Nuclear physicists from the U.S. Department of Energy Lawrence Berkeley National Laboratory (Berkeley Lab) played a leading role in analyzing data for a demonstration experiment that achieved record accuracy for a specific detector material.
The CUPID-Mo experiment belongs to an experimental field that uses different approaches to recognize a theorized particle process, the so-called neutrinoless double beta decay, which revises our understanding of the ghostly particles called neutrinos and their role in formation could of the universe.
Preliminary results from the CUPID-Mo experiment, based on Berkeley Lab̵
The new result sets the limit for the neutrinoless double beta decay half-life in Mo-100 to 1.4 times trillion years (that’s 14 followed by 23 zeros), which is a 30% improvement in sensitivity to the neutrino Ettore Majorana corresponds to Observatory 3 (NEMO 3), an earlier experiment that was carried out at the same location from 2003 to 2011 and also used Mo-100. A half-life is the time it takes a radioactive isotope to release half of its radioactivity.
The neutrinoless double beta decay process is believed to be very slow and rare, and after a year of data collection, not a single event was found in CUPID-Mo.
While both experiments used Mo-100 in their detector arrays, NEMO 3 used a foil form of the isotope, while CUPID-Mo used a crystal form that generates light flashes when certain particle interactions occur.
Larger experiments that use different detector materials and work over longer periods of time have achieved higher sensitivity, although the reported early success of CUPID-Mo creates the conditions for a planned follow-up experiment called CUPID with a 100 times larger detector array.
Berkeley Lab’s Contributions to CUPID-Mo
No experiment has yet confirmed whether the neutrinoless process exists. The existence of this process would confirm that neutrinos serve as their own antiparticles, and such evidence would also help explain why matter in our universe won against antimatter.
All data from the CUPID-Mo experiment – the acronym CUPID stands for CUORE Upgrade with Particle IDentification and “Mo” for the molybdenum contained in the detector crystal – is transferred from the Modane Underground Laboratory (Laboratoire souterrain de Modane) in France to the Cori supercomputer at the National Energy Research Scientific Computing Center at the Berkeley Lab.
Benjamin Schmidt, postdoctoral fellow in the Nuclear Science Division at Berkeley Lab, led all data analysis for the CUPID-Mo result and was supported by a team of researchers from Berkeley Lab and other members of the international collaboration.
Berkeley Lab also contributed 40 sensors that enabled the readout of signals picked up by CUPID-Mo’s 20-crystal detector array. The array was supercooled to about 0.02 Kelvin or minus 460 degrees Fahrenheit to maintain its sensitivity. Its cylindrical crystals contain lithium, oxygen and the isotope Mo-100 and generate tiny flashes of light when particles interact.
The international effort to achieve the CUPID-Mo result is remarkable, said Schmidt, given the context of the global pandemic that had raised uncertainty about how the experiment would continue.
“For a while, it looked like we had to end the CUPID-Mo experiment early in Europe due to the outbreak of COVID-19 in early March and the associated difficulties in supplying the experiment with the necessary cryogenic liquids,” he said .
He added, “Despite this uncertainty and the changes associated with office and school closures and restricted access to the underground laboratory, our employees have made every effort to keep the experiment going during the pandemic.”
Schmidt recognized 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 for them to be presented at Neutrino 2020, a virtual international conference on neutrino physics and astrophysics by Fermi National is hosted to introduce Accelerator Laboratory. Members of the CUPID-Mo collaboration plan to submit the results for publication in a peer-reviewed science journal.
Set ultrasonic sensitive detectors
A particular challenge in data analysis, according to Schmidt, was to ensure that the detectors were properly calibrated to record the “extremely elusive events” that are predicted to be associated with a neutrinoless double beta decay signal.
The neutrinoless decay process is expected to generate a very high-energy signal in the CUPID-Mo detector and a flash of light. The signal is expected to be free from interference from natural sources of radioactivity because of its high energy.
To test CUPID-Mo’s response to high energy signals, researchers placed other sources of high energy signals, including Tl-208, a radioactive isotope from thallium, near the detector array. The signals generated by the decay of this isotope are high in energy, but are not as high as the energy likely to be associated with the neutrinoless decay process in Mo-100, if any.
“So it was a big challenge to convince ourselves that we could calibrate our detectors with common sources, especially Tl-208,” said Schmidt, “and then extrapolate the detector response to our signal range and properly consider the uncertainties in this extrapolation can . “
To further improve calibration with high-energy signals, nuclear physicists used Berkeley Lab’s 88-inch cyclotron to make a wire containing Co-56, a cobalt isotope with low radioactivity, once the cyclotron was reopened last month after one temporary shutdown in response to the COVID-19 pandemic. The wire was shipped to France for testing with the CUPID Mo detector array.
Preparing for a next generation experiment in Italy
While CUPID-Mo may now lag sensitivity in measurements made by some other experiments – using different detector techniques and materials – because it is smaller and has not yet collected as much data, “with the full CUPID -Experiment that is used About 100 times more Mo-100 and with 10 years of operation, we have excellent prospects for the search and possible discovery of the neutrinoless double beta decay, “said Schmidt.
CUPID-Mo was installed at the Edelweiss III dark matter search site in a more than one mile deep tunnel in France near the Italian border and uses some Edelweiss III components. Meanwhile, CUPID is proposed to replace CUORE’s neutrinoless double beta decay detection experiment in 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 has completed data collection in two or three years, it can take four or five years to build the CUPID detector,” said Yury Kolomensky, US spokesman for the CUORE collaboration and senior faculty scientist at Berkeley Lab, who Cooperation between CUORE leads. “CUPID would be a relatively modest upgrade in terms of cost and technical challenges, but it will be a significant improvement in sensitivity.”
The collection of the physics data for CUPID-Mo ended on June 22nd, and new data that has not been included in the latest result means that the overall data will grow by approximately 20% to 30%. CUPID-Mo is supported by a group of French laboratories as well as laboratories in the USA, Ukraine, Russia, Italy, China and Germany.
NERSC is a user facility of 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 USA, Ukraine, Russia, Italy, China and Germany.
The experiment is funded by the U.S. Department of Nuclear Physics Office, the Berkeley Research Computing Program, the Agence Nationale de la Recherche, the IDEATE International Associated Laboratory (LIA), the Russian Science Foundation, the National Academy of Sciences of Ukraine, and the National Science supports the Foundation, the France-Berkeley Fund, the MISTI-France Fund and the Office for Science & Technology of the French embassy in the USA
The researchers are developing a novel approach to modeling rare core processes that have not yet been confirmed
Provided by Lawrence Berkeley National Laboratory
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