Neutrino’s Mass Smaller Than Previously Known

The neutrino, the tiniest of the fundamental particles, is even smaller than previously known. The Karlsruhe Tritium Neutrino Experiment (KATRIN), an international team of researchers including Carnegie Mellon physicist Diana Parno, has established a new upper limit on the neutrino’s mass at less than 0.8 electron volts (eV). The team’s findings were published in Nature Physics.

The weight of the neutrino is an important missing piece of Standard Model of particle physics. A theory that describes the fundamental forces of the universe and the known elementary particles, the Standard Model has been used to explain many phenomena seen in the universe and in particle physics. But the Standard Model as it was written some 50 years ago isn’t perfect. One flaw is that it assumed that neutrinos were massless.

While neutrinos may be extremely light — more than a half a million times lighter than the electron — there are an astounding number of neutrinos in the universe.

“Just like a pound of lead and a pound of feathers take up a different amount of space, their mass is still the same,” said Parno, assistant professor in Carnegie Mellon’s Department of Physics. “Neutrino mass plays an important role in the universe’s expansion and the formation of structures, like galaxy clusters. Knowing the mass of the neutrino is essential to understanding our universe.”

Detecting and weighing the neutrino is a difficult task. Neutrinos have no charge and they rarely interact with other matter.  

KATRIN, located at Germany’s Karlsruhe Institute of Technology, measures neutrino mass using the beta decay of tritium, which emits a pair of particles: one electron and one neutrino. The researchers measure the electrons’ energy and then use that to calculate the energy and mass of the neutrino. 

Parno’s team at Carnegie Mellon University contributed to the analysis of the experiment’s data. Among other work, Larisa Thorne and Ana Paula Vizcaya, who earned their Ph.D.’s from Carnegie Mellon in 2021, studied backgrounds from the rear of the experiment, investigated fitting techniques and demonstrated that the experiment could be run safely without contaminating sensitive equipment with tritium ions. Vizcaya, along with Parno, Carnegie Mellon doctoral candidate Byron Daniel and collaborators at the University of Washington, also experimentally validated the molecular theory used to extract the neutrino mass. Carnegie Mellon undergraduate student Aishwarya Vijai is now working to help understand the precise behavior of the tritium source over time.

The KATRIN experiment will continue to collect data for several years. The researchers hope that they can further narrow the parameters for the neutrino’s mass and continue to improve the Standard Model.

The KATRIN collaboration involves scientists from six countries and is funded by a number of international entities. The U.S. efforts are supported by the Department of Energy (DOE) Office of Nuclear Physics. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.