The detection of marine microseismic activity with the CUORE tonne-scale cryogenic experiment

This paper reports the first detection of marine microseismic vibrations using mK-scale calorimeters in the CUORE experiment, demonstrating their seasonal impact on detector performance and validating a noise decorrelation algorithm to mitigate such environmental noise for future rare-event searches.

The Gist

Imagine you are trying to hear a single, tiny whisper in a room that is supposed to be perfectly silent. That is what the CUORE experiment is trying to do. Located deep underground in Italy, CUORE is a giant, super-cold machine designed to listen for the faintest "whispers" of the universe—specifically, a rare nuclear event called "neutrinoless double beta decay." If they can hear this whisper, it would solve some of the biggest mysteries about how the universe works.

However, there is a problem: the machine is so sensitive that it can hear things that aren't whispers at all. It can hear the hum of the refrigerator, the footsteps of a scientist, and even the vibrations of the Earth itself.

The "Ocean's Footsteps"

In this study, the scientists discovered something surprising: the Mediterranean Sea is tapping on the door of their experiment.

Even though the lab is 1,400 meters underground, the ocean is still making noise. When waves crash against the coast of Italy, they create tiny, invisible vibrations called "microseisms." These vibrations travel through the rock and up into the lab, shaking the delicate equipment.

The researchers found a direct link between the weather and the machine's performance:

• In the Summer: The Mediterranean is calm. The sea is quiet, the vibrations are low, and the CUORE machine hears very clearly.
• In the Winter: Storms rage on the sea. The waves are huge and violent. This creates a "rumble" that travels underground. When this happens, the CUORE machine gets "noisy," and its ability to hear the faint whispers of physics gets worse.

Think of it like trying to listen to a radio station while someone is stomping on the floor above you. In the winter, the "stomping" (the storms) is so loud that the radio signal gets fuzzy. The study showed that during these stormy periods, the machine's ability to measure energy accurately dropped by as much as 40%.

The "Noise-Canceling Headphones"

Since they can't stop the ocean from making noise, the scientists had to get creative. They built a digital "noise-canceling" system for their data.

Here is how they did it:

1. The Sensors: They installed extra sensors around the machine, including seismometers (which feel the ground shake) and accelerometers (which feel movement). These act like "ears" specifically tuned to listen to the ocean's vibrations.
2. The Algorithm: They wrote a computer program that looks at what the "ocean ears" are hearing and compares it to what the main machine is hearing.
3. The Magic: The computer figures out exactly how much of the main machine's noise is coming from the ocean. It then subtracts that specific noise from the data, like a noise-canceling headphone cancelling out the sound of an airplane engine.

The Result: This trick worked incredibly well. By using this method, they reduced the total vibration noise by 74%. It's as if they turned down the volume on the ocean's stomping, allowing the machine to hear the faint whispers of the universe much more clearly.

Why This Matters

The paper concludes that even the most advanced, super-sensitive experiments are still being influenced by the natural world around them. By understanding that the ocean is "talking" to their machine, and by building a way to "tune out" that conversation, they have made their experiment much better.

This isn't just about one experiment; it's a lesson for all future physics experiments. If you want to hear the faintest signals in the universe, you have to learn how to ignore the noise of the Earth itself.

Technical Summary

Technical Summary: Detection of Marine Microseismic Activity with the CUORE Experiment

Problem Statement
Low-temperature calorimeters operating at the millikelvin (mK) scale are critical tools for searching for rare physics events, such as neutrinoless double beta ($0\nu\beta\beta$) decay and dark matter interactions. However, their sensitivity is frequently compromised by environmental noise, particularly vibrational perturbations. Even minute power depositions (~1–10 fW) from extrinsic vibrations can induce transient noise signals. In the CUORE experiment, which utilizes a tonne-scale array of $^{130}$TeO$_2$ crystals, high-impedance calorimeters are highly sensitive to noise in the sub-Hz regime. While passive and active mitigation strategies (such as mechanical decoupling and noise cancellation for pulse tube harmonics) have been implemented, the impact of faint, persistent environmental noise sources—specifically marine microseisms induced by sea swell motion—remained unquantified in this context.

Methodology
The study employs a multi-device correlation analysis spanning four years of data (January 2019 to April 2023) to investigate the coupling between marine activity and CUORE's performance. The methodology integrates three distinct data streams:

1. Marine Data: Spectral significant wave height ($VHM0$) data from the Copernicus Marine Environment Monitoring Service (CMEMS) for the Adriatic and Tyrrhenian Seas.
2. Seismic Data: Measurements from seismometers installed at the Gran Sasso National Laboratories (LNGS), approximately 50–150 km from the coastlines.
3. Calorimetric Data: Performance metrics from the full array of 988 CUORE detectors, specifically focusing on baseline energy resolution ($FWHM_{baseline}$) and low-energy mass exposure.

The analysis correlates the seasonal modulation of Mediterranean Sea activity with CUORE's energy resolution and low-energy thresholds. Furthermore, the paper evaluates a noise decorrelation (denoising) algorithm. This algorithm utilizes auxiliary sensors (seismometers, accelerometers, and microphones) to construct transfer functions in the frequency domain. By predicting the detector's response to mechanical vibrations based on auxiliary inputs, the algorithm subtracts the predicted noise from the raw calorimeter signals during post-processing.

Key Results

• Detection of Marine Microseisms: The study confirms a direct correlation between Mediterranean Sea storm activity and the microseismic noise recorded at LNGS. During storm outbreaks (lasting ~1–2 weeks), the correlation between sea wave amplitude and seismic activity is high, leading to a measurable degradation in CUORE's energy resolution (up to ~40% worsening during storms).
• Seasonal Modulation: A distinct seasonal pattern was identified. Winter months, characterized by more frequent and intense storms, result in higher microseismic noise. This causes:
  • A reduction in the low-energy mass exposure (detectors with thresholds <10 keV drops from ~76% in summer to ~48% in winter).
  • A degradation in the energy resolution at the 2615 keV $^{208}$Tl $\gamma$-ray peak, with variations exceeding 1 keV near the $0\nu\beta\beta$ Q-value (2527.5 keV).
  • An estimated sensitivity loss for $0\nu\beta\beta$ decay searches of $\gtrsim 4.3\%$ when comparing ideal summer conditions to the seasonally modulated reality.
• Denoising Efficacy: The implementation of the noise decorrelation algorithm, using auxiliary sensor data, significantly improved detector performance.
  • Total noise power was reduced by 74% across the detector array.
  • Noise in the [0.1, 1] Hz frequency interval was reduced by 56%, with specific reductions at peaks correlated with sea wave activity (0.6 Hz and 0.9 Hz).
  • The baseline amplitude resolution improved by a mean of 11.9% in 94% of the analyzed detectors, effectively lowering the energy threshold for the majority of channels.

Significance and Claims
The paper claims that the detection of marine microseisms via mK-scale calorimeters underscores the necessity of addressing faint environmental noise in ultra-sensitive experiments. The primary significance lies in demonstrating that even remote environmental phenomena (sea swells) can couple to underground detectors and measurably impact physics sensitivity.

The authors assert that understanding these noise coupling mechanisms and developing mitigation strategies, such as the demonstrated denoising algorithm, is essential for next-generation experiments. The work suggests that identifying unresolved noise sources and advancing suppression systems will directly inform the design of future rare-event searches, including the upcoming CUPID experiment. The study does not claim to have eliminated all noise but rather provides a validated method to reduce vibrational noise and improve the stability and resolution of low-temperature calorimeters, thereby enhancing their sensitivity to low-energy signatures.