An international research team that includes Lawrence Berkeley National Laboratory (Berkeley Lab) scientists has established a new upper limit of 0.8 electron volts (eV) for the mass of the neutrino, a milestone that will bear on future discoveries in nuclear and particle physics, and cosmology.

Indeed, without a measurement of the mass scale of neutrinos – extremely light subatomic particles once thought to be beyond measurement – physicists say their understanding of the universe would remain incomplete. An electron volt is defined as the energy that an electron gains when it travels through a potential of one volt.

The research team, known collectively as the Karlsruhe Tritium Neutrino Experiment (KATRIN), located at Germany’s Karlsruhe Institute of Technology (KIT), published their findings Feb. 14 in the journal Nature Physics. Some of the statistical analysis used to determine the neutrino mass was performed using the Cori supercomputer at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC).

This push into the sub-eV mass scale of neutrinos by a model-independent laboratory method, the KATRIN team says, has allowed them to constrain the mass of these so-called lightweights of the universe with unprecedented precision.

Berkeley Lab’s Nuclear Science Division (NSD) played a key role in this pathbreaking measurement of the mass of the neutrino. “The leadership of Berkeley Lab scientists in this critical area of modern physics combined with our unmatched computational and other technological resources have made us a natural partner in this enterprise,” said interim NSD Division Director, Volker Koch.

The team’s first reported measurement in 2019 yielded a result of 1.1 eV, said Berkeley Lab’s Alan Poon, NSD’s KATRIN group leader and Division Deputy Director for Science. Other NSD scientists, including Bjoern Lehnert, Ann-Kathrin Schuetz, and Rebecca Carney, also contributed to the research. Further measurement of the neutrino’s mass, Poon said, will continue through 2024.

To measure neutrino mass, KATRIN makes use of the beta decay of tritium, an unstable hydrogen isotope. The team was able to determine the mass of the neutrino via the measured energy of electrons released in the decay process. But to do so has necessitated a major technological effort: The experiment houses the world’s most intense tritium source as well as a giant spectrometer to measure the energy of decay electrons with unrivaled precision.

The expertise of scientists at the Tritium Laboratory Karlsruhe, which hosts KATRIN, has allowed safe handling of the chemical in quantities needed to reach experimental goals, the team says. What is more, KATRIN scientists have worked to reduce background noise in the gigantic KATRIN spectrometer, another key factor in establishing the new upper limit for the mass of the neutrino.

“KATRIN is an experiment with the highest technological requirements and is now running like perfect clockwork,” Guido Drexlin, project leader at KIT and one of its co-spokespersons, said of the experiment.

In-depth analysis of KATRIN’S experimental data demanded everything from the analysis team say coordinators, Magnus Schlösser of KIT, and Susanne Mertens of the Max Planck Institute of Physics and TU Munich. Each effect produced and recorded during the experiment, no matter how small, they said, had to be investigated in detail.

“Only by this laborious and intricate method were we able to exclude a systematic bias of our result due to distorting processes,” Schlösser and Mertens write in a KIT news release announcing the results. “We are particularly proud of our analysis team, which successfully took up this huge challenge with great commitment.”

Berkeley Lab’s Lehnert, for example, led the supercomputer data analysis effort in the US. The Lab’s analysis “provides an important input to other modern analyses in cosmology, it assigns probabilities to the neutrino mass values that have not yet been excluded previously,” he said.

Without the resources and support made available by NERSC, Poon said, a very computationally intensive statistical method known as Bayesian analysis “would have been very difficult.”

Poon says neutrinos are perhaps the most fascinating elementary particle in the universe. In cosmology, they play an important role in the formation of large-scale structures in the universe, while in nuclear and particle physics their tiny but non-zero mass points toward new physics phenomena beyond current theory.

Lehnert said that, since starting scientific measurements in 2019, the high quality of KATRIN data has continuously improved. He noted that while the concept of Bayesian analysis – a highly sophisticated interpretation of probabilities – is not new, “the applications are relatively modern because the computing power needed has only really been feasible in the last two decades or so.”

Indeed, experimental data from the first year of measurements and then modeling based on a vanishingly small neutrino mass matched perfectly, the team reports. From this, a new upper limit on the neutrino mass of 0.8 eV was determined. It is the first time that a direct neutrino mass experiment has entered the sub-eV mass range, they say, where the fundamental mass scale of neutrinos is suspected to reside.

KATRIN co-spokesperson, Christian Weinheimer of the University of Münster, added that “the increase of the signal rate and the reduction of background rate were decisive for the new result.”

Berkeley Lab’s Poon said, “in 2019, we shattered the neutrino-mass limit from previous experiments by a factor of two and now, at 0.8 electron volts, we have an even tighter constraint. In time, we will make even better measurements, more sensitive measurements, that help us understand the physical world better.”

This work was supported by the US Department of Energy, the European Research Council, the Helmholtz Association, Federal Ministry of Education and Research, Helmholtz Alliance for Astroparticle Physics, and Helmholtz Young Investigator Group; German Research Foundation; Max Planck Society of Germany; Ministry of Education, Czech Republic; and the Russian Federation Ministry of Science and Higher Education.

NERSC is a DOE Office of Science user facility at Berkeley Lab.

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