Wright Lab graduate student Samantha Pagan works on muon veto system taggers. Photo by Iris Ponce.
The DOE’s nuclear physics department is working on an international plan to finance three tonne-scale experiments–CUPID, nEXO, and LEGEND-1000–sensitive enough to look for evidence of neutrinoless double beta decay (0v). Two of the three experiments that will determine the future of the 0 effort – CUPID (Heeger and Maruyama) and nEXO (Moore) – are being led and built by Wright Lab researchers, including Yale Physics professors Karsten Heeger and Reina Maruyama, and assistant professor David Moore (Moore).

With neutrinoless double beta decay, scientists are looking for novel physics.

The generally recognized “standard model” of particle physics assumes that matter and antimatter are symmetric, meaning that anytime matter particles are formed in a laboratory, an equal amount of antimatter particles are created as well. However, observations demonstrate that the Beginning is made up of matter rather than antimatter, implying that some symmetry in the early universe was broken. The neutrino, a strange, ghostly particle that travels through most stuff in the cosmos without being impacted yet retains mass, might be at the center of this puzzle.

To understand more about the neutrino, Wright Lab researchers are directing and designing a range of experiments. CUPID and nEXO are two of them, and they’re looking for neutrinoless double beta decay, which is an as-yet-unobserved nuclear event that might point to novel physics outside the Standard Model.

The discovery of neutrinoless double beta decay would show if the neutrino is its own antiparticle and that neutrinos are majorana particles. This has ramifications for understanding the nature of neutrinos, how they acquire masses, and how the early universe’s modest excess of matter over antimatter was formed.

“The discovery of neutrinoless double beta decay would be a conclusive indicator that neutrinos are their own antiparticles and imply novel physics outside the standard model,” Heeger added.

The standard model’s conserved principle of lepton number would be broken if the neutrino is discovered to be a majorana particle, which might explain the observed matter/antimatter imbalance in the Universe.

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“Finding anything that breaches symmetry in particle interactions is interesting,” Maruyama added.

Experiments on double beta decay in the next generation

The DOE initiative’s three experiments are all looking for neutrinoless double beta decay by researching the decays of one element to another, but each is looking into a different isotope: Molybdenum (Mo-100) is used in CUPID, xenon (Xe-136) is used in nEXO, and germanium is used in LEGEND-1000 (Ge-76). CUPID’s forerunner, dubbed CUORE, is looking for 0 in tellurium (Te-130).

“Since the 2015 Nuclear Science Advisory Committee (NSAC) recommendation to search for neutrinoless double beta decay using multiple detector technologies and different isotopes,” Moore said, “there has been considerable support in the DOE for a multi-isotope strategy.” “Searching for half-life degradation is difficult, and looking for it in numerous methods decreases the risks,” he concluded. Even if we only observe a few candidate decays in ten years, if we see it in various detectors, we may be more certain that we are detecting the signal we are searching for.”

Particle Identification Upgrade for CUORE (CUPID)

The Cryogenic Underground Laboratory for Rare Events (CUORE), the world’s biggest functioning bolometric experiment, is being upgraded at the Gran Sasso National Laboratory (LNGS) near Assergi, Italy.

CUPID makes use of and leverages CUORE’s LNGS infrastructure and cryostat, as well as CUORE’s detector technology, which has showed remarkable capabilities to search for neutrinoless double beta decay. The initial CUORE experiment measures heat (phonons) produced by energy deposition caused by nuclear decay or particle interactions. CUPID will use scintillating crystals to enhance CUORE’s bolometric detectors, allowing it to sense light (photons) as well as heat. CUPID’s capacity to identify backdrops to the extremely infrequent signal will be improved because to this new functionality.

The bolometer technology used by CUPID also enables for the investigation of a variety of isotopes. Although the CUORE experiment used tellurium and CUPID will use molybdenum, the same technology and infrastructure might be used to look at additional isotopes if they become of interest in the hunt for neutrinoless double beta decay in the future.

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Wright Lab’s CUPID

Heeger joined CUORE in 2006, while Maruyama has been a member since 2004. The Wright Lab team was in charge of the CUORE Detector Calibration System’s design, building, and commissioning, as well as the analysis and modeling of CUORE data and collaborative leadership. Their work with CUPID builds on their previous work with CUORE. Heeger and Maruyama are now Principal Investigators, and Heeger is CUPID’s scientific co-spokesperson.

Along with researchers from Berkeley and MIT, Maruyama is in charge of organizing the work to build the thermal detectors that read out the bolometers, known as neutron transmutation doped (NTD) thermistors. NTD thermistors are created by sending germanium wafers to nuclear power plants. The neutrons produced in the reactors excite the germanium isotopes, causing gallium and arsenic to form. The resulting germanium doped with gallium and arsenic can be used to convert the chips into a semiconductor that can detect the amount of vibration/phonons produced in CUPID’s detector crystals and measure the change in the crystals’ temperature, which is the main signal that the experiment is looking for.

Ridge Liu, a Wright Lab graduate student working on the NTD themistors, is analyzing vibration data to assess the themisters’ performance and ensure that the experiment has the highest possible energy resolution.

Wright Lab is also developing CUPID’s muon veto system and is in charge of the detector’s calibrations, as well as its acoustic and vibration monitoring. The mountain that surrounds the subterranean lab shields cosmic-ray produced muons, yet some do get through. They may deposit energy that might be misinterpreted with neutrinoless double beta decay events when they do so. To further decrease the background, an additional layer of muon taggers is added around the primary detector.

Samantha Pagan and Iris Ponce, both graduate students, are working on muon tagger research and development. Pagan is working on the data collecting system and light collection simulation, while Ponce is working on the actual plastic scintillator panels inlaid with frequency-shifting light fibers.

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Research Scientist James Nikkel, Associate Research Scientists Tom Langford and Penny Slocum, Research Support Specialist James Wilhelmi, and Postdoctoral Associates Pranava and James Wilhelmi are among the Heeger-Maruyama Lab members engaged in CUPID. Jorge Torres and Teja Surukuchi


The nEXO detector will be a huge detector that will employ 5 tons of liquid Xenon (136Xe) in a radiopure time projection chamber (TPC) and will be built underground at SNOLAB in Sudbury, Ontario. It is the follow-up to the EXO-200 experiment, a 200 kg prototype at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico, that demonstrated the potential effectiveness of liquid noble gas technology for searching for neutrinoless double beta decay and imposed some of the most stringent constraints on the process to date.

Backgrounds from outside the detector will not reach the detector’s center, where an extremely low background rate will allow nEXO to see evidence for the decay even if it occurs with a half-life as long as 1028 years, making the detector significantly more sensitive to neutrinoless double beta decay signals than EXO and other existing experiments.

Wright Lab’s nEXO

Moore is the nEXO photon detector’s subsystem scientist, and the group at Wright Lab’s main task is to create the photon detector for nEXO’s enormous array (4.5 square meters) of extremely low background silicon photomultipler photosensors that detect xenon light. The Wright Lab team is collaborating closely with Brookhaven National Lab scientists, who will construct and test the final photodetector system.

Moore’s group also works on nEXO models and lab testing of innovative readout methods for massive liquid xenon detectors at Wright Lab. Avinay Bhat, a postdoctoral associate, is testing silicon photomultipliers, while graduate student Ako Jamil is simulating light transport and energy. Sierra Wilde and Glenn Richardson, graduate students, are working on light and charge collection in nEXO simulations, as well as silicon photomultiplier measurements and charge transport in liquid xenon.

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