Nuclear physicists affiliated with the Lawrence Berkeley National Laboratory of the United States Department of Energy (Berkeley Lab) were instrumental in analyzing data from a demonstration experiment that achieved record precision for detector material specialized.
The CUPID-Mo experiment is part of a field of experiments that use a variety of approaches to detect a theoretical particle process, called double beta decay without neutrinol, which could revise our understanding of ghostly particles called neutrinos and their role in the formation of the universe.
Preliminary results from the CUPID-Mo experiment, based on the Berkeley Lab-led analysis of data collected from March 2019 to April 2020, set a new global limit for the double beta-decay process without neutrinol in an isotope of molybdenum 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 fixes the limit of the double decay half-life without beta without neutrons in the Mo-100 at 1.4 times the trillion-trillion years (14 followed by 23 zeros), which represents an improvement of 30% sensitivity compared to the Neutrino Ettore Majorana Observatory 3 (NEMO 3), a previous experiment which worked on the same site from 2003 to 2011 and also used Mo-100. A half-life is the time it takes for a radioactive isotope to lose half of its radioactivity.
The double beta decay process without neutrinos is theoretically very slow and rare, and no event has been detected in CUPID-Mo after one year of data collection.
While both experiments used Mo-100 in their detector arrays, NEMO 3 used a sheet form of the isotope while CUPID-Mo used a crystal form which produces flashes of light in certain particle interactions.
Larger experiments using different detection materials and operating for longer periods have reached greater sensitivity, although the reported early success of CUPID-Mo paves the way for a planned successor experiment called CUPID with a network of detectors which will be 100 times larger.
Berkeley Lab’s contributions to CUPID-Mo
No experience has yet confirmed the existence of the neutrinol-free process. The existence of this process would confirm that neutrinos serve as their own antiparticles, and such evidence would also help explain why matter prevails over antimatter in our universe.
All the 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 crystal – is transmitted by Modane Underground Laboratory (Modane underground laboratory) in France to the Cori supercomputer from the National Center for Scientific Computing and Energy Research at the Berkeley Lab.
Benjamin Schmidt, postdoctoral researcher in the nuclear sciences division of the Berkeley Lab, led the overall data analysis effort for the CUPID-Mo result and was supported by a team of researchers affiliated with the Berkeley Lab and other members international collaboration.
Berkeley Lab also supplied 40 sensors which made it possible to read the signals picked up by the CUPID-Mo network of 20 crystal detectors. The network was supercooled to approximately 0.02 kelvin, or minus 460 degrees Fahrenheit, to maintain its sensitivity. Its cylindrical crystals contain lithium, oxygen and the isotope Mo-100 and produce tiny flashes of light in the interactions of particles.
The international effort to produce the CUPID-Mo result is remarkable, said Schmidt, given the context of the global pandemic which had thrown uncertainty over the continued operation of the experiment.
“For a while it seemed like we had to stop the CUPID-Mo experiment prematurely due to the COVID-19 epidemic in Europe in early March and the difficulties associated with providing the experiment with the necessary cryogenic liquids,” said he declared. .
He added: “Despite this uncertainty and the changes associated with the closure of offices and schools, as well as the restricted access to the underground laboratory, our collaborators did everything possible to keep the experiment going on during the pandemic. ”
Schmidt credited the efforts of the data analytics 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 physics and neutrino astrophysics organized by Fermi National Laboratory of accelerators. Members of the CUPID-Mo collaboration plan to submit the results for publication in a peer-reviewed scientific journal.
Adjustment of ultra-sensitive detectors
A particular challenge in analyzing the data, said Schmidt, was to ensure that the detectors were properly calibrated to record “the extremely elusive set of events” that should be associated with a double decay signal without beta without neutrinos .
The neutrino-free decay process should generate a very high energy signal in the CUPID-Mo detector and a flash of light. The signal, because it is at such a high energy, should be free from interference by 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 isotope of thallium, near the detector array. The signals generated by the decay of this isotope are at a high energy, but not as high as the energy expected to be associated with the decay process without neutrinos in the Mo-100, if it exists.
“Therefore, a big challenge was convincing us that we can calibrate our detectors with common sources, especially the Tl-208,” said Schmidt, “then extrapolating the detector response to our signal region and correctly take into account the uncertainties of this extrapolation. . ”
To further improve calibration with high-energy signals, nuclear physicists used the 88-inch cyclotron from Berkeley Lab to produce a wire containing Co-56, a cobalt isotope that has a low level of radioactivity, from the reopening of the cyclotron last month. following a temporary stop in response to the COVID-19 pandemic. The wire was shipped to France to be tested with the CUPID-Mo detector network.
Prepare for a new generation experience in Italy
While CUPID-Mo can now lag behind the sensitivity of measurements obtained 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, which will use approximately 100 times more Mo-100, and with 10 years of operation, we have excellent prospects for research and potential discovery of double decay without beta without neutrinos,” said Schmidt.
CUPID-Mo was installed on the site of the Edelweiss III dark matter research experiment in a tunnel more than a mile deep in France, near the Italian border, and uses certain components of Edelweiss III. CUPID, on the other hand, is proposing to replace the CUORE neutrino-free double beta decay research experiment at the National Laboratory of Gran Sasso (Laboratori Nazionali del Gran Sasso) in Italy. While CUPID-Mo only contains 20 detector crystals, CUPID is said to contain more than 1,500.
“After CUORE finishes collecting data in two or three years, the CUPID detector could take four or five years to build,” said Yury Kolomensky, US spokesperson for CUORE collaboration and principal investigator at Berkeley Lab, who leads CUORE’s 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 physical data acquisition for CUPID-Mo ended on June 22, and the new data which were not taken into account in the last result represent an increase of approximately 20% to 30% of the global data. CUPID-Mo is supported by a group of French laboratories and by laboratories in the United States, Ukraine, Russia, Italy, China and Germany.
NERSC is a user installation of the DOE Office of Science.
The CUPID-Mo collaboration brings together researchers from 27 institutions, including the French laboratories Irfu / CEA and IJCLab in Orsay; IP2I in Lyon; and the Néel Institute and SIMaP in Grenoble, as well as institutions in the United States, Ukraine, Russia, Italy, China and Germany.
The experiment is supported by the United States Department of Energy Office of Science’s Office of Nuclear Physics, Berkeley Research Computing program, National Research Agency, IDEATE International Associated Laboratory (LIA), Russian Science Foundation, National Academy of Sciences of Ukraine , 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
Researchers develop new approach to model rare but unconfirmed nuclear process
Supplied by Lawrence Berkeley National Laboratory
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