A new upper limit on neutrino mass


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February 23, 2022

The spectrometer of the Karlsruhe Neutrino experiment, or KATRIN.Michael Zacher

An international research team, including scientists from the University of Washington, has established a new upper mass limit for the neutrino, the lightest known subatomic particle.

In one paper published February 14 in Nature Physics, the collaboration – known as Karlsruhe Tritium Neutrino Experiment or KATRIN — indicates that the mass of the neutrino is less than 0.8 electron-volts, or 0.8 eV/c2. Focusing on the elusive value of neutrino mass will solve a major mystery in particle physics and provide scientists with a more complete view of the fundamental forces and particles that shape ourselves, our planet, and the cosmos.

KATRIN, based in Germany at the Karlsruhe Institute of Technology, has been tracking the neutrino’s mass since the experiment began collecting data in 2018. The team’s first measurement reported in 2019 nearly halved the upper limit of this value, 2 eV/ c2 at about 1.1 eV/c2. With the new findings reported this month, the upper limit falls below 1 eV/c2 for the first time.

the standard model in particle physics once predicted that neutrinos should have no mass. But experiments in the early 2000s at the Super-Kamiokande and detectors at the Sudbury Neutrino Observatory demonstrated that they actually had a small mass, a discovery recognized in 2015 with the Nobel Prize in Physics.

Although this mass is very small, it had a major impact because neutrinos are so numerous, according to the co-author Peter Doemember of the KATRIN team and teacher-researcher in physics at the UW.

“There are almost as many neutrinos in the universe as there are photons,” Doe said. “Thus, although the mass of neutrinos is tiny, their abundance means that they play an important role in the evolution of the large-scale structures of the universe, such as the distribution of galaxies. Determining the mass of neutrinos would also make it possible to further refine the standard models of particle physics and of cosmology. For these reasons, measuring the neutrino mass scale is of great importance for both particle physics and cosmology.

To measure the mass of neutrinos, KATRIN uses the beta decay of tritium, an unstable isotope of hydrogen. The team takes precision measurements of the energy spectrum of the electrons released by the decay process. The neutrino’s mass is revealed by a tiny distortion in this spectrum. But collecting data on these tiny particles is a big undertaking: the experiment uses the world’s most intense tritium source and a giant spectrometer to measure the energy of decay electrons with extremely high precision.

“KATRIN is an experiment with the highest technological demands and is now running like a perfect clock,” said KATRIN’s co-author and co-spokesperson. Guido Drexlin of the KIT.

The UW is a founding member of the KATRIN collaboration, established in 2001. Edited by co-author Hamish Robertson, professor emeritus of physics at UW, UW was the lead US institution for the design and acquisition of the KATRIN electron detection system. Directed by co-author Sanshiro EnomotoAn associate research professor of physics at UW, UW efforts are now focused on developing data analysis tools for KATRIN experiments, as well as understanding systematic errors in the detection system.

Data collected by the experiment in 2019 and 2021 allowed KATRIN scientists to reduce the upper limit of neutrino mass by more than a factor of two. The KATRIN experiment will continue to collect data until 2024, with the goal of achieving 4 times the sensitivity of what the collaboration has achieved to date.

Earlier indirect experiments by other groups suggest that the lower limit of neutrino mass at 0.02 eV/c2. But the technique used by KATRIN practically does not make it possible to determine a mass lower than 0.2 eV/c2. A new attempt, Project 8expects to achieve an upper limit sensitivity of 0.04 eV/c2, according to Doe. Project 8 will measure neutrino mass using an atomic tritium source — rather than molecular tritium — and track electron energy using a new detection technique recently demonstrated at UW.

Menglei Sun, a former postdoctoral fellow at UW’s Center for Experimental Nuclear Physics and Astrophysics, is also a co-author on the paper. KATRIN’s efforts in the United States are funded by the US Department of Energy’s Office of Nuclear Physics.

For more information, contact Doe at [email protected].

Adapted from a Press release by the Massachusetts Institute of Technology.

Tag(s): Center for Experimental Nuclear Physics and Astrophysics • College of Arts and Sciences • Department of Physics • Hamish Robertson • Peter Doe


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