⚛️ nuclear experiments

The CROSS experiment: detector construction, background projection, and sensitivity to $^{100}$ Mo $0\nu2\beta$ decay

This paper presents the construction of the CROSS experiment's 4.9 kg $^{100}$ Mo scintillating cryogenic calorimeter array at the Canfranc underground laboratory and reports Geant4-based simulations predicting a background level that enables world-leading sensitivity to neutrinoless double-beta decay within one year of data taking.

Original authors: D. Auguste, A. S. Barabash, G. Benato, V. Berest, L. Bergé, M. Buchynska, J. M. Calvo-Mozota, J. Cao, P. Carniti, M. Chapellier, D. Cintas, I. Cojocari, I. Dafinei, F. A. Danevich, M. De Deo, A. Drobi

Published 2026-07-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: D. Auguste, A. S. Barabash, G. Benato, V. Berest, L. Bergé, M. Buchynska, J. M. Calvo-Mozota, J. Cao, P. Carniti, M. Chapellier, D. Cintas, I. Cojocari, I. Dafinei, F. A. Danevich, M. De Deo, A. Drobizhev, L. Dumoulin, F. Ferri, A. Giuliani, C. Gotti, Ph. Gras, A. Ianni, V. V. Kobychev, Yu. G. Kolomensky, S. I. Konovalov, P. Loaiza, P. de Marcillac, S. Marnieros, C. A. Marrache-Kikuchi, M. Martinez, C. Nones, E. Olivieri, A. Ortiz de Solórzano, M. Pageot, Y. Peinaud, G. Pessina, D. V. Poda, Ph. Rosier, B. Schmidt, R. Serino, V. I. Tretyak, V. I. Umatov, M. Velazquez, 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 Great Atomic Ghost Hunt

Imagine the universe is a giant, bustling city where atoms are the citizens. Most of these citizens are perfectly stable, but a few are like shy ghosts that occasionally decide to vanish and reappear as something else. This process is called "double beta decay." Usually, when an atom decays, it spits out two tiny particles called electrons and two invisible, almost massless ghosts called neutrinos. This happens all the time, but it's so rare that it takes a trillion years for a single atom to do it.

But here is the big mystery that physicists are obsessed with: What if, in a very special, ultra-rare case, those two neutrino ghosts don't show up at all? If an atom could decay and release only the two electrons, with no neutrinos, it would mean that neutrinos are their own anti-neighbors (a property called being "Majorana" particles) and that the universe breaks a fundamental rule about how much "stuff" (lepton number) it has. Finding this "neutrinoless" decay would be like finding a ghost that can walk through walls without leaving a trace; it would rewrite the rulebook of physics and tell us why the universe is made of matter instead of just empty space.

To catch this ghost, scientists need to build a detector so quiet and sensitive that it can hear a single atom whispering in a hurricane. They need to listen for a very specific "ping" of energy that only happens if the neutrinos are missing. The challenge is that the universe is full of background noise—radioactive dust, cosmic rays from space, and even the tiny vibrations of the detector itself—that can sound exactly like the ghost we are looking for.

The CROSS Experiment: Building a Super-Sensitive Ear

This paper introduces the CROSS experiment, a high-tech listening device built deep underground in Spain to hunt for this neutrinoless double-beta decay in a specific atom called Molybdenum-100. Think of the CROSS detector not as a camera, but as a giant, super-cold "ear" made of 42 tiny modules. Each module holds a crystal cube (some made of Molybdenum, others of Tellurium) that acts as the listening surface.

The team built these crystals from ultra-pure materials, ensuring they are as clean as a surgical room. They are so cold that they are just a tiny fraction of a degree above absolute zero (the coldest temperature possible). At this temperature, if a particle hits the crystal, it doesn't just make a sound; it creates a tiny ripple of heat and a flash of light. The detector is designed to catch both the heat and the light. By comparing the two, the scientists can tell the difference between a "real" ghost signal (the decay they want) and a fake noise (like a cosmic ray hitting the side).

The paper details how they constructed this massive, delicate machine. They used a special robotic arm to glue tiny sensors onto the crystals with microscopic precision, using a glue that doesn't glow with radioactivity. The whole thing was built in a clean room, wrapped in layers of lead and copper to block out the outside world, and then lowered into a deep underground laboratory called Canfranc. This lab is buried under 2,450 meters of rock equivalent, which acts like a giant shield against cosmic rays from space. Even then, they installed a "muon veto"—a fence of sensors around the detector that acts like a bouncer, kicking out any event if a cosmic muon tries to sneak in.

What They Found (and What They Predict)

The paper doesn't claim to have caught the ghost yet. Instead, it presents the blueprint and the weather forecast for the hunt. The authors built a super-computer simulation (a digital twin of their experiment) to predict how much "noise" or background radiation their detector would hear.

Here is the good news: Their simulations predict that the CROSS detector will be incredibly quiet. They estimate that in the specific energy range where the ghost signal should appear (around 3034 keV), the background noise will be only 3.2(5) × 10⁻³ counts per keV per kg per year. To put that in perspective, that is an almost silent room.

Based on this predicted silence, the paper calculates that if the CROSS experiment runs for just one year, it could become the most sensitive experiment in the world for finding this decay in Molybdenum-100. It could potentially set a new record, pushing the limit of how long an atom can live before decaying to 4 × 10²⁴ years.

However, the authors are very careful not to overpromise. They acknowledge that real life is messy. They simulate a "worst-case scenario" where the materials might be slightly more radioactive than expected, or the detector might not work quite as perfectly as the simulations hope. Even if the background noise is 3 to 10 times worse than their best prediction, they calculate that running the experiment for two years would still allow them to compete with the best experiments in the world.

The Bottom Line

The CROSS experiment is now live and taking data as of April 2026. This paper is the "user manual" and the "performance report" for the machine. It confirms that the detector was built correctly, the simulations show it should be quiet enough to hear the faintest whisper of a neutrinoless decay, and the timeline looks promising. While they haven't found the ghost yet, they have built the best possible trap for it, and they are ready to start the hunt with a high chance of success. If the universe is hiding this secret, CROSS is the tool that might finally reveal it.

Technical Summary of the CROSS Experiment Paper

Problem Statement
The search for neutrinoless double-beta ( $0\nu2\beta$ ) decay in the $^{100}\text{Mo}$ isotope is a primary avenue for probing physics beyond the Standard Model, specifically testing the Majorana nature of neutrinos and lepton number violation. While previous experiments like NEMO-3, LUMINEU, CUPID-Mo, and AMoRE-I have established significant limits, the field requires detectors with larger masses and lower background indices to reach the sensitivity required to probe the inverted neutrino mass hierarchy. The CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) experiment aims to address this by utilizing an array of cryogenic calorimeters containing enriched $^{100}\text{Mo}$ . A key challenge addressed in this work is the degradation of performance observed in large-volume, surface-coated crystals, necessitating a new detector architecture that relies on paired light detectors for particle identification without surface coatings.

Methodology
The paper details the construction, installation, and background modeling of the CROSS detector array at the Canfranc Underground Laboratory (LSC) in Spain.

Detector Construction: The array consists of 42 modules of cryogenic calorimeters with dual heat-light readout. The core absorbers include 32 cubic crystals (45 mm side) of lithium molybdate ( $\text{Li}_2^{100}\text{MoO}_4$ , LMO) enriched to $\sim98\%$ in $^{100}\text{Mo}$ (totaling 8.93 kg of LMO, or 4.9 kg of $^{100}\text{Mo}$ ), alongside 6 $\text{TeO}_2$ crystals and 4 LMO samples for background studies.
Light Detection and Readout: Each crystal is coupled to a cryogenic light detector (LD) made of high-purity Germanium (Ge) or Silicon (Si) wafers. These LDs feature Aluminum electrodes designed to exploit the Neganov-Trofimov-Luke (NTL) effect for signal amplification, enhancing particle identification capabilities. The mechanical structure minimizes passive materials, utilizing a low Cu-to-LMO mass ratio (6%) and 3D-printed polylactide (PLA) components.
Installation and Environment: The detector was assembled in a cleanroom at IJCLab (Orsay, France) and installed in the LSC's dedicated low-background cryogenic facility (C2U) in mid-2025. The setup includes massive passive shielding (lead and copper), an anti-radon box, and a muon veto system comprising 174 plastic scintillator modules. The system operates at $\sim20$ mK using a dilution refrigerator.
Background Modeling: The authors performed extensive Geant4-based Monte Carlo simulations to project the background index (BI) in the region of interest (ROI). The model included detailed geometries of the detector components, shielding, and the muon veto. It accounted for radioactive contaminants ( $^{226}\text{Ra}$ , $^{228}\text{Th}$ ) in materials, $2\nu2\beta$ decays of $^{100}\text{Mo}$ , random pile-up events, and cosmic muons. Event selection criteria were applied, including single-crystal hits, anti-coincidence with the muon veto (2 ms window), and Light-to-Heat Ratio (LHR) discrimination.

Key Contributions and Results

Detector Performance: The paper reports the successful construction and commissioning of the 42-module array. The crystals demonstrate high optical quality and radiopurity, with internal $^{228}\text{Th}$ and $^{226}\text{Ra}$ activities below $1\,\mu\text{Bq/kg}$ for enriched samples. The LDs achieved an RMS baseline noise of $\sim10$ eV with a signal-to-noise ratio gain of approximately 9 under 80 V bias.
Background Projection: The Monte Carlo simulations predict a background index in a 100-keV-wide interval centered at the $Q$ -value of $^{100}\text{Mo}$ (3034 keV) of $3.2(5) \times 10^{-3}$ counts/keV/kg/yr. The dominant background source is identified as muon-induced events (approx. 80% of the total), followed by contributions from the cryostat screens and electronic pins.
Sensitivity Estimates:
- Assuming the projected background index of $3.2(5) \times 10^{-3}$ counts/keV/kg/yr, a duty cycle of 90%, and an 18% dead time from the muon veto, the experiment is projected to reach a world-leading sensitivity to $^{100}\text{Mo}$ $0\nu2\beta$ decay ( $T_{1/2} \sim 4 \times 10^{24}$ yr) within 1 year of data taking.
- The authors present conservative scenarios accounting for potential unpredictable radioactive contamination or performance degradation. Even with a background index 3 times worse ( $1 \times 10^{-2}$ counts/keV/kg/yr), the experiment could reach world-best sensitivity in 1.5 years. In a worst-case scenario (10 times worse, $5 \times 10^{-2}$ counts/keV/kg/yr), a 2-year operation would still yield competitive sensitivity.

Significance
The paper establishes the CROSS experiment as a viable and highly sensitive next-generation search for $0\nu2\beta$ decay in $^{100}\text{Mo}$ . By successfully transitioning from surface-coated crystals to a paired crystal-light detector architecture, the collaboration has demonstrated a scalable solution that maintains high radiopurity and particle identification capabilities. The projected background levels are comparable to or better than current leading experiments, positioning CROSS to set the most stringent limits on the $^{100}\text{Mo}$ $0\nu2\beta$ half-life in the near term. The work also validates the mechanical and assembly techniques (low-mass structures, robotic gluing) intended for future large-scale experiments like CUPID-1T. Regular physics data taking began in April 2026, marking the transition from R&D to full-scale physics operation.

The Great Atomic Ghost Hunt

The CROSS Experiment: Building a Super-Sensitive Ear

What They Found (and What They Predict)

The Bottom Line

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