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New research imposes new constraints on neutrinos’ strange behavior


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. Last week, researchers at the Cryogenic Underground Observatory for Rare Events (CUORE) reported that they have set some of the strictest 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.

“We want to understand how matter is created,” said Carlo Bucci, CUORE’s spokesperson and researcher at Italy’s Laboratori Nazionali del Gran Sasso. “We’re looking for a process that violates a fundamental symmetry in nature,” said Roger Huang

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.

For a long time, scientists have debated whether neutrinos themselves are antiparticles. There are antiparticles for every particle: electrons, quarks, neutrons, and protons all have their own antiparticles, which make up the nuclei of atoms, respectively. Neutrinos, on the other hand, have the potential to be their own antiparticles. Majorana fermions, initially proposed in 1937 by Italian physicist Ettore Majorana, are particles that are their own antiparticles.

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.

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“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, since neutrinos interact so seldom, it is impossible to tell whether they are their own antiparticles or not. Neutrinoless double beta decay is the finest instrument available to physicists for searching for Majorana neutrinos. A neutron is converted into a proton via beta decay, altering the chemical element of the atom and producing an electron and an anti-neutrino. Beta decay is a rather frequent kind of decay in certain atoms. When two neutrons decay into one proton, two electrons are also released in the process, which is referred to as a “double beta decay.” Even though the neutrino is an antiparticle, it might possibly behave as its own antiparticle in double beta decay if it is a Majorana fermion. Only two electrons would escape from the nucleus of an atom. Although it’s been hypothesized for decades, neutrinoless twofold disintegration has never been seen.

Tellurium atoms are decaying in the CUORE experiment, and the team has gone to tremendous efforts to capture them. Tellurium oxide crystals weighing more than 700 kg are used in the experiment. A single unstable atom of tellurium takes billions of times longer than the present age of the universe to undergo conventional double beta decay, necessitating thus large a quantity of tellurium. However, since each of the CUORE crystals contains billions of trillions of tellurium atoms, the detector experiences normal twofold beta decay on a daily basis, roughly a few times each day. The CUORE team must work hard to eliminate as many sources of background radiation as possible in order to reduce the likelihood of neutrinoless double beta decay. A big mountain, Gran Sasso in Italy, serves as the detector’s barrier from cosmic rays. Additional protection is supplied by numerous tons of lead. But the lead used to protect the most sensitive component of CUORE is largely lead retrieved from a drowned ancient Roman ship that is approximately 2,000 years old, owing to radioactive contamination by uranium and other elements.

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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.”


A neutrino passing through the heart

CUORE has failed to discover neutrinoless double beta decay. It turns out that this decay happens in a single tellurium atom around every 22 trillion trillion years. Its half-life is more than a million billion billion times longer than the age of the universe, even if this happens, CUORE may not be sensitive enough to detect it, therefore verification is required. When physics has unanticipated implications, we learn the most. CUORE may not find evidence of neutrinoless double-beta decay, but it paves the way for future research. CUPID, CUORE’s replacement, is already in the works. Due of CUPID’s sensitivity, it may detect 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, a 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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