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A study of neutrinoless double electron capture in $^{40}$ Ca from the AMoRE experiment

The AMoRE-I experiment analyzed 7.32 kg $\cdot$ yr of exposure from thirteen $^{40}$ Ca $^{100}$ MoO $_4$ crystals to search for neutrinoless double electron capture in $^{40}$ Ca, finding no significant signal and setting a lower half-life limit of $1.7 \times 10^{22}$ years at 90% confidence level.

Original authors: AMoRE Collaboration, A. Agrawal, V. V. Alenkov, P. Aryal, J. Beyer, B. Bhandari, R. S. Boiko, K. Boonin, O. Buzanov, C. R. Byeon, N. Chanthima, M. K. Cheoun, J. S. Choe, Seonho Choi, S. Choudhury, J.

Published 2026-07-10

📖 3 min read🧠 Deep dive

CC BY 4.0

Original authors: AMoRE Collaboration, A. Agrawal, V. V. Alenkov, P. Aryal, J. Beyer, B. Bhandari, R. S. Boiko, K. Boonin, O. Buzanov, C. R. Byeon, N. Chanthima, M. K. Cheoun, J. S. Choe, Seonho Choi, S. Choudhury, J. S. Chung, F. A. Danevich, M. Djamal, D. Drung, C. Enss, A. Fleischmann, A. M. Gangapshev, L. Gastaldo, Y. M. Gavrilyuk, A. M. Gezhaev, O. Gileva, V. D. Grigorieva, V. I. Gurentsov, C. Ha, D. H. Ha, E. J. Ha, D. H. Hwang, E. J. Jeon, J. A. Jeon, H. S. Jo, J. Kaewkhao, C. S. Kang, W. G. Kang, V. V. Kazalov, S. Kempf, A. Khan, S. Khan, D. Y. Kim, G. W. Kim, H. B. Kim, Ho-Jong Kim, H. J. Kim, H. L. Kim, H. S. Kim, M. B. Kim, S. C. Kim, S. K. Kim, S. R. Kim, W. T. Kim, Y. D. Kim, Y. H. Kim, K. Kirdsiri, Y. J. Ko, V. V. Kobychev, V. Kornoukhov, V. V. Kuzminov, D. H. Kwon, C. H. Lee, DongYeup Lee, E. K. Lee, H. J. Lee, H. S. Lee, J. Lee, J. Y. Lee, K. B. Lee, M. H. Lee, M. K. Lee, S. W. Lee, Y. C. Lee, D. S. Leonard, H. S. Lim, B. Mailyan, E. P. Makarov, P. B. Nyanda, Y. Oh, S. L. Olsen, S. I. Panasenko, H. K. Park, H. S. Park, K. S. Park, S. Y. Park, O. G. Polischuk, H. Prihtiadi, S. Ra, S. S. Ratkevich, G. Rooh, E. Sala, M. B. Sari, J. Seo, K. M. Seo, B. Sharma, K. A. Shin, V. N. Shlegel, K. Siyeon, J. So, N. V. Sokur, J. K. Son, J. W. Song, N. Srisittipokakun, V. I. Tretyak, R. Wirawan, K. R. Woo, H. J. Yeon, Y. S. Yoon, Q. Yue

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

Imagine the universe is a giant, quiet library where particles are the books. Most of the time, these books follow strict rules: you can't just make a book disappear without a trace, and you can't create one out of thin air. But physicists have a wild theory that sometimes, a very special kind of "ghost particle" called a neutrino might be its own twin, a Majorana particle. If this is true, it would break the library's most sacred rule: the conservation of lepton number.

To catch this rule-breaker in the act, a team of scientists called the AMoRE collaboration built a super-sensitive trap deep underground, under 700 meters of rock in Yangyang, South Korea. They didn't use a net; they used a set of thirteen giant, super-cold crystals made of calcium molybdate (CaMoO₄). Think of these crystals as ultra-quiet, frozen ears waiting to hear a whisper from the atomic world.

Their target was a specific atom, Calcium-40. Usually, atoms are stable, but sometimes, two electrons inside the atom's nucleus can decide to vanish at the exact same time. This is called "double electron capture." If neutrinos are their own twins, this event could happen without sending out any neutrinos at all. It would be like two people sneaking out of a room without making a sound, leaving behind only a tiny, specific "thump" of energy.

The scientists ran their experiment for a long time, collecting data equivalent to 7.32 kilogram-years of exposure. They were looking for a very specific "thump" at an energy level of 193.5 keV. It's like listening for a single, perfect note in a noisy room.

The Big Reveal:
After all that listening, the crystals stayed silent. They didn't hear the special "thump" of the neutrinoless double electron capture. The team found no evidence that this specific event happened during their observation time. Instead of catching the event, they set a new "speed limit" for how fast it could be happening. They calculated that if this event does occur, it must be incredibly rare: it happens less than once every 1.7 × 10²² years. To put that in perspective, that number is so huge it makes the age of the universe look like a blink of an eye. This new limit is slightly stricter than what previous experiments using different crystals (CaWO₄) had found, meaning the AMoRE team has tightened the noose around this mystery.

What About the Future?
The paper doesn't say the search is over; it just says this round of the game didn't yield a winner. The team is already planning a bigger, better version called AMoRE-II. They expect to use more crystals and have a much quieter background (less noise in the library). Based on computer simulations, they predict that in the next phase, they could push that "speed limit" down even further, to about 9 × 10²² years.

They also noted that their current equipment wasn't sensitive enough to hear an even quieter whisper: a process involving two neutrinos (2ν2K) that happens at a much lower energy of 6.4 keV. That's a job for the future, improved detectors.

For now, the main takeaway is a solid "no" for this specific experiment. They didn't find the neutrinoless double electron capture in Calcium-40, but by proving it doesn't happen that often, they've helped map out the boundaries of our universe's rulebook a little more clearly. The search continues, but for now, the Calcium atoms are keeping their secrets.

Technical Summary: A study of neutrinoless double electron capture in 40Ca from the AMoRE experiment

Problem Statement
The search for neutrinoless double electron capture ( $0\nu2EC$ ) serves as a critical probe for lepton-number violation and the Majorana nature of neutrinos. While the primary physics goal of the AMoRE (Advanced Mo-based Rare process Experiment) collaboration is the search for neutrinoless double beta decay ( $0\nu\beta\beta$ ) of $^{100}$ Mo, the experiment utilizes Calcium Molybdate ( $^{40}$ Ca $^{100}$ MoO $_4$ or CMO) crystals. These crystals contain $^{40}$ Ca as a naturally occurring isotope with a high isotopic concentration ( $\delta \approx 96.94\%$ ). Although the decay energy for $^{40}$ Ca $0\nu2EC$ ( $Q_{2EC} = 193.50$ keV) does not favor resonant enhancement, the experiment is well-suited to search for this rare process. Previous limits on this decay were set by the CRESST collaboration using CaWO $_4$ bolometers. This paper presents an investigation into the $0\nu2EC$ decay of $^{40}$ Ca using data from the AMoRE-I phase.

Methodology
The analysis utilizes a physics dataset from the AMoRE-I experiment, conducted at the Yangyang Underground Laboratory (700 m rock overburden) between December 2020 and May 2023. The dataset corresponds to a total exposure of 7.32 kg·yr from thirteen CMO crystals, yielding a specific $^{40}$ Ca isotopic exposure of 1.43 kg·yr.

Detector System: The experiment employs cryogenic detectors equipped with metallic magnetic calorimeters (MMCs) for simultaneous phonon and photon readout. This dual-readout scheme allows for particle identification to reject $\alpha$ -decay backgrounds.
Data Processing: The data was reanalyzed with optimized event triggers to lower the energy threshold, optimal filtering for signal amplitude, and corrections for signal instabilities. The dataset was divided into two periods: "before-power-outage" (BPO, Dec 2020–Nov 2021) and "after-power-outage" (APO, Jan 2022–May 2023). The power outage incidentally led to improved energy resolution in most detectors and the activation of a previously unresponsive module (CMO 03).
Energy Calibration and Resolution: Calibration utilized $\gamma$ -ray peaks from a $^{232}$ Th source (583, 911, 969, 2615 keV) and a 75 keV Pb X-ray peak. Peak shapes were fitted using the Bukin function to account for asymmetric tails. The Full Width at Half Maximum (FWHM) energy resolution at the region of interest (ROI) was modeled, with most detectors achieving 2.5–4 keV FWHM in the APO dataset.
Signal Modeling: The $0\nu2EC$ process in $^{40}$ Ca proceeds via atomic capture (dominantly $f_{KK} \approx 85\%$ , $f_{KL} \approx 14\%$ ). For ground-to-ground state transitions, the KK channel proceeds via Internal Conversion (IC) electrons, while KL channels may emit real photons or IC electrons. GEANT4 Monte Carlo simulations were used to determine containment efficiencies ( $\varepsilon_{cont} \approx 0.91–0.93$ ) and total detection efficiencies ( $\varepsilon \approx 0.86–0.92$ ), accounting for the lower density of CMO compared to CaWO $_4$ .
Statistical Analysis: A binned-extended likelihood fit was performed on the energy spectrum within a $\pm 12\sigma$ window around $Q_{2EC} = 193.5$ keV. The background model included a linear component and a peak at 186 keV (attributed to $^{226}$ Ra/ $^{235}$ U). The signal was modeled using the Bukin probability density function. Systematic uncertainties were constrained via Gaussian terms.

Key Contributions

First AMoRE-I Limit on $^{40}$ Ca: The paper establishes the first lower limit on the half-life of $^{40}$ Ca $0\nu2EC$ using CMO crystals, demonstrating the capability of the AMoRE detector array to search for rare processes beyond its primary $^{100}$ Mo $0\nu\beta\beta$ program.
Improved Sensitivity: The analysis refines the energy scale reconstruction and utilizes a larger dataset with improved energy resolution (post-power outage) compared to previous studies.
Background Characterization: The study details the background composition in the ROI, identifying the 186 keV peak and quantifying the background index for individual detectors, noting that temporary aluminum shielding in the BPO period contributed to higher background levels.

Results

Observation: No significant excess of events was observed in the region of interest (193.5 keV). The best-fit decay rate ( $\Gamma_{best} \sim 1.2 \times 10^{-23}$ yr $^{-1}$ ) is attributed to statistical fluctuations in the background.
Half-Life Limit: At a 90% confidence level (CL), the experiment sets a lower limit on the half-life of:
$T_{1/2}^{0\nu} > 1.7 \times 10^{22} \text{ yr}$
This corresponds to an upper limit on the decay rate of $\Gamma_{90} = 4.1 \times 10^{-23}$ yr $^{-1}$ .
Comparison: This result represents a slight improvement over the previous limit of $T_{1/2}^{0\nu} > 1.4 \times 10^{22}$ yr set by the CRESST collaboration using CaWO $_4$ bolometers.
Sensitivity Projections: The median expected sensitivity for the current dataset is $T_{1/2}^{0\nu} = 2.0^{+0.8}_{-0.6} \times 10^{22}$ yr.

Significance and Future Outlook
The paper concludes that the AMoRE experiment successfully demonstrates the potential of CaMoO $_4$ detectors to explore rare decay processes. While the current AMoRE-I dataset does not reach the low energy threshold (6.4 keV) required to search for the two-neutrino double K-capture ( $2\nu2K$ ) process in $^{40}$ Ca, the results validate the detector technology for such searches.

Looking toward the upcoming AMoRE-II experiment, which will utilize 90 kg of $^{100}$ Mo and retain at least 13 CMO crystals, the authors project a significant improvement in sensitivity. Assuming a 4.6 kg CMO mass, 5 years of live time, and reduced background levels, the expected median exclusion sensitivity is projected to reach $T_{1/2}^{0\nu} \sim 9 \times 10^{22}$ yr at 90% CL. This represents an improvement of approximately a factor of five over the current AMoRE-I limit, driven by a fivefold reduction in background and a threefold increase in exposure. These findings highlight the broader physics potential of the AMoRE project, extending from few-keV rare-event searches to MeV-scale probes of physics beyond the Standard Model.

Technical Summary: A study of neutrinoless double electron capture in 40Ca from the AMoRE experiment

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