Scientists demonstrate new experiment in search for theorized ‘neutrino-free’ processes

Nuclear physicists affiliated with the United States Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) played a leading role in data analysis for a demonstration experiment that has achieved record precision for a specialized detector material.

The CUPID-Mo experiment is among a field of experiments that is using a variety of approaches to detect a theorized particle process, called neutrino-free double beta decay, that could revise our understanding of ghost particles called neutrinos, and their role. in the formation of the universe.

Preliminary results from the CUPID-Mo experiment, based on analysis of data collected by the Berkeley Lab from March 2019 to April 2020, set a new global limit for the neutrino-free double beta decomposition process in a molybdenum isotope known as Mo -100. Isotopes are forms of an element that carry a different number of uncharged particles called neutrons in their atomic nuclei.

The new result sets the neutrino-free beta double decay half-life limit at Mo-100 at 1.4 times a trillion trillion years (i.e. 14 followed by 23 zeros), which is a 30% improvement in sensitivity on the Neutrino Ettore Majorana Observatory 3 (NEMO 3), a previous experiment that worked on the same site from 2003-2011 and also used Mo-100. A half-life is the time it takes for a radioactive isotope to remove half of its radioactivity.

The neutrino-free double beta decay process is theorized to be very slow and rare, and not a single event was detected in CUPID-Mo after one year of data collection.

While both experiments used Mo-100 in their detector sets, NEMO 3 used a laminated form of the isotope, while CUPID-Mo used a crystal form that produces flashes of light in certain particle interactions.

Larger experiments using different detector materials and operating for longer periods of time have achieved increased sensitivity, although the reported early success of CUPID-Mo sets the stage for a planned successor experiment called CUPID with a detector array that will be 100 times bigger.

Berkeley Lab contributions to CUPID-Mo

No experiment has yet confirmed whether the neutrino-free process exists. The existence of this process would confirm that neutrinos serve as their own antiparticles, and such a test would also help explain why matter beat antimatter in our universe.

All data from the CUPID-Mo experiment (the acronym CUPID stands for CUORE Upgrade with Particle Identification, and “Mo” is for the molybdenum contained in the detector glass) is transmitted from the Modane Underground Laboratory (Laboratoire souterrain de Modane) in France to the Cori supercomputer at the Berkeley Lab National Center for Scientific Research in Energy Research.

Benjamin Schmidt, a postdoctoral researcher in the Berkeley Lab Nuclear Science Division, led the overall data analysis effort for the CUPID-Mo outcome, and was supported by a team of researchers affiliated with Berkeley Lab and other members of the collaboration. international.

Berkeley Lab also contributed 40 sensors that enabled the reading of signals captured by the CUPID-Mo 20 crystal detector set. The matrix was supercooled to approximately 0.02 degrees Kelvin, or minus 460 degrees Fahrenheit, to maintain its sensitivity. Its cylindrical crystals contain lithium, oxygen, and the Mo-100 isotope, and produce small flashes of light in particle interactions.

The international effort to produce the CUPID-Mo result is remarkable, Schmidt said, given the context of the global pandemic that had created uncertainty about the continued operation of the experiment.

“For a time it seemed that we would have to shut down the CUPID-Mo experiment prematurely due to the COVID-19 outbreak in Europe in early March and the associated difficulties in supplying the experiment with the required cryogenic fluids,” he said. .

He added: “Despite this uncertainty and the changes associated with the closure of office and school spaces, as well as restricted access to the underground laboratory, our collaborators did everything possible to maintain the experiment during the pandemic.”

Schmidt credits the efforts of the data analysis group he led to find a way to work from home and produce the results of the experiment in time to present them at Neutrino 2020, a virtual International Conference on Neutrino Physics and Astrophysics organized by Fermi National Accelerator Laboratory . Members of the CUPID-Mo collaboration plan to present the results for publication in a peer-reviewed scientific journal.

Tuning ultrasensitive detectors

A particular challenge in data analysis, Schmidt said, was ensuring that detectors were properly calibrated to record the “extremely elusive set of events” that are predicted to be associated with a neutrino-free double beta decomposition signal.

The neutrino-free decomposition process is expected to generate a very high-energy signal at the CUPID-Mo detector and a flash of light. The signal, due to its high energy, is expected to be free from interference from natural sources of radioactivity.

To test CUPID-Mo’s response to high-energy signals, the researchers placed other sources of high-energy signals, including Tl-208, a radioactive thallium isotope, near the detector array. The signals generated by the decomposition of this isotope are high energy, but not as high as the energy predicted to be associated with the neutrino-free decomposition process at Mo-100, if it exists.

“Therefore, a major challenge was convincing ourselves that we can calibrate our detectors with common sources, particularly Tl-208,” said Schmidt, “and then extrapolate the detector response to our signal region and adequately explain the uncertainties in this extrapolation”. “

To further enhance calibration with high-energy signals, nuclear physicists used the Berkeley Lab’s 88-inch cyclotron to produce a cable containing Co-56, a cobalt isotope that has a low level of radioactivity, as soon as the Cyclotron reopened last month. after a temporary shutdown in response to the COVID-19 pandemic. The cable has been sent to France for testing with the CUPID-Mo detector set.

Preparing for the next generation experiment in Italy

While CUPID-Mo may now lag behind sensitivity in measurements made by other experiments, which use different detection techniques and materials, because it is smaller and has not yet collected as much data, “with the full CUPID experiment, it will use approximately 100 times more Mo-100, and with 10 years of operation, we have excellent prospects for the search and potential discovery of neutrino-free double beta decay, “said Schmidt.

CUPID-Mo was installed at the site of the Edelweiss III dark matter search experiment in a tunnel more than a mile deep in France, near the Italian border, and uses some components of Edelweiss III. Meanwhile, CUPID intends to replace the CUORE neutral neutral double beta decay search experiment at the Gran Sasso National Laboratory (Laboratori Nazionali del Gran Sasso) in Italy. While CUPID-Mo contains only 20 detector crystals, CUPID would contain more than 1,500.

“After CUORE finishes collecting data in two to three years, it could take four to five years for the CUPID detector to build,” said Yury Kolomensky, United States spokesman for the CUORE collaboration and chief scientist at Berkeley Lab, who leads the CUORE collaboration in the United States. . “CUPID would be a relatively modest upgrade in terms of costs and technical challenges, but it will be a significant improvement in terms of sensitivity.”

The physics data collection for CUPID-Mo concluded on June 22, and the new data that was not considered in the last result represents a growth of approximately 20% to 30% in the overall data. CUPID-Mo is supported by a group of French laboratories and laboratories in the USA, Ukraine, Russia, Italy, China and Germany.

NERSC is a user facility from the DOE Office of Science.

The CUPID-Mo collaboration brings together researchers from 27 institutions, including the French Irfu / CEA and IJCLab laboratories in Orsay; IP2I in Lyon; and Institut Néel and SIMaP in Grenoble, as well as institutions in the United States, Ukraine, Russia, Italy, China and Germany.

The experiment is supported by the Office of Nuclear Physics, Office of Science, US Department of Energy, The Berkeley Research Informatics Program, The National Research Agency, The IDEATE International Associated Laboratory (LIA) , the Russian Science Foundation, the National Academy of Sciences of Ukraine, the National Science Foundation, the France-Berkeley Fund, the MISTI-France fund and the Office of Science and Technology of the French Embassy in the United States.