January 2022 (Volume 31, Number 1)

# FYI: Science Policy News From AIP

## DOE Eyes Expansive Hunt for Neutrinoless Double Beta Decay

By William Thomas

The Department of Energy’s Nuclear Physics program recently began advocating for a multi-nation search for neutrinoless double beta decay, a theorized phenomenon that, if detected, would offer a long-sought opening into physics beyond the Standard Model.

At a November meeting, program head Tim Hallman asserted, “The potential discovery of a neutrinoless double beta decay would be every bit as much of a game-changer as the discovery of supersymmetry at CERN, and as compelling as any accelerator-based research currently underway.”

The existence of the decay would indicate the neutrino is a Majorana fermion, meaning that it is its own antiparticle. That discovery could in turn offer clues about the origins of the neutrino’s slight mass as well as the prevalence of matter over antimatter in the universe.

Because the decay would be a low-energy radioactive process, albeit an exceptionally rare one, the search for it would not require a particle accelerator, only a detector large and sensitive enough to find one. Currently, experimenters are focused on building a “ton-scale” experiment to press beyond the limits of prior efforts.

Initiating such a project is among the top priorities identified in a 2015 long-range plan for nuclear science. However, there is also a risk DOE might not be able to undertake the project as it pursues other high priorities such as the Electron-Ion Collider at Brookhaven National Lab. Hallman has previously suggested that finding the needed funding could prove “challenging.”

Yet, at the November meeting he made the case that an international coalition could band together to fund up to three such experiments simultaneously.

Because even a ton-scale experiment could only be expected to count about one neutrinoless double beta decay per year, Hallman said it is desirable that any positive result be confirmed. He further noted that a contemporaneous confirmation would “greatly increase the chance of a Nobel Prize.”

To scope out a multi-experiment effort, DOE conducted a program review last year of potential ton-scale experiments, focusing on three top contenders: CUPID, nEXO, and LEGEND-1000. Estimates for the overall costs of those experiments were $64 million,$406 million, and $442 million, with DOE covering, respectively, 55%, 85%, and 60% of the total. Then, at the end of September, potential participants in an international effort convened near Italy’s Gran Sasso National Laboratory. Hallman said that the attendees ultimately agreed a multi-experiment effort would provide the best chance of success and that a formalized collaboration should be explored. Setting the prospective effort in perspective, Hallman said that building all three prioritized experiments would involve spending$1 billion spread over 10 years. He remarked that it is “not a small amount of money,” but neither is it “exceedingly outrageous” if several countries contribute. He also put the figure in context with the much larger sums spent on accelerator-based physics.

Hallman argued that in addition to multiplying efforts and dividing the cost burden, an international effort would help to build an “ecosystem” of institutions and researchers around the neutrinoless double beta decay search. That, in turn, could pave the way toward a subsequent generation of experimentation if it turns out to be needed.

He remarked, “If this really is a campaign, if the next round of experiments is not going to be decisive, because nature is not kind, then you need sort of an ecosystem to carry this through to conclusion. And it’s not going to be a two-decade outlook, it’s more like a four-decade outlook, and you want that ecosystem to be able to carry through.”

The author is a Senior Science Policy Analyst for FYI.

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