The Surukuchi Lab is helping solve mysteries of the universe with CUORE

The coldest cubic meter in the universe is the Cryogenic Underground Observatory for Rare Events (CUORE).

A group of researchers from the University of Pittsburgh - led by Pranava Teja Surukuchi, assistant professor in the Kenneth P. Dietrich School of Arts and Sciences' Department of Physics and Astronomy - has been working with CUORE's multi-institutional, international research team on this chilly nuclear physics experiment. They're seeking events with a specific energy value, indicated by a well-defined change in temperature change, resulting from a never-before-seen process called "neutrinoless double beta decay." If observed, this could explain one of the biggest mysteries in the universe, namely why it is full of matter.

With the largest dataset of its kind, double the amount previously published, the CUORE collaboration restricted how often neutrinoless double beta decay occurs in an atom of tellurium: on average, no more than once every 50 septillion - that's a trillion trillion - years.

If matter and antimatter were created in equal amounts during the Big Bang, they should have annihilated each other and left us with an empty universe. But if neutrinos can also act as antineutrinos, they could have potentially tipped the scales in favor of matter.

"If we see neutrinoless double beta decay for the first time, that would be a monumental discovery showing that neutrinos can act as their own antiparticles," said Carlo Bucci, head spokesperson for CUORE and a physicist at Gran Sasso National Laboratory, where the experiment is operated. "That, in turn, could help explain why the universe evolved into the form we see today."

The CUORE detector uses 988 crystals of tellurium dioxide cooled to 10 millikelvin, or about -459 degrees Fahrenheit. If a nucleus in one of the crystals experiences the sought-after particle decay, the crystal observes a distinctive temperature change. These ultra-sensitive detectors, called bolometers, have a strong Pitt connection-they were invented in late 19th century at the Allegheny Observatory by Samuel Pierpont Langley, who used them to measure the Moon's temperature and map the Sun's spectrum, a remarkable feat for the late 19th century. Since then, they've become indispensable in physics, helping scientists explore everything from the cosmic microwave background to quantum materials.

The extreme rarity of the decay process demands strict reduction of external interferences that could mimic the neutrinoless double beta decay or impair detector performance. Some reduction is handled by the location. CUORE sits beneath nearly a mile of rock that blocks particles coming from space. However, CUORE also records other activity: the sounds of scientists talking, the pulse of waves crashing on the shore 50 kilometers away, and earthquakes on the other side of the world.

CUORE's latest data releaseuses a new algorithm to clean their data that is similar to what happens in noise-canceling headphones. "Instead of the handful of microphones you'd find on a headset, CUORE - and the area around the detector - has more than two dozen different sensors that measure temperature, sound, vibration and electrical interference," said Bicocca University of Milano postdoc Simone Quitadamo, who studied how sea waves show up in the detector while a graduate student at the Gran Sasso Science Institute. Among these "witness" sensors are microphones to pick up noise from researchers talking near the detector, accelerometers to monitor vibrations from pumps cooling the experiment and seismometers to pick up low-frequency movements like earthquakes.

Scientists matched information from the sensors with the recorded data, learning which activity in the detector should be ignored. The new algorithm was applied to CUORE's previously collected data and can also be used for future datasets.

The algorithm is just one part of the process. To search for neutrinoless double beta decay, temperature fluctuations measured by nearly 1,000 detectors over more than five years had to be detected, converted into energy and then combined across all the crystals for the entire period. That's a challenge, because each detector responds slightly differently to the same energy.

Surukuchi co-led a team of more than 20 researchers to carry out this analysis and served as one of three editors for the final publication in Science. And Vivek Sharma, now a postdoctoral researcher with Surukuchi, improved one of the software programs used to process the data during his time as a graduate student at Virginia Tech.

The data analysis methods used in this search can be applied to many other areas of physics as well. "The techniques we have developed to subtract the noise could be useful for other sensitive detectors, including experiments studying dark matter and gravitational waves," said Chiara Brofferio, a professor at the University of Milano-Bicocca and the Italian spokesperson for CUORE.

The approach will also be useful for CUORE Upgrade with Particle Identification (CUPID), the next-generation experiment that could follow CUORE and continue the hunt for neutrinoless double beta decay.

Surukuchi's group is currently focused on further improving noise reduction to help CUPID reach its full potential. Tristan Hurst, a graduate student in the Surukuchi Lab is working with Sharma on adapting accelerometers to operate at extremely low temperatures. These accelerometers are being investigated for use in CUPID as witness sensors, measuring vibrations closer to the crystals to improve signal matching. Sharma is also developing machine learning software to further enhance noise reduction.

In addition to neutrinos and dark matter, CUORE's sensitivity means it can also be used to study the physics of Earth itself. Researchers are already collaborating with geophysicists to understand how windstorms in Italy can affect the underground environment for sensitive detectors.

Megan Gibbons, an undergraduate in the Dietrich School, is studying how CUORE crystals respond to earthquakes in the Surukuchi Lab. The research aims to use earthquakes to better understand the crystals' response to external vibrations and explore how seismic waves travel both on the Earth's surface and through its interior.

"We built this to be a particle detector, but it's also an amazing seismometer," said Yury Kolomensky, a scientist at Berkeley Lab and professor at UC Berkeley. "One vision is to have technology like CUPID and CUORE deployed in multiple locations around the world that could study the propagation of earthquakes and the Earth's core as well as dark matter and neutrinoless double beta decay. We're seeing how these very cool techniques for our detector can have implications for other fields of science."

Surukuchi's lab is centered around studying neutrinos, which were proven to be massless only about 25 years ago. "So far, we've only determined that their masses are incredibly small compared to all other known particles." he said.

CUORE is one path to learning more about these subatomic particles. "There are a couple of reasons confirming neutrinoless double beta decay would be impactful." he said. "First, it would mean that, as their unusually small mass suggests, neutrinos truly are different from any other known fundamental particle."

"What's even more exciting is that if neutrinoless double beta decay does take place, it produces two matter particles without producing any antimatter." And while other factors need to be in place, "this could help explain one of the most fundamental questions: Why is there more matter than antimatter in the universe?"