DOE nuclear physics program strategy includes neutrinoless double beta decay experiments at Wright Lab
The US Department of Energy’s (DOE) nuclear physics program is pursuing an international strategy to fund three ton-scale experiments – CUPID, nEXO and LEGEND-1000 – that are sensitive enough to search for evidence of a double beta decay without neutrinos (0νββ). Wright Lab researchers, including Yale physics professors Karsten Heeger and Reina Maruyama, and assistant professor David Moore, are involved in directing and building two of the three experiments that will define the future of the 0νββ effort – CUPID (Heeger and Maruyama) and nEXO (Moore).
The search for new physics with neutrinoless double beta decay
The widely accepted “standard model” of particle physics has a symmetry between matter and antimatter – whenever matter particles are created in a laboratory, an equal number of antimatter particles are also created. However, observations show that the Universe is made of matter and not antimatter, so a process in the early universe must have violated this symmetry. The neutrino, a mysterious, ghostly particle that passes through most of the matter in the universe unaffected but still has mass, could be at the heart of this mystery.
Wright Lab researchers lead and build various experiments to learn more about the neutrino. Among them, CUPID and nEXO are specifically looking for neutrinoless double beta decay, a yet unobserved nuclear process that would point to new physics beyond the Standard Model of particle physics.
Detecting neutrinoless double beta decay would reveal whether the neutrino is its own antiparticle and demonstrate that neutrinos are so-called majorana particles. This would have implications for understanding the nature of neutrinos, how neutrinos acquire their masses, and how the small excess of matter over antimatter was generated in the early universe.
Heeger said, “The observation of neutrinoless double beta decay would be a definitive sign that neutrinos are their own antiparticles and point to new physics beyond the Standard Model.”
If the neutrino turns out to be a majorana particle, the conserved lepton number principle of the Standard Model would be violated and could explain the matter/antimatter asymmetry observed in the Universe.
Maruyama said, “Finding anything that breaks symmetries in particle interactions is exciting.”
Next-gen beta double decay experiments
Each of the three experiments in the DOE initiative searches for neutrinoless double beta decay by studying the decays of one element to another element, but each studies a different isotope: CUPID uses molybdenum (Mo-100), nEXO uses xenon (Xe-136) and LEGEND-1000 uses Germanium (Ge-76). CUPID’s predecessor, called CUORE, search for 0νββ in tellurium (Te-130).
Moore said that “there has been good support at DOE for a multi-isotope approach since the 2015 Nuclear Science Advisory Committee (NSAC) report to search for neutrinoless double beta decay with different detection technologies and different isotopes. He continued: “Searching for half-life decay is difficult and searching for it in multiple ways reduces risk. If we see it in different detectors, we can be more certain that we are seeing the signal we are looking for, even if we only see a few candidate decays in 10 years.
CUORE Upgrade with Particle Identification (CUPID)
CUPID is an upgrade of the Cryogenic Underground Laboratory for Rare Events (CUORE), the largest operational bolometric experiment, located at the Gran Sasso National Laboratory (LNGS) in Assergi, Italy.
CUPID uses and leverages the infrastructure and cryostat that CUORE has built over many years at LNGS, as well as CUORE’s detection technology that has demonstrated an exceptional ability to search for neutrinoless double beta decay. The original CUORE experiment detects heat (phonons) resulting from energy deposition from nuclear decay or particle interactions. CUPID will upgrade CUORE’s bolometric detectors using scintillating crystals so that CUPID can also detect light (photons) as well as heat. This additional feature will improve CUPID’s ability to distinguish backgrounds from the very rare signal.
CUPID’s bolometer technology also allows flexibility in studying a number of different isotopes. The CUORE experiment used tellurium and CUPID will use molybdenum, but it is possible to use the same technology and the same infrastructure to examine other isotopes, if others become interesting in the search for double beta decay without neutrinos.
CUPID at the Wright Lab
Maruyama has been involved with CUORE since 2004 and Heeger joined in 2006. The Wright Lab team was responsible for the design, construction and commissioning of the CUORE detector calibration system; analysis and simulation of CUORE data; and collaborative leadership. Their efforts in CUPID build on years of experience with CUORE. Currently, Heeger and Maruyama are the Principal Investigators and Heeger is the CUPID Scientific Co-Spokesperson.
Maruyama is responsible for coordinating efforts to develop thermal detectors, called neutron transmutation doped (NTD) thermistors, that read bolometers, with collaborators at Berkeley and MIT. NTD thermistors are made by bringing slices of germanium into nuclear reactors. The neutrons generated in the reactors excite the germanium isotopes and transform them into gallium and arsenic. The resulting germanium doped with gallium and arsenic can be used to transform the chips into a semiconductor capable of detecting the amount of vibrations/phonons produced in the crystals of the CUPID detector and measuring the temperature change of the crystals, which is the main signal sought by the experiment.
Wright Lab graduate student Ridge Liu works on NTD thermistors and examines vibration data to characterize thermistor performance and ensure the experiment has the best possible energy resolution.
Wright Lab is also building the muon veto system for CUPID and is responsible for calibrations and acoustic and vibration monitoring of the detector. The cosmic ray-induced muons are shielded by the mountain surrounding the underground laboratory, but some pass through. When they do, they can deposit energy that could be confused with neutrinoless double beta decay events. An additional layer of muon markers is placed around the main detector to further reduce this background noise.
Graduate students Samantha Pagan and Iris Ponce both work in research and development (R&D) for muon markers. Pagan develops the physical plastic scintillator panels embedded with frequency-shifted light fibers, while Ponce works on the data acquisition system and the simulation of light collection.
Other members of the Heeger-Maruyama lab who are involved in CUPID are research scientist James Nikkel; Associate Researchers Tom Langford and Penny Slocum; research support specialist James Wilhelmi; and Pranava postdoctoral associates Teja Surukuchi and Jorge Torres.
nEXO will be a large detector that uses 5 tons of liquid xenon (136Xe) in a radiopure time projection chamber (TPC), currently planned to be installed underground at SNOLAB in Sudbury, Ontario. It was the successor to the EXO-200 experiment, which was a 200 kg prototype at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM that demonstrated the potential effectiveness of noble gas technology liquids for the search for neutrinoless double beta decay, and established some of the most stringent constraints on this process to date.
Due to the large size and homogeneity of nEXO, backgrounds from outside the detector will not reach its center, where an extremely low background rate will allow nEXO to see evidence of decay even if it occurs with a half-life as long as 1028, making the detector significantly more sensitive to neutrinoless double beta decay signals than EXO and other existing experiments.
nEXO at Wright Lab
Moore is the subsystem scientist for the nEXO photon detector, and the main responsibility of the Wright Lab group is to build the photon detector for nEXO’s large (4.5 square meter) array of silicon photomultiplier photosensors at ultra low background noise that detect light emitted by xenon. The Wright Lab group is working closely with scientists at Brookhaven National Lab, where the final photodetector system will be assembled and tested.
The Moore Group also performs simulations for nEXO as well as laboratory testing at Wright Lab of new readout techniques for large liquid xenon detectors. Postdoctoral associate Avinay Bhat is working on silicon photomultiplier testing, while graduate student Ako Jamil is doing simulations of light and energy transport. Graduate students Sierra Wilde and Glenn Richardson work on light and charge collection in nEXO simulations and measurements of silicon photomultipliers and charge transport in liquid xenon.