CUORE

A search for neutrinoless double beta decay

Ettore Fiorini left us on April 9, 2023. He was born in Verona on April 19, 1933. Endowed with exquisite sympathy and politeness as well as great intelligence, Ettore is certainly a legend in the world of science. With him, we lose a father of Italian physics who raised many generations of scientists who carry on his legacy.

After graduating with Giovanni Polvani, he immediately began working in his group on making measurements in high mountains with Wilson chambers for the study of cosmic radiation. In the same years he manifested his love for the research of neutrinoless double beta decay, of which he can be considered one of the pioneers as well as the father of the Germanium diode technique. He conducted the first experiment in Turin, together with Antonino Pullia using the first prototypes of Germanium diodes developed in Ispra. In the following years he began his collaboration with the Lagarrigue group, where he learned the bubble chamber technique. It was from this collaboration that Gargamelle was born, the heavy-liquid bubble chamber with which he would be a major player in the discovery of weak neutral interactions in 1973. In the following years, while he continued his collaboration with his colleagues at CERN, his tireless spirit and insatiable curiosity pushed him to search for new techniques to experimentally verify whether fundamental properties of nature, now considered obvious, are true or not: lepton and baryon numbers, electric charge, and the stability of the constituents of electrons and nucleons. He thus proposed an experiment at Legnaro to search for parity violation in nuclei, and in the same years he headed an international collaboration to carry out an experiment to search for nucleon decay in the Mont Blanc tunnel. NUSEX, as the experiment was called, represented the first application of Iarocci's tubes and, despite its small size (a cube of about 3 meters on a side), produced results competitive with those of the leading experiments of the day: IMB and Kamiokande. Moreover, NUSEX was one of the first experiments conducted underground, when real laboratories of this kind did not yet exist in the western world. In 1979, however, a new underground laboratory dedicated to fundamental physics research was about to be born: in fact, Antonino Zichichi, then INFN president, presented the "Gran Sasso Project" to the Italian Senate, which was approved by Parliament with two funding laws in 1982 and 1984.

While NUSEX was taking data at garage 17 in the Mont Blanc tunnel, Ettore then spearheaded interest in the new Abruzzo laboratory: he began a campaign of site characterization measurements while the laboratory was still being excavated. At the same time he carried out a new campaign of measurements on the double beta decay of 76Ge at garage 27 of Mont Blanc employing two of the first commercially produced Germanium diodes. The experiment can certainly be considered the progenitor of a long family of experiments of increasing size and sensitivity that, passing through Heidelberg-Moscow, IGEX, MJD and GERDA, culminate today with the proposed LEGEND 1000.

In 1986, when the Gran Sasso Laboratories were not yet completed, he proposed to install a high-pressure gaseous Xenon detector (a multiproportional chamber) in one of the highway bypasses near the laboratories. He then entered with his group into one of the first major experiments proposed for the new laboratories, named GALLEX, for the measurement of solar neutrinos produced in proton-proton fusion reactions. The success of this experiment, with the first experimental demonstration of the mechanisms of energy production in the sun, gave a strong impetus to the Gran Sasso laboratories, which became the standard for underground experiments for the study of neutrinos and the search for rare events.

In the same year (1984), Ettore proposed to develop a new technique, that of very low temperature calorimeters, to be applied to studies such as double beta decay, the search for dark matter, and the direct measurement of the neutrino mass. At the time of the proposal, the masses of the detectors were a few fractions of a gram, and his friend Frank Avignone was not ashamed to call it a "crazy" idea. With his characteristic tenacity, Ettore decided that it was not only a viable technique, but also a promising one. Two separate lines of research were born, one for the development of large-mass bolometers for the study of double beta decay, the other for the realization of microbolometers for the determination of neutrino mass. After about 20 years of tireless work, in 1998 Ettore proposed together with Frank Avignone the construction of CUORE, an array of about 1,000 bolometers with a total mass on the order of a tonne. Its construction would begin only a decade later after the success of Cuoricino, a smaller version of the experiment.

CUORE has been taking data at Gran Sasso since 2017 and represents, as Ettore often said, the realization of one of his great dreams and also the conclusion of his scientific adventure in elementary particle physics. Even so, he never stopped supporting developments in the technique that led to the proposal of CUPID, the version of CUORE improved with the use of scintillating bolometers.

Blessed with a gracious disposition and innate friendliness, we always heard him say of each colleague that he was a great friend of them. Indeed Ettore was able to collaborate with scientists from all over the world and manage the first international collaborations of a certain size. He was also a man of great culture, always had a Latin quote or motto ready and delighted his interlocutors with anecdotes and stories.

His returns from vacations or missions around the world were overwhelming. Excited with boyish enthusiasm, he would tell his collaborators about new ideas or new proposals, drawing them into new adventures.

Professor emeritus at the University of Milan Bicocca, Ettore authored several hundred scientific papers. He was director of the Milan INFN Section and a professor beloved by his students at both the University of Milan and Milan Bicocca. He was among the founders of the INFN Section and the Physics Department of Milano Bicocca. A national member of the Accademia dei Lincei since 1988, he devoted himself fervently to promoting its initiatives. He was the recipient of important awards, such as the Feltrinelli Prize for the discovery of weak neutral currents, the Pontecorvo Prize for his studies on neutrino physics, and the Benemeriti medal of Culture and Science.

Ettore should also be remembered for some of his forays into other scientific fields, such as physics applied to the environment, medicine and cultural heritage. Among other things, he was co-director of the school of medical physics at the University of Milan for many years. In addition, from the accident at the Chernobyl nuclear power plant in 1986, through the minor accidents in Spain in '98 to the Fukushima accident in 2011, Ettore has always made available to society the knowledge and instrumentation used in particle physics to assess the impact that the release of radioactive contaminants has on human health and the environment.

In his later years he cultivated a passion for archaeometry, which began with the recovery of more than a thousand ancient lead ingots from the Roman era, found by a diver in the late 1980s off the island of Mal di Ventre in Sardinia. This was an exceptional find of an ancient Roman ship, sunk off the coast of Sardinia more than 2,000 years ago whose cargo, due to its low radioactivity content (210Pb) could be an exceptionally useful element in rare-event physics experiments. After reading the news in the newspapers, Ettore immediately took off on a spearhead, involving INFN and then-president Cabibbo, who immediately embraced the idea of funding the recovery of the ingots. From there began a whole series of activities, such as the study of the hair of Napoleon and his contemporaries to determine whether or not the emperor had been poisoned with arsenic, or such as the measurements of lead isotope ratios to establish the provenance of archaeological finds from the Nuragic Site of Sant'Imbenia (or the Lupa Capitolina). In each of these activities Ettore combined the enthusiasm of a child, with which he transmitted positive energy to all who collaborated with him, and the rigorousness of an experimental physicist typical of research in particle physics.

Ettore had an eye capable of going beyond immediate contingencies and could sense the scope of an experiment even at the level of its impact on the general public. His guidance, jokes, anecdotes, and proverbial insight as an experimental physicist will be missed. As he often liked to remind us: of the companions of our lives when they leave us we should not say "they are no more," but with gratitude "they have been with us" (Anton Chekhov).

Physicists are closing in on the true nature of the neutrino — and might be closer to answering a fundamental question about our own existence

Press release from LBNL

In a Laboratory under a mountain, physicists are using crystals far colder than frozen air to study ghostly particles, hoping to learn secrets from the beginning of the universe. Researchers at the Cryogenic Underground Observatory for Rare Events (CUORE) announced this week that they had placed some of the most stringent limits yet on the strange possibility that the neutrino is its own antiparticle. Neutrinos are deeply unusual particles, so ethereal and so ubiquitous that they regularly pass through our bodies without us noticing. CUORE has spent the last three years patiently waiting to see evidence of a distinctive nuclear decay process, only possible if neutrinos and antineutrinos are the same particle. CUORE’s new data shows that this decay doesn’t happen for trillions of trillions of years, if it happens at all. CUORE’s limits on the behavior of these tiny phantoms are a crucial part of the search for the next breakthrough in particle and nuclear physics – and the search for our own origins.

“Ultimately, we are trying to understand matter creation,” said Carlo Bucci, researcher at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy and the spokesperson for CUORE. “We’re looking for a process that violates a fundamental symmetry of nature,” added Roger Huang, a postdoctoral researcher at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and one of the lead authors of the new study.

CUORE – Italian for “heart” – is among the most sensitive neutrino experiments in the world. The new results from CUORE are based on a data set ten times larger than any other high-resolution search, collected over the last three years. CUORE is operated by an international research collaboration, led by the Istituto Nazionale di Fisica Nucleare (INFN) in Italy and Berkeley Lab in the US. The CUORE detector itself is located under nearly a mile of solid rock at LNGS, a facility of the INFN. U.S. Department of Energy-supported nuclear physicists play a leading scientific and technical role in this experiment. CUORE’s new results were published today in Nature.

Peculiar Particles

Neutrinos are everywhere — there are trillions of neutrinos passing through your thumbnail alone as you read this sentence. They are invisible to the two strongest forces in the universe, electromagnetism and the strong nuclear force, which allows them to pass right through you, the Earth, and nearly anything else without interacting. Despite their vast numbers, their enigmatic nature makes them very difficult to study, and has left physicists scratching their heads ever since they were first postulated over 90 years ago. It wasn’t even known whether neutrinos had any mass at all until the late 1990s — as it turns out, they do, albeit not very much.

One of the many remaining open questions about neutrinos is whether they are their own antiparticles. All particles have antiparticles, their own antimatter counterpart: electrons have antielectrons (positrons), quarks have antiquarks, and neutrons and protons (which make up the nuclei of atoms) have antineutrons and antiprotons. But unlike all of those particles, it’s theoretically possible for neutrinos to be their own antiparticles. Such particles that are their own antiparticles were first postulated by the Italian physicist Ettore Majorana in 1937, and are known as Majorana fermions.

If neutrinos are Majorana fermions, that could explain a deep question at the root of our own existence: why there’s so much more matter than antimatter in the universe. Neutrinos and electrons are both leptons, a kind of fundamental particle. One of the fundamental laws of nature appears to be that the number of leptons is always conserved — if a process creates a lepton, it must also create an anti-lepton to balance it out. Similarly, particles like protons and neutrons are known as baryons, and baryon number also appears to be conserved. Yet if baryon and lepton numbers were always conserved, then there would be exactly as much matter in the universe as antimatter — and in the early universe, the matter and antimatter would have met and annihilated, and we wouldn’t exist. Something must violate the exact conservation of baryons and leptons. Enter the neutrino: if neutrinos are their own antiparticles, then lepton number wouldn’t have to be conserved, and our existence becomes much less mysterious.

“The matter-antimatter asymmetry in the universe is still unexplained,” said Huang. “If neutrinos are their own antiparticles, that could help explain it.”

Nor is this the only question that could be answered by a Majorana neutrino. The extreme lightness of neutrinos, about a million times lighter than the electron, has long been puzzling to particle physicists. But if neutrinos are their own antiparticles, then an existing solution known as the “seesaw mechanism” could explain the lightness of neutrinos in an elegant and natural way.

A Rare Device for Rare Decays

But determining whether neutrinos are their own antiparticles is difficult, precisely because they don’t interact very often at all. Physicists’ best tool for looking for Majorana neutrinos is a hypothetical kind of radioactive decay called neutrinoless double beta decay. Beta decay is a fairly common form of decay in some atoms, turning a neutron in the atom’s nucleus into a proton, changing the chemical element of the atom and emitting an electron and an anti-neutrino in the process. Double beta decay is more rare: instead of one neutron turning into a proton, two of them do, emitting two electrons and two anti-neutrinos in the process. But if the neutrino is a Majorana fermion, then theoretically, that would allow a single “virtual” neutrino, acting as its own antiparticle, to take the place of both anti-neutrinos in double beta decay. Only the two electrons would make it out of the atomic nucleus. Neutrinoless double-beta decay has been theorized for decades, but it’s never been seen.

The CUORE experiment has gone to great lengths to catch tellurium atoms in the act of this decay. The experiment uses nearly a thousand highly pure crystals of tellurium oxide, collectively weighing over 700 kg. This much tellurium is necessary because on average, it takes billions of times longer than the current age of the universe for a single unstable atom of tellurium to undergo ordinary double beta decay. But there are trillions of trillions of atoms of tellurium in each one of the crystals CUORE uses, meaning that ordinary double beta decay happens fairly regularly in the detector, around a few times a day in each crystal. Neutrinoless double beta decay, if it happens at all, is even more rare, and thus the CUORE team must work hard to remove as many sources of background radiation as possible. To shield the detector from cosmic rays, the entire system is located underneath the mountain of Gran Sasso, the largest mountain on the Italian peninsula. Further shielding is provided by several tons of lead. But freshly mined lead is slightly radioactive due to contamination by uranium and other elements, with that radioactivity decreasing over time — so the lead used to surround the most sensitive part of CUORE is mostly lead recovered from a sunken ancient Roman ship, nearly 2000 years old.

Perhaps the most impressive piece of machinery used at CUORE is the cryostat, which keeps the detector cold. To detect neutrinoless double beta decay, the temperature of each crystal in the CUORE detector is carefully monitored with sensors capable of detecting a change in temperature as small as one ten-thousandth of a Celsius degree. Neutrinoless double beta decay has a specific energy signature and would raise the temperature of a single crystal by a well-defined and recognizable amount. But in order to maintain that sensitivity, the detector must be kept very cold — specifically, it’s kept around 10 mK, a hundredth of a degree above absolute zero. “This is the coldest cubic meter in the known universe,” said Laura Marini, a research fellow at Gran Sasso Science Institute and CUORE’s Run Coordinator. The resulting sensitivity of the detector is truly phenomenal. “When there were large earthquakes in Chile and New Zealand, we actually saw glimpses of it in our detector,” said Marini. “We can also see waves crashing on the seashore on the Adriatic Sea, 60 kilometers away. That signal gets bigger in the winter, when there are storms.”

A Neutrino Through The Heart

Despite that phenomenal sensitivity, CUORE hasn’t yet seen evidence of neutrinoless double beta decay. Instead, CUORE has established that, on average, this decay happens in a single tellurium atom no more often than once every 22 trillion trillion years. “Neutrinoless double beta decay, if observed, will be the rarest process ever observed in nature, with a half-life more than a million billion times longer than the age of the universe,” said Danielle Speller, Assistant Professor at Johns Hopkins University and a member of the CUORE Physics Board. “CUORE may not be sensitive enough to detect this decay even if it does occur, but it’s important to check. Sometimes physics yields surprising results, and that’s when we learn the most.” Even if CUORE doesn’t find evidence of neutrinoless double-beta decay, it is paving the way for the next generation of experiments. CUORE’s successor, the CUORE Upgrade with Particle Identification (CUPID) is already in the works. CUPID will be over 10 times more sensitive than CUORE, potentially allowing it to glimpse evidence of a Majorana neutrino.

But regardless of anything else, CUORE is a scientific and technological triumph — not only for its new bounds on the rate of neutrinoless double beta decay, but also for its demonstration of its cryostat technology. “It’s the largest refrigerator of its kind in the world,” said Paolo Gorla, a staff scientist at LNGS and CUORE’s Technical Coordinator. “And it’s been kept at 10 mK continuously for about three years now.” Such technology has applications well beyond fundamental particle physics. Specifically, it may find use in quantum computing, where keeping large amounts of machinery cold enough and shielded from environmental radiation to manipulate on a quantum level is one of the major engineering challenges in the field.

Meanwhile, CUORE isn’t done yet. “We’ll be operating until 2024,” said Bucci. “I’m excited to see what we find.”

CUORE is supported by the U.S. Department of Energy, Italy’s National Institute of Nuclear Physics (Instituto Nazionale di Fisica Nucleare, or INFN), and the National Science Foundation (NSF). CUORE collaboration members include: INFN, University of Bologna, University of Genoa, University of Milano-Bicocca, and Sapienza University in Italy; California Polytechnic State University, San Luis Obispo; Berkeley Lab; Johns Hopkins University; Lawrence Livermore National Laboratory; Massachusetts Institute of Technology; University of California, Berkeley; University of California, Los Angeles; University of South Carolina; Virginia Polytechnic Institute and State University; and Yale University in the US; Saclay Nuclear Research Center (CEA) and the Irène Joliot-Curie Laboratory (CNRS/IN2P3, Paris Saclay University) in France; and Fudan University and Shanghai Jiao Tong University in China.

Press release from LBNL

As the COVID-19 outbreak took hold in Italy, researchers working on a nuclear physics experiment called CUORE at an underground laboratory in central Italy scrambled to keep the ultrasensitive experiment running and launch new tools and rules for remote operations.

This Cryogenic Underground Observatory for Rare Events experiment – designed to find a never-before-seen process involving ghostly particles known as neutrinos, to explain why matter won out over antimatter in our universe, and to also hunt for signs of mysterious dark matter – is carrying on with its data-taking uninterrupted while some other projects and experiments around the globe have been put on hold.

Finding evidence for these rare processes requires long periods of data collection – and a lot of patience. CUORE has been collecting data since May 2017, and after upgrade efforts in 2018 and 2019 the experiment has been running continuously.

Before the pandemic hit there were already tools in place that stabilized the extreme cooling required for CUORE’s detectors and provided some remote controls and monitoring of CUORE systems, noted Yury Kolomensky, senior faculty scientist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the U.S. spokesperson for CUORE.

The rapid global spread of the disease, and related restrictions on access to the CUORE experiment at Gran Sasso National Laboratory (Laboratori Nazionali del Gran Sasso, or LNGS, operated by the Italian Nuclear Physics Institute, INFN) in central Italy, prompted CUORE leadership and researchers – working in three continents – to act quickly to ramp up the remote controls to prepare for an extended period with only limited access to the experiment.

Just days before the new restrictions went into effect at Gran Sasso, CUORE leadership on March 4 made the decision to rapidly deploy a new remote system and to work out the details of how to best maintain the experiment with limited staffing and with researchers monitoring in different time zones. The new system was fully operational about a week later, and researchers at Berkeley Lab played a role in rolling it out.

“We were already planning to transition to remote shift operations, whereby a scientist at a home institution would monitor the systems in real time, respond to alarms, and call on-site and on-call personnel in case an emergency intervention is needed,” Kolomensky said, adding, “We were commissioning the system at the time of the outbreak.”

Brad Welliver, a postdoctoral researcher, served as Berkeley Lab’s lead developer for the new remote monitoring system, and Berkeley Lab staff scientist Brian Fujikawa was the overall project lead for the enhanced remote controls, collectively known as CORC, for CUORE Online/Offline Run Check.

Fujikawa tested controls for starting and stopping the data collection process, and also performed other electronics testing for the experiment from his home in the San Francisco Bay Area.

He noted that the system is programmed to send email and voice alarms to the designated on-shift CUORE researcher if something is awry with any CUORE system. “This alarm system is particularly important when operating CUORE remotely,” he said, as in some cases on-site workers may need to visit the experiment promptly to perform repairs or other needed work.

Development of so-called “slow controls,” which allow researchers to monitor and control CUORE equipment such as pumps and sensors, was led by Joe Johnston at the Massachusetts Institute of Technology.

“Now we can perform most of the operations from 6,000 miles away,” Kolomensky said.

And many participants across the collaboration continue to play meaningful roles in the experiment from their homes, from analyzing data and writing papers to participating in long-term planning and remote meetings.

Despite access restrictions at Gran Sasso, experiments are still accessible for necessary work and checkups. The laboratory remains open in a limited way, and its staff still maintains all of its needed services and equipment, from shuttles to computing services.

Laura Marini, a postdoctoral researcher at UC Berkeley who serves as a run coordinator for CUORE and is now living near Gran Sasso, is among a handful of CUORE researchers who still routinely visits the lab site.

“As a run coordinator, I need to make sure that the experiment works fine and the data quality is good,” she said. “Before the pandemic spread, I was going underground maybe not every day, but at least a few times a week.” Now, it can be about once every two weeks.

Sometimes she is there to carry out simple fixes, like a stuck computer that needs to be restarted, she said. Now, in addition to the requisite hard hat and heavy shoes, Marini – like so many others around the globe who are continuing to work – must wear a mask and gloves to guard against the spread of COVID-19.

The simple act of driving into the lab site can be complicated, too, she said. “The other day, I had to go underground and the police stopped me. So I had to fill in a paper to declare why I was going underground, the fact that it was needed, and that I was not just wandering around by car,” she said. Restrictions in Italy prevent most types of travel.

CUORE researchers note that they are fortunate the experiment was already in a state of steady data-taking when the pandemic hit. “There is no need for continuous intervention,” Marini said. “We can do most of our checks by remote.”

She said she is grateful to be part of an international team that has “worked together on a common goal and continues to do so” despite the present-day challenges.

Kolomensky noted some of the regular maintenance and upgrades planned for CUORE will be put off as a result of the shelter-in-place restrictions, though there also appears to be an odd benefit of the reduced activity at the Gran Sasso site. “We see an overall reduction in the detector noise, which we attribute to a significantly lower level of activity at the underground lab and less traffic in the highway tunnel,” he said. Researchers are working to verify this.

CUORE already had systems in place to individually and remotely monitor data-taking by each of the experiment’s 988 detectors. Benjamin Schmidt, a Berkeley Lab postdoctoral researcher, had even developed software that automatically flags periods of “noisy” or poor data-taking captured by CUORE’s array of detectors.

Kolomensky noted that work on the CORC remote tools is continuing. “As we have gained more experience and discovered issues, improvements and bug fixes have been implemented, and these efforts are still ongoing,” he said.

CUORE is supported by the U.S. Department of Energy Office of Science, Italy’s National Institute of Nuclear Physics (Instituto Nazionale di Fisica Nucleare, or INFN), and the National Science Foundation (NSF). CUORE collaboration members include: INFN, University of Bologna, University of Genoa, University of Milano-Bicocca, and Sapienza University in Italy; California Polytechnic State University, San Luis Obispo; Berkeley Lab; Lawrence Livermore National Laboratory; Massachusetts Institute of Technology; University of California, Berkeley; University of California, Los Angeles; University of South Carolina; Virginia Polytechnic Institute and State University; and Yale University in the US; Saclay Nuclear Research Center (CEA) and the Irène Joliot-Curie Laboratory (CNRS/IN2P3, Paris Saclay University) in France; and Fudan University and Shanghai Jiao Tong University in China.

Press release from LBNL

In an underground laboratory deep beneath a mountain in Central Italy, an array of crystals, chilled to within a hair of absolute zero – the coldest possible temperature in the universe – has been steadily compiling one of the most precise measurements to date in pursuit of a rare particle process. If it is proven to exist, this process may well be the “smoking gun” of how matter was created in the universe.

The experiment that is designed to seek out this process, called CUORE (Cryogenic Underground Observatory for Rare Events), is at Gran Sasso National Laboratories, part of the Italian National Institute for Nuclear Physics (INFN).

The observation of this process, known as neutrinoless double-beta decay, would have profound implications for understanding the properties of ghostly, abundant particles called neutrinos that pass through most matter unaffected. U.S. Department of Energy-supported nuclear physicists play a leading scientific and technical role in this experiment.

The latest results represent more than a 2-year span of data collection – from April 2017 to July 2019. This dataset, which is about four times larger than the initial results announced in October 2017 (see a related article), sets even more stringent limits on the theoretical ultra-rare process that CUORE is designed to seek out.

The CUORE detector is surrounded by a layer of highly pure lead that was recovered from a 2,000-year-old Roman shipwreck. (Credit: CUORE Collaboration)

Double-beta decay is a proven particle process in which two neutrons, which are uncharged particles in an atom’s nucleus, morph into two protons and emit two electrons and two antineutrinos. Antineutrinos are the antiparticles, or antimatter counterparts, to neutrinos.

CUORE is designed to detect the signature of a theoretical neutrinoless double-beta decay process in which no antineutrinos are created. This is because they would erase each other in the decay process, proving that the neutrino is its own antiparticle, as the Italian scientist Ettore Majorana hypothesized in 1937.

“We have now more than quadrupled the collected data, reaching one of the best sensitivities worldwide for the discovery of this rare particle process,” said Oliviero Cremonesi, senior researcher at INFN Milano Bicocca and spokesperson of the CUORE Collaboration.

Yury Kolomensky, U.S. spokesperson for the CUORE collaboration and senior faculty scientist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), said, “The decay process in the CUORE crystals is a matter-creating process relevant to the Big Bang at the inception of our universe, and could help us to explain how matter won out over antimatter in its evolution.” Kolomensky is also a physics professor at UC Berkeley.

Discovery of this neutrinoless process would mean that a neutrino and an antineutrino, which are both electrically neutral, are essentially the same particle (called a Majorana neutrino) and only differ in a mirroring property known as helicity. Helicity is somewhat analogous to a person being either left-handed or right-handed, and a Majorana neutrino could switch handedness – akin to an ambidextrous person.

The CUORE detector array consists of 988 cube-shaped crystals made of a highly purified, natural form of tellurium dioxide that are stacked in 19 copper-cladded towers.

The CUORE detector array during assembly. (Credit: Yura Suvorov)

While no sign of neutrinoless double-beta decay is found in the data at this time, CUORE improved by a factor of two, compared to the previous results, the bound on the rate of this process in the nuclei of tellurium-130 atoms that are contained in the CUORE crystals. The interpretation of this result is a tighter bound on the allowed value of the neutrino mass in the Majorana hypothesis, which now extends below one-tenth of an electronvolt, at least 5 million times lighter than an electron.

The results incorporate a sophisticated new algorithm that helps to amplify CUORE’s detected signals while cutting out unwanted background “noise.” The algorithm helps identify and reject signals caused by small energy deposits in the detectors, such as those left by some other, well-known particle decays. This could provide a cleaner signature of neutrinoless double-beta decay.

The new algorithm would also allow CUORE to hunt for theoretical particles of dark matter known as WIMPs, or weakly interacting massive particles, in its nearly 1-ton detector.

“This is the largest, most sensitive detector of its kind in the world,” said Thomas O’Donnell, professor of physics at Virginia Tech University and a member of the CUORE Physics Board that organized and coordinated the data analysis. “Each month we are accumulating as much data as some detectors get in a year.”
CUORE detector rendering

An illustration of the cross-section of the CUORE experiment with a human figure for scale. The chambers encasing the central detector are used to deeply chill the detector. (Credit: CUORE Collaboration)

CUORE’s latest results represent the largest dataset collected by a particle detector that uses solid crystals, rather than the more common tank of liquid, in an effort to find this particle process. It is the first example of a solid-state detector with nearly a ton of mass.

Solid-state detectors have the ability to very accurately measure the energy of particle decays. But it is challenging to scale up a solid-state detector to very large sizes when compared to a liquid-based detector.

“We are delighted that we are now operating the detectors at close to 90% efficiency,” added Carlo Bucci, senior researcher at INFN’s Gran Sasso, who is the Italian spokesperson and technical coordinator of the experiment. “All of the work invested in the last two years to bring the system to this performance has paid off. Warming up and cooling back down takes several months, so we have to get it right each time.”

The crystal array is extremely sensitive to a very slight and narrow energy signature that is predicted for the neutrinoless decay process. Chilling the array to below minus 459 degrees Fahrenheit makes the entire array, which weighs about 1,650 pounds, sensitive to an incredibly slight rise in temperature arising from a particle interaction with a detector crystal. The tellurium-130 in the crystals, which is the decaying component in the detector, accounts for about 450 pounds of that weight.

This heightened sensitivity, which enables CUORE to look for signatures of dark matter particles – could possibly help to understand a periodic signal that a dark matter experiment called DAMA/LIBRA, installed at the same Gran Sasso site, has reported.

After CUORE’s 5-year run, a planned next-gen upgrade called CUPID will exchange the tellurium crystals with new crystals that will use a form of molybdenum with light-emitting properties. These crystals can produce both temperature-based and light-based signals that will further extend the sensitivity of the detector’s measurements.

“It is an exciting time for neutrino physics,” said Claudia Tomei, a member of the CUORE Executive Board and a researcher at INFN Roma, “with numerous complementary experiments that will help us better understand the properties of neutrinos.”

Cremonesi added, “We know we’ll learn something. We’re aiming for definitive measurements.”

CUORE is supported by the U.S. Department of Energy, Italy’s National Institute of Nuclear Physics (Instituto Nazionale di Fisica Nucleare, or INFN), and the National Science Foundation (NSF). CUORE collaboration members include: INFN, University of Bologna, University of Genoa, University of Milano-Bicocca, and Sapienza University in Italy; California Polytechnic State University, San Luis Obispo; Berkeley Lab; Lawrence Livermore National Laboratory; Massachusetts Institute of Technology; University of California, Berkeley; University of California, Los Angeles; University of South Carolina; Virginia Polytechnic Institute and State University; and Yale University in the US; Saclay Nuclear Research Center (CEA) and the Irène Joliot-Curie Laboratory (CNRS/IN2P3, Paris Saclay University) in France; and Fudan University and Shanghai Jiao Tong University in China.

Press release from INFN

The first glimpse of data from the full array of a deeply chilled particle detector operating beneath a mountain in Italy sets the most precise limits yet on where scientists might find a theorized process to help explain why there is more matter than antimatter in the universe. This new result, submitted today to the journal Physical Review Letters, is based on two months of data collected from the full detector of the CUORE (Cryogenic Underground Observatory for Rare Events) experiment at the Italian National Institute for Nuclear Physics’ (INFN’s) Gran Sasso National Laboratories (LNGS) in Italy. CUORE means “heart” in Italian.

CUORE is considered one of the most promising efforts to determine whether tiny elementary particles called neutrinos, which interact only rarely with matter, are “Majorana particles” – identical to their own antiparticles. Most other particles are known to have antiparticles that have the same mass but a different charge, for example. CUORE could also help us home in on the exact masses of the three types, or “flavors,” of neutrinos – neutrinos have the unusual ability to morph into different forms.

“This is the first preview of what an instrument this size is able to do,” said Oliviero Cremonesi, a senior faculty scientist at INFN and spokesperson for the CUORE collaboration. Already, the full detector array’s sensitivity has exceeded the precision of the measurements reported in April 2015 after a successful two-year test run that enlisted one detector tower. Over the next five years CUORE will collect about 100 times more data.

Yury Kolomensky, a senior faculty scientist in the Nuclear Science Division at Lawrence Berkeley National Laboratory (Berkeley Lab) and U.S. spokesperson for the CUORE collaboration, said, “The detector is working exceptionally well and these two months of data are enough to exceed the previous limits.” Kolomensky is also a professor in the UC Berkeley Physics Department.

The new data provide a narrow range in which scientists might expect to see any indication of the particle process it is designed to find, known as neutrinoless double beta decay.

“CUORE is, in essence, one of the world’s most sensitive thermometers,” said Carlo Bucci, technical coordinator of the experiment and Italian spokesperson for the CUORE collaboration. Its detectors, formed by 19 copper-framed “towers” that each house a matrix of 52 cube-shaped, highly purified tellurium dioxide crystals, are suspended within the innermost chamber of six nested tanks.

Cooled by the most powerful refrigerator of its kind, the tanks subject the detector to the coldest known temperature recorded in a cubic meter volume in the entire universe: minus 459 degrees Fahrenheit (10 milliKelvin).

The detector array was designed and assembled over a 10-year period. It is shielded from many outside particles, such as cosmic rays that constantly bombard the Earth, by the 1,400 meters of rock above it, and by thick lead shielding that includes a radiation-depleted form of lead rescued from an ancient Roman shipwreck. Other detector materials were also prepared in ultrapure conditions, and the detectors were assembled in nitrogen-filled, sealed glove boxes to prevent contamination from regular air.

“Designing, building, and operating CUORE has been a long journey and a fantastic achievement,” said Ettore Fiorini, an Italian physicist who developed the concept of CUORE’s heat-sensitive detectors (tellurium dioxide bolometers), and the spokesperson-emeritus of the CUORE collaboration. “Employing thermal detectors to study neutrinos took several decades and brought to the development of technologies that can now be applied in many fields of research.”

Together weighing over 1,600 pounds, CUORE’s matrix of roughly fist-sized crystals is extremely sensitive to particle processes, especially at this extreme temperature. Associated instruments can precisely measure ever-slight temperature changes in the crystals resulting from these processes. The measurements carry the telltale signature of specific types of particle interactions or particle decays – a spontaneous process by which a particle or particles transform into other particles.

In double beta decay, which has been observed in previous experiments, two neutrons in the atomic nucleus of a radioactive element become two protons. Also, two electrons are emitted, along with two other particles called antineutrinos.

Neutrinoless double beta decay, meanwhile – the specific process that CUORE is designed to find or to rule out – would not produce any antineutrinos. This would mean that neutrinos are their own antiparticles. During this decay process the two antineutrino particles would effectively wipe each other out, leaving no trace in the CUORE detector. Evidence for this type of decay process would also help scientists explain neutrinos’ role in the imbalance of matter vs. antimatter in our universe.

Neutrinoless double beta decay is expected to be exceedingly rare, occurring at most (if at all) once every 100 septillion (1 followed by 26 zeros) years in a given atom’s nucleus. The large volume of detector crystals is intended to greatly increase the likelihood of recording such an event during the lifetime of the experiment.

There is growing competition from new and planned experiments to resolve whether this process exists using a variety of search techniques, and Kolomensky noted, “The competition always helps. It drives progress, and also we can verify each other’s results, and help each other with materials screening and data analysis techniques.”

Lindley Winslow of the Massachusetts Institute of Technology, who coordinated the analysis of the CUORE data, said, “We are tantalizing close to completely unexplored territory and there is great possibility for discovery. It is an exciting time to be on the experiment.”

CUORE is supported jointly by the Italian National Institute for Nuclear Physics Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and the U.S. Department of Energy’s Office of Nuclear Physics, the National Science Foundation, and the Alfred P. Sloan Foundation in the U.S. The CORE collaboration includes about 150 scientists from Italy, U.S., China, France, and Spain, and is based in the underground Italian facility called INFN Gran Sasso National Laboratories (LNGS) of the INFN. Berkeley Lab leads the U.S. nuclear physics effort for the international CUORE collaboration.

CUORE collaboration members include: Italian National Institute for Nuclear Physics (INFN), University of Bologna, University of Genoa, University of Milano-Bicocca, and Sapienza University in Italy; California Polytechnic State University, San Luis Obispo; Berkeley Lab; Lawrence Livermore National Laboratory; Massachusetts Institute of Technology; University of California, Berkeley; University of California, Los Angeles; University of South Carolina; Virginia Polytechnic Institute and State University; and Yale University in the US; Saclay Nuclear Research Center (CEA) and the Center for Nuclear Science and Materials Science (CNRS/IN2P3) in France; and the Shanghai Institute of Applied Physics and Shanghai Jiao Tong University in China.