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Muon veto system for the CROSS double-beta decay search experiment

This paper details the design, construction, and validation of a nine-sector muon veto system for the CROSS experiment at the Canfranc underground laboratory, demonstrating its ability to reduce muon-induced background in the region of interest to an acceptable level for neutrinoless double-beta decay searches despite an 18% dead time.

Original authors: A. S. Barabash, L. Bergé, M. Buchynska, J. M. Calvo-Mozota, A. Candela, P. Carniti, M. Chapellier, D. Cintas, A. Corsi, I. Dafinei, F. A. Danevich, M. De Deo, L. Dumoulin, F. Ferri, A. Giuliani, C. Go

Published 2026-07-28

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: A. S. Barabash, L. Bergé, M. Buchynska, J. M. Calvo-Mozota, A. Candela, P. Carniti, M. Chapellier, D. Cintas, A. Corsi, I. Dafinei, F. A. Danevich, M. De Deo, L. Dumoulin, F. Ferri, A. Giuliani, C. Gotti, P. Gras, A. Ianni, V. V. Kobychev, S. I. Konovalov, P. Loaiza, P. de Marcillac, S. Marnieros, C. A. Marrache-Kikuchi, M. Martinez, C. Nones, E. Olivieri, A. Ortiz de Solórzano, V. Perez, G. Pessina, D. V. Poda, B. Romualdi, Ph. Rosier, R. Serino, V. I. Tretyak, V. I. Umatov, G. Ursini, M. M. Zarytskyy, A. Zolotarova

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Rain and the Silent Search

Imagine the universe is constantly raining. But instead of water, it's a storm of invisible, super-fast particles called cosmic muons, zipping through space at nearly the speed of light. These particles are like tiny, ghostly bullets that can punch right through mountains, buildings, and even your body without you feeling a thing. For most of us, this is just background noise. But for a special group of scientists hunting for one of the biggest mysteries in physics, this rain is a massive problem.

They are looking for a rare event called "neutrinoless double-beta decay." Think of it as trying to hear a single, perfect whisper in the middle of a roaring stadium. The "whisper" is a specific type of atomic decay that could prove neutrinos are their own antiparticles, a discovery that would rewrite the laws of physics and explain why the universe exists. To hear this whisper, scientists build ultra-sensitive detectors deep underground to block out the noise. However, even deep underground, the "cosmic rain" is still heavy enough to drown out the whisper. If a cosmic muon hits the detector, it creates a loud bang that looks exactly like the whisper they are hunting for. To solve this, the scientists need a way to spot the muons the moment they arrive and instantly tell their detectors, "Ignore that noise! It's just a muon!" This is the story of how they built a giant, high-tech umbrella to catch the cosmic rain.

Catching the Ghosts: The CROSS Experiment's New Shield

The paper you are reading details the creation and testing of a "muon veto system" for an experiment called CROSS, which is being set up in a deep underground laboratory in Spain. The CROSS team is preparing to hunt for that elusive neutrinoless double-beta decay using crystals made of a special material called lithium molybdate. Because the mountain above their lab isn't thick enough to stop all the cosmic muons (there are still about 20 muons hitting every square meter every hour), they needed a custom-made shield to act as a bouncer for their experiment.

Building the Umbrella
The scientists designed a giant, three-dimensional cage around their delicate cryogenic equipment. Imagine a room where the detectors live; the team built a wall of sensors around the sides, a floor of sensors underneath, and a roof of sensors on top.

The Walls (Lateral Sectors): They used four curved sections made of 28 long, clear plastic bars (polystyrene) wrapped around the detector. When a muon zips through these bars, it makes a tiny flash of light, which is caught by super-sensitive eyes called Silicon Photomultipliers (SiPMs).
The Floor (Bottom Sectors): Four more sections of plastic bars were laid out underneath the detector to catch muons coming from below.
The Roof (Top Sectors): Two large panels with traditional light-sensing tubes (PMTs) were placed on the roof of the hut to catch muons coming from above.

In total, this system has nine different "sectors" that can act as independent tripwires. If a muon triggers any of these sectors, the system screams "MUON!" and tells the main detectors to ignore any signal that happens at the exact same time.

The Simulation and the Strategy
Before building the real thing, the team used powerful computer simulations (like a video game for physics) to figure out the best way to set up their sensors. They discovered a tricky problem: if they only looked for muons that hit two different sectors at once, they would miss too many. It's like trying to catch a thief by only arresting them if two different security guards see them at the same time; the thief might slip past if only one guard sees them.

The simulations showed that to catch 99.7% of the muons, they had to be more aggressive. They decided to trigger the veto if any single sector saw a muon. This is a very strict rule. It means that if even one tiny plastic bar on the roof or floor gets hit, the whole system shuts down the main detectors for a tiny fraction of a second. The paper notes that this strictness comes with a cost: it creates a "dead time" of about 18%. This means the experiment is "blind" for roughly 18% of the time because it's busy ignoring muon noise. However, the trade-off is worth it because it reduces the background noise in the critical energy range (around 3 MeV) down to a level of about 0.002 counts per keV per kg per year. This low noise level is essential for the experiment to have a chance of finding the rare decay.

Testing the System
The team didn't just rely on computer games; they built the system and tested it in the real underground lab. They installed the plastic bars and the electronics and ran a "dress rehearsal" using two high-performance detector modules (borrowed from a previous experiment called CUPID-Mo). They cooled these detectors down to a temperature colder than outer space (20 millikelvin) and watched them for hundreds of hours.

The results were a success. The system operated stably, with the sensors firing at the rates the computer simulations predicted. When they looked at the data, they initially found that the muon veto identified and rejected about 71-75% of the muon-induced events that would have otherwise looked like the signal they were hunting for. However, they discovered a small technical glitch during the test: a specific "trigger bit" in the electronics had been temporarily deactivated. Once they corrected for this issue, the efficiency jumped to roughly 85-90%, matching their computer models almost perfectly.

What This Means
The paper concludes that the CROSS muon veto system is ready for the main event. By using a "single-sector" trigger logic (where any hit counts), they have built a shield that can filter out the overwhelming majority of cosmic noise, even though it means the experiment has to pause for a bit more than usual. This setup allows the CROSS experiment to reach the ultra-low background levels required to search for neutrinoless double-beta decay. The team has proven that their "umbrella" works, and they are now confident that when the full experiment runs, they will be able to listen for that cosmic whisper without being drowned out by the rain.

Technical Summary: Muon Veto System for the CROSS Double-Beta Decay Search Experiment

Problem Statement
The CROSS (Cryogenic Rare event Observatory with Surface Sensitivity) collaboration is preparing a high-sensitivity experiment to search for neutrinoless double-beta ( $0\nu\beta\beta$ ) decay of $^{100}$ Mo using low-temperature detectors with heat-scintillation readout. The experiment is located at the Canfranc Underground Laboratory (LSC) in Spain. While deep underground facilities typically suppress cosmic muon flux, the specific topography of the LSC site—specifically a valley (Rioseta) with a minimal rock depth of $\sim$ 200 m—results in a relatively high residual cosmic muon flux of approximately 20 $\mu$ /m $^2$ /h. This flux creates a dominant background in the region of interest (ROI) around the $Q$ -value of the $^{100}$ Mo decay ( $\sim$ 3.0 MeV). Without mitigation, muon-induced secondary particles would overwhelm the signal, limiting the experiment's sensitivity.

Methodology
To address this background, the authors designed, constructed, and validated a dedicated muon veto system surrounding the CROSS cryogenic facility. The methodology involved three primary phases:

System Design and Construction:
The veto system comprises nine sectors: four lateral, one top, and four bottom.
- Lateral and Bottom Sectors: Constructed from polystyrene scintillating bars (120 $\times$ 4 $\times$ 1 cm for lateral; 52 $\times$ 4 $\times$ 1 cm for bottom) coupled to Silicon Photomultipliers (SiPMs). These are arranged in 28 modules for the lateral sectors and 15 modules for the bottom sectors.
- Top Sector: Consists of two large panels (100 $\times$ 50 $\times$ 1 cm) read out by conventional Photomultiplier Tubes (PMTs).
- Electronics and DAQ: The system utilizes a custom signal processing chain involving SiPM amplifiers, discriminator boards, and a logic unit operating in OR mode. A Raspberry Pi controller manages bias voltages and thresholds for each channel. The trigger logic was optimized to operate in two modes: a coincidence of any two sectors or a single-sector trigger. The final adopted strategy uses a single-sector trigger to maximize background rejection.
Monte Carlo (MC) Simulations:
Using Geant4 v11.1, the authors simulated muon interactions within the CROSS setup geometry. The simulations incorporated:
- Realistic angular distributions of muons based on LSC measurements (peaking at a zenith angle of 40 $^\circ$ and azimuth of 150 $^\circ$ ).
- Slant depth calculations accounting for the valley profile.
- Initial energy spectra derived from surface-level data.
- Three detector geometries: a 10-crystal test tower, a two-module CUPID-Mo tower, and the full 42-crystal CROSS array.
  The simulations aimed to validate the veto efficiency and optimize the energy thresholds to balance background rejection with dead time.
Experimental Validation:
Prior to the full CROSS experiment, the system was tested during "RUN 13" using two high-performance CUPID-Mo modules (LMO-5 and LMO-12) inside the CROSS cryostat. The study involved:
- Calibrating veto module thresholds using a $^{60}$ Co source.
- Characterizing background rates in the veto modules to establish homogeneous energy thresholds.
- Measuring coincidences between the thermal detectors and the muon veto over 555 hours of background data.
- Comparing experimental rejection rates with MC predictions.

Key Contributions and Results

System Implementation: The paper details the successful construction of a 9-sector muon veto system capable of detecting muons with high efficiency. The system operates with a stable average trigger rate compatible with MC simulations.
Trigger Logic Optimization: MC studies demonstrated that a trigger logic requiring coincidences between two sectors rejects 99.2% of muon-induced events in the ROI. However, a single-sector trigger logic (rejecting any event coincident with any of the nine sectors) increases rejection to 99.7%.
Background Reduction: The adopted single-sector strategy introduces a dead time of approximately 18%. Despite this, the system reduces the muon-induced background in the ROI to $\sim$ 2 $\times$ 10 $^{-3}$ counts/keV/kg/yr. This level is deemed acceptable for the high-sensitivity $0\nu\beta\beta$ search.
Experimental Validation: During the validation run with CUPID-Mo modules, the veto system demonstrated stable operation. The experimental rejection of muon-like events ( $E > 3$ MeV) was measured at 71% (corrected to 85% accounting for a temporary DAQ issue), which aligns well with the simulation prediction of 88%. For events $E > 10$ MeV, the experimental rejection was 75% (corrected to 90%), matching the simulation's 89%.
Detector Performance: The validation run confirmed that the CROSS cryogenic facility can operate CUPID-Mo modules with energy resolutions of $\sim$ 6–7 keV FWHM at 2615 keV, comparable to their performance in the EDELWEISS setup.

Significance
The paper establishes that a muon veto system is essential for the success of the CROSS experiment at the Canfranc Underground Laboratory due to the site's specific geological constraints. By implementing a single-sector trigger logic, the collaboration achieves a background index low enough to proceed with a competitive search for $0\nu\beta\beta$ decay in $^{100}$ Mo. The work validates the design, construction, and operation of the veto system and confirms through both simulation and experimental data that the system can effectively suppress muon-induced backgrounds to the required level, despite the associated dead time. This validation provides confidence for the commissioning of the full 42-crystal CROSS detector array.

The Cosmic Rain and the Silent Search

Catching the Ghosts: The CROSS Experiment's New Shield

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