The search for neutrinoless double beta decay gets some noise cancelling headphones

Deep under a mountain in Italy, researchers continue to push the boundaries of science with an experiment that could rewrite the Standard Model of Particle Physics.

Their experiment, known as the Cryogenic Underground Observatory for Rare Events (CUORE), which includes researchers from Yale, has now collected two ton-years of data (the equivalent of collecting data for two years if the cube-shaped crystals in the CUORE detector weighed one ton) in a years-long effort to document a theory of rare nuclear particle decay called neutrinoless double beta decay.

Standard double beta decay is already a proven particle process. When it occurs, two neutrons, which are uncharged particles in the nucleus of an atom, transform into two protons and emit two electrons and two antineutrinos. Antineutrinos are the antimatter counterpart to neutrinos.

Neutrinoless double-beta decay is a theorized process in which no antineutrinos are created. According to the theory, this would prove that neutrinos and antineutrinos are the same—that a neutrino is its own antiparticle.

In a new study in the journal Science, CUORE researchers used their latest dataset to place new limits on how often neutrinoless double beta decay occurs in an atom of tellurium. They say it occurs no more than once every 50 septillion years—or once every trillion trillion years.

Aiding the researchers’ work was a specially designed algorithm that filtered out extraneous “noise”—i.e., vibrations, including the muffled sounds of researchers talking nearby, ocean waves hitting the Italian coast, and earthquakes sending out seismic shockwaves anywhere in the world. Think noise-cancelling headphones, but on a much larger scale.

“The focus of this data release is understanding sources of external vibrations and learning how to subtract that from our data to better search for this extremely rare decay,” said Reina Maruyama, a professor of physics and astronomy in the Yale Faculty of Arts and Sciences (FAS) and CUORE member.

The CUORE site is located in the Gran Sasso National Laboratory in central Italy. The laboratory sits beneath nearly a mile of rock and is surrounded by low-radiation shielding made from lead ingots salvaged from a 2,000-year-old Roman shipwreck.

Even with its noise-blocking setting and shielding, a certain amount of vibrational noise gets through—prompting CUORE’s new noise-cancelling measures.

As part of their ongoing research, the research team installed more than two dozen sensors that measure temperature, sound, vibration, and electrical interference near the detector. Scientists matched information from the sensors with the recorded data, learning which activity in the detector was “noise” and should be ignored. The new algorithm was applied to CUORE’s previously collected and new data.

The experiment, led by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and featuring more than 20 research institutions, including Yale, began operations in 2017, after many years of planning and development.

CUORE will continue its data collection through the rest of this year. Its successor, the CUORE Upgrade with Particle Identification (CUPID), will then take over the search for neutrinoless double beta decay at the same location.

The CUPID team will add enhanced light sensors to the experiment’s thermal detectors to improve event identification and background discrimination. It will also use enriched molybdenum crystals in place of tellurium.

“Detecting neutrinoless double beta decay would reveal that neutrinos are their own antiparticles, known as Majorana particles,” said Karsten Heeger, the Eugene Higgins Professor of Physics in Yale FAS, director of Yale’s Wright Laboratory, and a recent international spokesperson for the CUPID experiment.

“This unique nature of neutrinos may explain the matter-antimatter asymmetry in the universe—the fact that there is more matter than antimatter,” Heeger added. “It would also violate a fundamental principle of the Standard Model of Particle Physics called the lepton number and provide unambiguous evidence for new physics.”

Yale researchers lead several efforts in CUORE and CUPID, including the development of low-energy analysis and the reduction of muon-induced backgrounds. In addition to Maruyama and Heeger, Yale researchers involved in the experiments are research scientist Penny Slocum, postdoctoral researcher Tyler Johnson, and engineer James Wilhelmi, all in the Department of Physics; graduate students Ridge Liu, Maya Moore, and Zach Muraskin, all in the Yale Graduate School of Arts and Sciences; and former graduate student Samantha Pagan.