Researchers have released the first collection of data from the full detector of the CUORE (Cryogenic Underground Observatory for Rare Events) experiment. Located at the Italian National Institute for Nuclear Physics’ (INFN’s) Gran Sasso National Laboratories (LNGS) in Italy, CUORE is considered to be one of the most promising efforts to determine whether tiny elementary particles called neutrinos are Majorana particles—that is, identical to their own antiparticles. Most other particles are known to have antiparticles that have the same mass but a different charge, for example. Scientists also expect CUORE to help reveal the exact masses of the three types, or flavors, of neutrinos.
“This is the first preview of what an instrument this size is able to do,” says Oliviero Cremonesi, a senior faculty scientist at INFN and spokesperson for the CUORE collaboration. The result, published in Physical Review Letters, is based on two months of data with the full detector. The array’s sensitivity has already exceeded the precision of the measurements reported in April 2015, after a successful two-year test run that enlisted one detector tower, according to Cremonesi. Over the next five years, CUORE is expected to collect about 100 times more data.
“CUORE is, in essence, one of the world’s most sensitive thermometers,” says 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 an innermost chamber of six nested tanks. The tanks cool the detector to 10 mK, the coldest known temperature recorded in a cubic meter volume in the entire universe.
The detector array was designed and assembled over a 10-year period. It is shielded from many outside particles, such as the cosmic rays that constantly bombard the Earth, by 1,400 meters of rock 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,” says 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 [led] to the development of technologies that can now be applied in many fields of research.”
CUORE’s matrix of roughly fist-sized crystals—together weighing over 1,600 pounds—is extremely sensitive to particle processes, especially at extreme temperatures. Associated instruments can precisely measure very 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, 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, the specific process that CUORE is designed to find or to rule out, would not produce any antineutrinos, meaning 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. “We are tantalizingly close to completely unexplored territory and there is great possibility for discovery,” says Lindley Winslow of the Massachusetts Institute of Technology, who coordinated the analysis of the CUORE data. “It is an exciting time to be on the experiment.”