In a laboratory under a mountain, scientists are studying ethereal particles with crystals much colder than frozen air, hoping to unearth secrets from the birth of the universe. This week, researchers at the Cryogenic Underground Observatory for Rare Events (CUORE) reported that they have set some of the most strict limitations yet on the unusual possibility that the neutrino is its own antiparticle. Neutrinos are very uncommon particles that are so ethereal and pervasive that they travel through our bodies without our knowledge. CUORE has been patiently waiting for three years for evidence of a separate nuclear decay mechanism, which is only feasible if neutrinos and antineutrinos are the same particle. According to CUORE’s new statistics, this deterioration does not occur until billions of trillions of years, if at all. The restrictions imposed by CUORE on the behavior of these small phantoms are an important aspect of the hunt for the next breakthrough in particle and nuclear physics—as well as the search for our own beginnings.

“Ultimately, we’re seeking to comprehend matter creation,” said Carlo Bucci, CUORE’s spokesman and researcher at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. “We’re searching for a mechanism that breaks a basic symmetry of nature,” said Roger Huang, a postdoctoral researcher at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and one of the study’s primary authors.

CUORE—Italian for “heart”—is one of the world’s most sensitive neutrino experiments. The latest CUORE findings are based on a data collection 10 times bigger than any prior high-resolution search in the past three years. CUORE is run by a multinational research cooperation directed by Italy’s Istituto Nazionale di Fisica Nucleare (INFN) and the United States’ Berkeley Lab. The CUORE detector is housed under almost a mile of solid rock at LNGS, an INFN facility. Nuclear physicists financed by the US Department of Energy play a key scientific and technical role in this endeavor. The latest CUORE findings were published in Nature today.

Strange particles

Neutrinos are everywhere around us—trillions of neutrinos are passing through your thumbnail as you read this phrase. They are invisible to the universe’s two biggest forces, electromagnetism and the strong nuclear force, allowing them to travel straight through you, the Earth, and practically everything else without interfering. Despite their immense numbers, their cryptic nature makes them very difficult to investigate, and has kept physicists perplexed since they were initially proposed over 90 years ago. Until the late 1990s, no one knew if neutrinos had any mass at all—as it turns out, they do, but not very much.

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One of the many unanswered problems surrounding neutrinos is whether they are antiparticles in their own right. Antiparticles, or antimatter counterparts, exist for all particles: electrons have antielectrons (positrons), quarks have antiquarks, and neutrons and protons (which make up the nucleus of atoms) have antineutrons and antiprotons. However, unlike all of other particles, neutrinos have the potential to be their own antiparticles. Majorana fermions are particles that are their own antiparticles, which were proposed by the Italian scientist Ettore Majorana in 1937.

If neutrinos are Majorana fermions, it might solve a fundamental puzzle at the heart of our existence: why is there so much more matter in the cosmos than antimatter. Neutrinos and electrons are both types of leptons, which are basic particles. One of nature’s basic principles seems to be that the number of leptons is always conserved—if a process produces a lepton, it must also produce an anti-lepton to balance it out. Similarly, particles like as protons and neutrons are referred to as baryons, and the number of baryons seems to be preserved. However, if baryon and lepton quantities were constantly preserved, there would be precisely as much matter as antimatter in the cosmos—and in the early beginning, matter and antimatter would have collided and destroyed, and humans would not exist. Something has to go against the precise conservation of baryons and leptons. Enter the neutrino: if neutrinos are their own antiparticles, the lepton number does not need to be preserved, and our existence becomes much less mysterious.

“The universe’s matter-antimatter imbalance remains unexplained,” Huang remarked. “It might help explain it if neutrinos are their own antiparticles.”

This isn’t the only question that a Majorana neutrino may answer. The tremendous lightness of neutrinos, which are nearly a million times lighter than electrons, has long perplexed particle scientists. However, if neutrinos are their own antiparticles, an existing solution known as the “seesaw mechanism” might explain neutrinos’ lightness in a simple and natural manner.

A one-of-a-kind gadget for one-of-a-kind decays

However, identifying whether neutrinos are their own antiparticles is challenging since they seldom interact. The finest instrument physicists have for searching for Majorana neutrinos is a hypothetical kind of radioactive decay known as neutrinoless double beta decay. In certain atoms, beta decay converts a neutron in the nucleus into a proton, altering the chemical element of the atom and releasing an electron and an anti-neutrino in the process. Double beta decay is more uncommon: instead of one neutron becoming a proton, two neutrons do so, generating two electrons and two anti-neutrinos in the process. However, if the neutrino is a Majorana fermion, then a single “virtual” neutrino, operating as its own antiparticle, may possibly replace both anti-neutrinos in double beta decay. Only two electrons would escape from the atomic nucleus. For decades, neutrinoless double-beta decay has been postulated, but it has never been seen.

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The CUORE experiment went to tremendous efforts to capture tellurium atoms in the process of decay. Nearly a thousand exceedingly pure crystals of tellurium oxide weighing over 700 kg are used in the experiment. This much tellurium is required because typical double beta decay of a single unstable atom of tellurium takes billions of times longer than the present age of the universe. However, since there are billions of trillions of tellurium atoms in each of the crystals used by CUORE, conventional double beta decay occurs very often in the detector, about a few times per day in each crystal. Neutrinoless double beta decay is extremely rarer, thus the CUORE team must work hard to eliminate as many sources of background radiation as possible. To protect the detector from cosmic rays, the complete system is housed under the Gran Sasso mountain, the biggest on the Italian peninsula. Several tons of lead give further protection. However, owing to contamination by uranium and other elements, newly extracted lead is mildly radioactive, with that radioactivity diminishing with time—so the lead used to surround the most sensitive component of CUORE is primarily lead retrieved from a sunken ancient Roman ship approximately 2000 years old.

The cryostat, which keeps the detector cold, is perhaps the most astounding piece of equipment employed at CUORE. To detect neutrinoless double beta decay, the temperature of each crystal in the CUORE detector is meticulously monitored using sensors capable of detecting temperature changes as tiny as one tenth of a degree Celsius. The energy signature of neutrinoless double beta decay is well-defined and unmistakable, and it would elevate the temperature of a single crystal by a well-defined and recognizable amount. To maintain such sensitivity, the detector must be kept extremely cold—specifically, approximately 10 mK, or a tenth of a degree above absolute zero. “This is the coldest cubic meter in the known cosmos,” stated Laura Marini, CUORE’s Run Coordinator and a research fellow at Gran Sasso Science Institute. The detector’s sensitivity as a consequence is simply astounding. “We really got glimpses of it in our detector when there were huge earthquakes in Chile and New Zealand,” Marini added. “We can also see waves smashing on the Adriatic Sea’s shoreline from 60 kilometers distant. When there are storms in the winter, that signal becomes stronger.”

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A neutrino passing through the heart

Despite its incredible sensitivity, CUORE has failed to detect neutrinoless double beta decay. Instead, CUORE has determined that this decay occurs in a single tellurium atom little more than once every 22 trillion trillion years on average. “If observed, neutrinoless double beta decay will be the most rare process ever observed in nature, with a half-life more than a million billion billion times longer than the age of the universe,” said Danielle Speller, Assistant Professor at Johns Hopkins University and member of the CUORE Physics Board. “Even if this deterioration occurs, CUORE may not be sensitive enough to identify it, thus it’s necessary to verify. Sometimes physics produces unexpected consequences, and it is at these moments that we learn the most.” Even if CUORE does not discover evidence of neutrinoless double-beta decay, it is setting the road for future investigations. The CUORE Upgrade with Particle Identification (CUPID), CUORE’s successor, is already in the works. CUPID will be almost ten times more sensitive than CUORE, perhaps detecting evidence of a Majorana neutrino.

CUORE, on the other hand, is a scientific and technical accomplishment, not only for its new limitations on the rate of neutrinoless double beta decay, but also for its demonstration of cryostat technology. “It’s the world’s biggest refrigerator of its sort,” said Paolo Gorla, an LNGS staff scientist and CUORE’s Technical Coordinator. “And it’s remained stable at 10 mK for almost three years now.” Such technology has far-reaching applications that go beyond basic particle physics. It might be used in quantum computing, where one of the key technical hurdles is keeping massive quantities of equipment cool enough and insulated from ambient radiation to control on a quantum level.

Meanwhile, CUORE is far from finished. “We’ll be open till 2024,” Bucci remarked. “I’m looking forward to seeing what we discover.”

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