UW News

February 23, 2022

A new upper limit on the mass of neutrinos

The spectrometer for the Karlsruhe Neutrino Experiment, or KATRIN.Michael Zacher

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

In a paper published Feb. 14 in Nature Physics, the collaboration — known as the Karlsruhe Tritium Neutrino Experiment or KATRIN — reports that the neutrino’s mass is below 0.8 electron volts, or 0.8 eV/c2. Honing in on the elusive value of the neutrino’s mass will solve a major outstanding mystery in particle physics and equip 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 hunting for the neutrino’s mass since the experiment began collecting data in 2018. The team’s first reported measurement in 2019 cut the upper limit for this value almost in half, from 2 eV/c2 to about 1.1 eV/c2. With the new findings reported this month, the upper limit drops below 1 eV/c2 for the first time.

The Standard Model of particle physics once predicted that neutrinos shouldn’t have a mass. But experiments in the early 2000s at the Super-Kamiokande and the Sudbury Neutrino Observatory detectors demonstrated that they actually do have a small mass, a discovery recognized in 2015 with the Nobel Prize in Physics.

Though that mass is very small, it has had a major impact because neutrinos are so numerous, according to co-author Peter Doe, a KATRIN team member and research professor of physics at the UW.

“There are almost as many neutrinos in the universe as there are photons,” said Doe. “So, although the neutrino mass is tiny, their abundance results in them playing an important role in the evolution of the large-scale structures of the universe, such as the distribution of galaxies. Determining the neutrino mass would also enable further refinement of the standard models of particle physics and of cosmology. For these reasons, the measurement of the mass scale of the neutrino is of great importance to both particle physics and cosmology.”

To measure neutrino mass, KATRIN makes use of the beta decay of tritium, an unstable isotope of hydrogen. The team takes precision measurements of the energy spectrum of electrons released by the decay process. The neutrino mass is revealed in a minute distortion within that spectrum. But collecting data about these small particles is a big undertaking: The experiment utilizes the world´s most intense tritium source as well as a giant spectrometer to measure the energy of decay electrons with extremely high precision.

“KATRIN is an experiment with the highest technological requirements and is now running like perfect clockwork,” said co-author and KATRIN co-spokesperson Guido Drexlin of the KIT.

The UW is a founding member of the KATRIN collaboration, which was formed in 2001. Under the direction of co-author Hamish Robertson, a UW professor emeritus of physics, the UW was the lead U.S. institution for designing and acquiring KATRIN’s electron detection system. Led by co-author Sanshiro Enomoto, a UW research associate professor of physics, UW efforts now focus on developing data analysis tools for KATRIN experiments, as well as understanding systematic errors in the detector system.

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

Previous, indirect experiments by other groups suggest that the lower limit for the neutrino’s mass at 0.02 eV/c2.  But the technique employed by KATRIN cannot practically determine a mass below 0.2 eV/c2. A new endeavor, Project 8, plans to reach an upper limit sensitivity of 0.04 eV/c2, according to Doe. Project 8 will measure the neutrino’s mass by making use of an atomic tritium source — rather than molecular tritium — and will track the electron energy using a novel detection technique that was recently demonstrated at the UW.

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

For more information, contact Doe at pdoe@uw.edu.

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