it, and proved the camera can weigh that coin with surprising accuracy.

Technical Summary

Technical Summary: Characterizing the Energy Resolution of the MicroBooNE LArTPC at the MeV Scale

Problem Statement
Liquid Argon Time Projection Chambers (LArTPCs) offer millimeter-scale spatial resolution and low energy thresholds, making them ideal for MeV-scale neutrino physics, including solar and supernova neutrino detection. However, while LArTPC performance at GeV scales is well-established, their energy resolution capabilities at the MeV scale had previously been characterized only through simulation. Experimental validation of the energy resolution and reconstruction fidelity for low-energy deposits (specifically "blips" from MeV-scale charged particles) was lacking. Without this empirical data, the potential for precise low-energy calorimetry and calibration in future experiments (such as DUNE and the SBN program) remained unverified.

Methodology
The MicroBooNE Collaboration utilized a dataset of 653,367 unbiased, beam-external events collected during Run 3 (June–July 2018). The analysis focused on identifying monoenergetic features arising from the decay of $^{208}\text{Tl}$, a radiogenic isotope present in the detector's G10 structural struts.

1. Signal Source: The study targeted 2.614 MeV $\gamma$-rays emitted by $^{208}\text{Tl}$. While Compton scattering is the dominant interaction, the analysis focused on the pair-production channel (5.24% probability), which produces an electron-positron ($e^+e^-$) pair with a fixed total kinetic energy of 1.592 MeV (2.614 MeV minus two electron masses).
2. Topology Selection: The selection strategy leveraged the unique topology of pair production. The positron ($e^+$) annihilates at rest, producing two back-to-back 0.511 MeV $\gamma$-rays. These photons subsequently interact via Compton scattering or photoelectric absorption, creating secondary electron clusters.
   • The primary signal is a "blip" (a compact 3D charge deposition) corresponding to the initial $e^+e^-$ pair.
   • This blip is accompanied by two collection-plane clusters from the annihilation photons, forming a nearly linear topology ($\approx 180^\circ$) relative to the blip.
3. Reconstruction and Cuts: The analysis applied a series of strict selection criteria to isolate this topology from background:
   • Signal Cuts: The accompanying clusters were required to have reconstructed energies between 0.1 and 0.35 MeV, be within 10 cm of the blip, and form an angle $\theta > 175^\circ$ with the blip.
   • Noise Reduction: Cuts were applied to reject candidates with excessive coincident clusters, coherent noise patterns, or hits on known noisy wires.
   • Background Reduction: Events with high total energy (indicating showers), proximity to cosmic muon tracks, or excessive nearby blip activity were vetoed to suppress cosmogenic and radiogenic backgrounds.
4. Background Subtraction: To further isolate the signal, the analysis defined "signal" and "background" regions based on the spatial distribution of $^{208}\text{Tl}$ hot spots (near G10 struts) versus regions distant from them. The background energy spectrum was rescaled and subtracted from the signal spectrum.

Key Contributions

• First Experimental Measurement: This work presents the first direct measurement of LArTPC energy resolution at the MeV scale.
• Monoenergetic Calibration Technique: It demonstrates a novel method for calibrating LArTPC energy response using the monoenergetic pair-production peak of $^{208}\text{Tl}$ decays, a technique applicable to future experiments lacking high-flux through-going muons for calibration.
• Blip Reconstruction Validation: The study validates the "blip" reconstruction algorithms (specifically the BlipReco toolkit) in the sub-MeV to MeV regime, confirming their ability to resolve compact, isolated energy depositions.

Results
The analysis identified 640 pair-production blip candidates in the data after background subtraction, centered at a reconstructed energy of 1.54 $\pm$ 0.01 (stat) $\pm$ 0.02 (syst) MeV.

• Measured Energy Resolution: The fractional energy resolution ($\sigma_E/E$) was determined to be 7.52 $\pm$ 0.78 (stat) $\pm$ 0.92 (syst)%.
• Simulation Comparison: A corresponding Monte Carlo simulation of the same process yielded a resolution of 9.70 $\pm$ 0.65 (stat)%.
• Consistency: The data and simulation results are consistent within 1.6$\sigma$.
• Energy Scale: The data peak (1.54 MeV) is slightly lower than the simulation peak (1.44 MeV), and both are slightly lower than the theoretical true value (1.592 MeV). The paper notes a data-simulation offset in peak location of approximately 7.2%, which is larger than previously observed offsets at the Compton edge (~2.4 MeV), suggesting complex detector response effects.

Significance and Claims
The paper claims that this study provides a critical benchmark for the fidelity of low-energy reconstruction in LArTPCs. By demonstrating that the MicroBooNE detector can resolve a monoenergetic MeV-scale feature with ~7.5% resolution, the work validates the detector's capability to perform the low-threshold physics required for supernova and solar neutrino detection.

The authors emphasize that this result serves as a "pathway for monoenergetic energy calibrations in future experiments." They modestly conclude that while the resolution is consistent with simulation, the observed data-simulation offsets in both resolution and energy scale warrant further investigation with higher-statistics samples to fully understand the systematic effects (such as charge deposition on sub-threshold wires and non-uniformities) governing MeV-scale energy reconstruction.