An worldwide team of scientists, including MIT experts, revealed today that they have set a new upper limit on the mass of the neutrino subatomic particle: 0.8 eV/c2. This is a significant step forward for future nuclear and particle physics discoveries.

According to physicists, a precise grasp of the mass scale of neutrinos would leave their theory of the cosmos incomplete. With the new discovery, published in Nature Physics, scientists from the Karlsruhe Tritium Neutrino Experiment (KATRIN) at Germany’s Karlsruhe Institute of Technology (KIT) have expanded our knowledge of neutrinos’ mass.

Since 2005, MIT has been a part of the KATRIN project. MIT researchers contributed to the development of the simulation framework used to study the detector as well as the data processing capabilities.

The team’s first published measurement in 2019 came in at 1.1 eV/c2. Scientists subsequently broke the 1 eV/c2 barrier, demonstrating the technique’s accuracy. The mass of the neutrino will be measured in greater detail until 2024.

The beta decay of tritium, an unstable hydrogen isotope, is used by KATRIN to detect neutrino mass. The mass of the neutrino was determined by measuring the energy of electrons emitted during the decay process. However, doing so has needed a significant technical effort: The experiment includes the world’s most powerful tritium source as well as a massive spectrometer that can precisely detect the energy of decay electrons.

According to Joseph Formaggio, the primary investigator at MIT, “the notion of employing radioactive decays to estimate neutrino masses is as ancient as the idea of the neutrino itself.” “The approach was invented by [Enrico] Fermi himself in the 1930s.” But it is only now that we have the capability to apply the approach to precisely extract the neutrino mass.”

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According to the team, scientists at the Tritium Laboratory Karlsruhe, which houses KATRIN, have made it possible to safely handle the chemical in the proportions required to achieve experimental aims. Scientists at KATRIN have also worked to minimize background noise in the KATRIN spectrometer, which is another important component in determining the new upper limit for neutrino mass.

“KATRIN is a technologically demanding experiment that is currently functioning like a well-oiled machine,” said Guido Drexlin, project head at KIT and one of the experiment’s co-spokespersons.

The team reveals that experimental results from the first year of observations aligned exactly with models based on a vanishingly tiny neutrino mass. As a result, a new upper limit on neutrino mass of 0.8 eV/c2 has been established. According to the researchers, this is the first time a direct neutrino mass experiment has penetrated the sub-eV mass region, which is thought to be where the basic mass scale of neutrinos resides.

Over the next few years, the KATRIN project will continue to collect data, ultimately resulting to a sensitivity that is up to four times better than what we describe now. “Hopefully, a positive neutrino mass measurement is on the way,” Formaggio adds. “It will really flesh up our understanding of the basic particles in our Standard Model.”

Formaggio is also working on Project 8, a future experiment that aims to improve neutrino mass sensitivity even further.

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