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Physicists Don’t Know the Mass of a Neutrino, But Now They Know it’s No Larger Than 1 Electron Volt

The Standard Model of Particle Physics is one of science’s most impressive feats. It’s a rigorous, precise effort to understand and describe three of the four fundamental forces of the Universe: the electromagnetic force, the strong nuclear force, and the weak nuclear force. Gravity is absent because so far, fitting it into the Standard Model has been extremely challenging.

But there are some holes in the Standard Model, and one of them involves the mass of the neutrino.

The existence of the neutrino was first proposed in 1930, then detected in 1956. Since then, physicists have learned there are three types of neutrinos, and they’re abundant and elusive. Only special facilities can detect them because they rarely interact with other matter. There are several sources for them, and some of them have been zipping through space since the Big Bang, but most of the neutrinos near Earth come from the Sun.

The Standard Model predicts that neutrinos have no mass, like photons. But physicists have found that the three types of neutrinos can transform into one another as they move. According to physicists, they should only be able to do that if they do have mass.

The Standard Model of Elementary Particles. Image: By MissMJ – Own work by uploader, PBS NOVA [1], Fermilab, Office of Science, United States Department of Energy, Particle Data Group, CC BY 3.0

But how much mass? That’s a question that’s been dogging particle physicists. And answering that question is part of what drives scientists at KATRIN (Karlsruhe Tritium Neutrino Experiment.)

The 10 meter high-resolution spectrometer at the heart of KATRIN (Karlsruhe Tritium Neutrino Experiment. Image Credit: KATRIN Collaboration.

“These findings by the KATRIN collaboration reduce the previous mass range for the neutrino by a factor of two…”


A team of researchers have come up with part of an answer to that: the mass of the neutrino can be no larger than 1.1 electron volts (eV.) This is a reduction of the upper limit of a neutrino’s mass by nearly 1 eV; from 2 eV down to 1.1 eV. By building on previous experiments that set the lower mass limit at 0.02 eV, these researchers have set a new range for the neutrino’s mass. It shows that a neutrino has less than 1/500,000th the mass of an electron. This is an important step in the advancement of the Standard Model.

“Knowing the mass of the neutrino will allow scientists to answer fundamental questions in cosmology, astrophysics and particle physics…”

Hamish Robertson, KATRIN scientist and professor emeritus of physics at the University of Washington.

The researchers behind this work come from 20 different research institutions around the world. They’re working with KATRIN at the Karlsruhe Institute of Technology in Germany. The KATRIN facility features a 10 meter high-resolution spectrometer which allows it to measure electron energies with great precision.

The KATRIN instrument features a high-resolution spectrometer that allows it to measure electron volts with extreme precision. This diagram shows the layout and major features of the KATRIN experimental facility at the Karlsruhe Institute of Technology. Image Credit: Karlsruhe Institute of Technology

The KATRIN team presented their results at the 2019 Topics in Astroparticle and Underground Physics conference in Toyama, Japan, on September 13th.

“Knowing the mass of the neutrino will allow scientists to answer fundamental questions in cosmology, astrophysics and particle physics, such as how the universe evolved or what physics exists beyond the Standard Model,” said Hamish Robertson, a KATRIN scientist and professor emeritus of physics at the University of Washington. “These findings by the KATRIN collaboration reduce the previous mass range for the neutrino by a factor of two, place more stringent criteria on what the neutrino’s mass actually is, and provide a path forward to measure its value definitively.”

Neutrinos are notoriously difficult to detect, even though they’re abundant. Only photons are more abundant. Like their name says, they’re electrically neutral. This makes detecting them extremely difficult. There are neutrino observatories sunk deep in the Antarctic ice, and also deep in abandoned mines. They often use heavy water to entice the neutrinos to interact. When a neutrino does interact, it produces Cherenkov radiation that can be measured.

Neutrinos are nearly impossible to detect. One neutrino observatory, called the IceCube Neutrino Laboratory, ) tries to detect them by sinking strings of detectors deep into the cold, dark, Antarctic ice, where it tries to observe the rare times when neutrinos interact with other matter. This image shows a visual representation of one of the highest-energy neutrino detections superimposed on a view of the IceCube Lab at the South Pole. Credit: IceCube Collaboration.

“If you filled the solar system with lead out to fifty times beyond the orbit of Pluto, about half of the neutrinos emitted by the sun would still leave the solar system without interacting with that lead,” said Robertson.

The history of the neutrino has evolved over time with experiments such as KATRIN. Originally, the Standard Model predicted neutrinos would have no mass. But in 2001, two different detectors showed their mass is non-zero. The 2015 Nobel Prize in Physics was awarded to two scientists who showed that neutrinos can oscillate between types, showing they have mass.

The KATRIN facility measures the mass of neutrinos indirectly. It works by monitoring the decay of tritium, which is a highly-radioactive form of hydrogen. As the tritium isotope decays, it emits pairs of particles: an electron and an anti-neutrino. Together, they share 18,560 eV of energy.

The tell-tale blue glow of Cherenkov radiation from the Advanced Test Reactor in Idaho. Image Credit: By Argonne National Laboratory – originally posted to Flickr as Advanced Test Reactor core, Idaho National LaboratoryUploaded using F2ComButton, CC BY-SA 2.0,

In most cases, the pair of particles share the 18,560 eV equally. But in rare instances, the electron hogs most of the energy, leaving the neutrino with very little. These rare instances are what scientists are focused on.

Due to E=mC2, the tiny amount of energy left for the neutrino in these rare cases must also equal its mass. Because KATRIN has the power to measure the electron accurately, it’s also able to determine the neutrino’s mass.

“Solving the mass of the neutrino would lead us into a brave new world of creating a new Standard Model,” said Peter Doe, a research professor of physics from the University of Washington who works on KATRIN.

This new Standard Model that Doe mentions may have the potential to account for dark matter, which makes up most of the matter in the Universe. Efforts like KATRIN may one day detect another, fourth type of neutrino called the sterile neutrino. So far this fourth type is only conjecture, but it is a candidate for dark matter.

A computer simulation of the distribution of matter in the Universe. Orange regions host galaxies; blue structures are gas and dark matter. It’s possible that there’s a fourth type of undiscovered neutrino called a sterile neutrino that could conceivably account for all dark matter in the Universe. Credit: TNG Collaboration

“Neutrinos are strange little particles,” said Doe. “They’re so ubiquitous, and there’s so much we can learn once we determine this value.”

Showing that neutrinos have mass, and constraining the range of that mass, are both important. But particle physicists still don’t know how they gain their mass. It’s probably different than how other particles gain theirs.

Results like this from KATRIN are helping close a hole in the Standard Model, and in our overall understanding of the Universe. The Universe is full of ancient neutrinos from the Big Bang, and every advancement in the mass of the neutrino helps us understand how the Universe formed and evolved.


Evan Gough

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