An international research team, including scientists from the 91Ě˝»¨, has established a new upper limit on the mass of the neutrino, the lightest known subatomic particle.

In a published Feb. 14 in Nature Physics, the collaboration — known as the 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 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 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 , 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 and of . 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 of the KIT.

The 91Ě˝»¨is a founding member of the KATRIN collaboration, which was formed in 2001. Under the direction of co-author , a 91Ě˝»¨professor emeritus of physics, the 91Ě˝»¨was the lead U.S. institution for designing and acquiring KATRIN’s electron detection system. Led by co-author , a 91Ě˝»¨research associate professor of physics, 91Ě˝»¨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, , 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 91Ě˝»¨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 by the Massachusetts Institute of Technology.

An international team of scientists has announced a breakthrough in its quest to measure the mass of the neutrino, one of the most abundant, yet elusive, elementary particles in our universe.

At the conference in Toyama, Japan, leaders from the KATRIN experiment reported Sept. 13 that the estimated range for the rest mass of the neutrino is no larger than about 1 , or eV. These inaugural results obtained earlier this year by — or KATRIN — cut the mass range for the neutrino by more than half by lowering the upper limit of the neutrino’s mass from 2 eV to about 1 eV. The lower limit for the neutrino mass, 0.02 eV, was set by previous experiments by other groups.

“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 , a KATRIN scientist and professor emeritus of physics at the 91Ě˝»¨. “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.”

The KATRIN experiment is based at the Karlsruhe Institute of Technology in Germany and involves researchers at 20 research institutions around the globe. In addition to the 91Ě˝»¨, KATRIN member institutions in the United States are:

• The University of North Carolina at Chapel Hill, led by professor of physics and astronomy , a former 91Ě˝»¨faculty member
• The Massachusetts Institute of Technology, led by professor of physics
• The Lawrence Berkeley National Laboratory, led by Nuclear Science Division deputy director
• Carnegie Mellon University, led by assistant professor of physics
• Case Western Reserve University, led by associate professor of physics

Under Robertson and Wilkerson, the 91Ě˝»¨ became one of KATRIN’s founding member institutions in 2001. Wilkerson later moved to the University of North Carolina at Chapel Hill. Formaggio and Parno began their involvement with KATRIN as 91Ě˝»¨researchers and later moved to their current institutions. In addition to Robertson, other current 91Ě˝»¨scientists working on the KATRIN experiment are research professor of physics , research associate professor of physics and Menglei Sun, a postdoctoral researcher in the 91Ě˝»¨.

Neutrinos are abundant. They are one of the most common fundamental particles in our universe, second only to photons. Yet neutrinos are also elusive. They are neutral particles with no charge and they interact with other matter only through the aptly named “weak interaction,” which means that opportunities to detect neutrinos and measure their mass are both rare and difficult.

“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.

Neutrinos are also mysterious particles that have already shaken up physics, cosmology and astrophysics. The of particle physics had once predicted that neutrinos should have no mass. But by 2001, scientists had shown with two detectors, Super-Kamiokande and the Sudbury Neutrino Observatory, that they actually do have a nonzero mass — a breakthrough with the Nobel Prize in Physics. Neutrinos have mass, but how much?

“Solving the mass of the neutrino would lead us into a brave new world of creating a new Standard Model,” said Doe.

The KATRIN discovery stems from direct, high-precision measurements of how a rare type of electron-neutrino pair share energy. This approach is the same as neutrino mass experiments from the 1990s and early 2000s in Mainz, Germany, and Troitsk, Russia, both of which set the previous upper limit of the mass at 2 eV. The heart of the KATRIN experiment is the source that generates electron-neutrino pairs: gaseous tritium, a highly radioactive isotope of hydrogen. As the tritium nucleus undergoes radioactive decay, it emits a pair of particles: one electron and one neutrino, both sharing 18,560 eV of energy.

KATRIN scientists cannot directly measure the neutrinos, but they can measure electrons, and try to calculate neutrino properties based on electron properties.

Most of the electron-neutrino pairs emitted by the tritium share their energy load equally. But in rare cases, the electron takes nearly all the energy — leaving only a tiny amount for the neutrino. Those rare pairs are what KATRIN scientists are after because — thanks to E = mc² — scientists know that the miniscule amount of energy left for the neutrino must include its rest mass. If KATRIN can accurately measure the electron’s energy, they can calculate the neutrino’s energy and therefore its mass.

The tritium source generates about 25 billion electron-neutrino pairs each second, only a fraction of which are pairs where the electron takes nearly all the decay energy. The KATRIN facility in Karlsruhe uses a complex series of magnets to channel the electron away from the tritium source and toward an electrostatic spectrometer, which measures the energy of the electrons with high precision. An electric potential within the spectrometer creates an “energy gradient” that electrons must “climb” in order to pass through the spectrometer for detection. Adjusting the electric potential allows scientists to study the rare, high-energy electrons, which carry information concerning the neutrino mass.

U.S. institutions have made broad contributions to KATRIN, including providing the electron-detector system — the “eye” of KATRIN — which looks into the heart of the spectrometer, an instrument built at the UW. The University of North Carolina at Chapel Hill led the development of the detector’s data acquisition system, the “brains” of KATRIN. MIT’s contribution was the design and development of the simulation software used to model the response of KATRIN. The Lawrence Berkeley National Laboratory contributed to the creation of the physics analysis program and provided access to national computing facilities, and quick analysis was enabled by a suite of applications that originated at the UW. The Case Western Reserve University was responsible for the design of the electron gun, central to calibrating the KATRIN apparatus. Carnegie Mellon University contributed primarily to analysis, with special attention to background and to fitting, and assisted in analysis coordination for the experiment.

With tritium data acquisition now underway, U.S. institutions are focused on analyzing these data to further improve our understanding of neutrino mass. These efforts may also reveal the existence of sterile neutrinos, a possible candidate for the dark matter that, though accounting for 85% of the matter in the universe, remains undetected.

“KATRIN is not only a shining beacon of fundamental research and an outstandingly reliable high-tech instrument, but also a motor of international cooperation, which provides first-class training of young researchers,” said KATRIN co-spokespersons Guido Drexlin of the Karlsruhe Institute of Technology and Christian Weinheimer of the University of MĂĽnster in a statement.

Now that KATRIN scientists have set a new upper limit for the mass of the neutrino, project scientists are working to narrow the range even further.

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

The U.S. Department of Energy’s Office of Nuclear Physics has funded the U.S. participation in the KATRIN experiment since 2007.

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For more information, contact Robertson at 206-616-2745 or rghr@uw.edu and Doe at 206-543-8862 or pdoe@uw.edu.

Neutrinos may be small, but when it comes to prizes, they pack quite a punch.

In October, it was announced that two scientists who headed international projects to study these miniscule, seemingly ephemeral subatomic particles will . On Nov. 8, these same scientists joined five of their colleagues from other neutrino projects to accept the . The $3 million prize will be shared among the over 1,300 scientists, including 91Ě˝»¨ researchers, who participated in these years-long efforts to understand neutrinos.

91Ě˝»¨scientists contributed to three of these projects. Physics professor led the team of 91Ě˝»¨scientists with the Sudbury Neutrino Observatory in Canada, while headed 91Ě˝»¨efforts with the Super-Kamiokande and K2K/T2K collaborations, which were both based in Japan. Wilkes was also a U.S. co-spokesperson for K2K, while Robertson served the same role for the Sudbury experiments. The prize also honored the KamLAND project in Japan and Daya Bay in China.

All of these endeavors explored the fundamental properties of neutrinos, which are among the smallest and most mysterious of fundamental particles that make up the universe. They can form when particles collide or undergo decay, and are the second most common particle in the universe, after photons. But scientists struggled for decades to understand whether neutrinos have mass and gather other basic information about them. Experiments at the Sudbury Neutrino Observatory and Super-Kamiokande in particular showed that neutrinos have mass and can oscillate among three different “flavors.” The K2K and T2K experiments have verified these oscillations and studied them in greater detail.

In addition to Robertson and Wilkes, dozens of 91Ě˝»¨professors, researchers and graduate students from the Department of Physics worked on these experiments over the years, including the late professor , who began the UW’s involvement with Super-Kamiokande.

This is the third year that the prize — founded by Sergey Brin, Anne Wojcicki, Jack Ma, Cathy Zhang, Yuri Milner, Julia Milner, Mark Zuckerberg and Priscilla Chan — was presented at a . Prizes were also presented for achievement in life sciences and mathematics.

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The prize recipients, Takaaki Kajita and Arthur McDonald, respectively led the in Japan and the in Canada. As the Emerald City awoke to the news, the two teams of 91Ě˝»¨ researchers who were members of these multinational, decades-long scientific groups began to celebrate.

“It feels great,” said 91Ě˝»¨physics professor . “We’re glad the recognition came for the hard work everyone has done.”

Meanwhile, physics professor has led the 91Ě˝»¨team working with the Sudbury Neutrino Observatory, or SNO, which includes over thirty professors, students and other researchers from the 91Ě˝»¨on experiments that were in 2006.The endeavors honored by the 2015 Nobel Prize in Physics were large-scale, multiyear experiments to measure the fundamental properties of neutrinos. Wilkes and the late 91Ě˝»¨physics professor began the UW’s involvement with the experiments at Super-Kamiokande, or Super-K, when the facility was still under construction in the early 1990s. A team of about a dozen 91Ě˝»¨postdoctoral researchers, graduate students and engineers later joined them on the Super-K Collaboration.

“Before these experiments, people used to think neutrinos had no mass, and there were many other unanswered questions about their behavior,” said 91Ě˝»¨physics professor , who worked on the SNO collaboration and still conducts neutrino research.

As early risers for the Oct. 6 announcement learned, neutrinos are the second most common type of particle in our universe — after light. Yet neutrinos are difficult to detect and measure with precision. They are ubiquitous and easily form when other types of particles collide or undergo radioactive decay. But they are tiny, electrically neutral — neutrino means “little neutral one” — and can easily pass through light years of matter without interacting with it. Neutrinos also come in three “flavors,” but the differences between flavors have proven difficult to tease out. At times neutrinos, especially those generated by the sun, seemed to disappear as they traveled to Earth.

In the 1990s, two international groups of researchers constructed elaborate facilities to learn some basic facts about neutrino behavior. Both teams wanted to detect neutrinos that form far above our heads — Super-K focused on neutrinos generated in Earth’s atmosphere while SNO sought out neutrinos from the sun. But to do this, they had to conduct their experiments in underground mines.

“Cosmic rays sometimes hit the upper atmosphere, creating a shower of particles that you and I don’t notice, but they completely block out any signal from neutrinos on the surface,” said Detwiler. “So you have to take your neutrino detectors far underground.”

Both Super-K and SNO used massive tanks of ultrapure liquids to detect neutrinos. In the SNO facility, in a spherical tank made of clear acrylic served as the detection medium.

“When a neutrino comes in and interacts with the water, it makes little flashes of light,” said Detwiler. “Outside this huge spherical tank, they mounted almost 10,000 light detectors to measure these flashes.”

Super-K used a tank of purified water to detect neutrinos. Both facilities required years of construction and calibration before experiments could begin. 91Ě˝»¨teams pored over and analyzed data along with their collaborators at other institutions. But within just a few years of each other, both international teams announced major discoveries.

The Super-K collaboration discovered that atmospheric neutrinos can “oscillate” between at least two flavors. In addition, months before Young’s death in 1998, the group that neutrinos do indeed have mass. Three years later, SNO confirmed neutrino oscillation among all three flavors, which explained why solar neutrinos seemed to vanish by the time they reach Earth. On Oct. 6, the Nobel committee specifically cited these groundbreaking discoveries during the prize announcement.

91Ě˝»¨researchers are not yet finished with the subject.

“There are still many questions about neutrinos,” said Wilkes. “For example, we don’t know their mass — just that they have mass, it’s very small and we don’t know why.”

Neutrinos may also be a route toward detecting new types of physical interactions, added Detwiler.

Unanswered questions aside, the 91Ě˝»¨Department of Physics plans to celebrate the SNO and Super-K experiments. On Monday Oct. 12 at 4:00 p.m., the department will hold a colloquium in the in room A102 to revisit the contributions of 91Ě˝»¨researchers.

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For more information, contact Wilkes at wilkes@u.washington.edu or 206-543-4232 and Robertson at rghr@uw.edu or 206-616-2745.

A paper published this month in shows that 91Ě˝»¨researchers and collaborators have managed to detect the radiation flung off by a single electron, a key step in their new strategy to pin down the neutrino’s mass.

The ultimate goal is to measure the weight of the neutrino. Scientists used to think the subatomic particle had no mass, but results starting in 1998 turned that idea on its head.

“Neutrino mass not being zero is the only definitive example where a prediction of the standard model of particle physics has been contradicted by data,” said co-author , a 91Ě˝»¨professor of physics.

The 91Ě˝»¨scientists are part of , a group of six institutions that seeks to measure the mass of the neutrino. Collaborators on the new study are at the Massachusetts Institute of Technology; the Pacific Northwest National Laboratory; the University of California, Santa Barbara; the National Radio Astronomy Observatory in Virginia; and the Karlsruhe Institute of Technology in Germany.

The new paper shows they were able to track electrons from radioactive decay inside an apparatus on the 91Ě˝»¨campus for several milliseconds, long enough to make a rough measurement of a single electron’s energy without disturbing it. Next will be relating that electron’s energy to the neutrino’s mass.

“We want to know the mass of the neutrino because we want to understand how the enormous skeins and clusters of galaxies formed from the uniform early universe,” Robertson said.

Measuring the neutrino’s mass could also help explain the physics of very-high-energy reactions and help develop a new of particle physics, he added.

The core of the apparatus, located in the UW’s physics building, is a low-pressure gas cell about the size of an espresso cup inside a strong 1-tesla magnet. The cell contains a small amount of metastable krypton-83 gas, a radioactive isotope that spews out electrons as its nucleus undergoes radioactive decay. These ejected electrons are then forced into a circular orbit by the magnetic field and emit cyclotron radiation, the electromagnetic waves emitted by charged particles as they move through a magnetic field. A detector measures the waves’ frequency, around 25 billion cycles per second, to gauge the electron’s energy.

Neutrinos also are created in . Their existence, first predicted by theoretical physicist Wolfgang Pauli in 1930, is needed to ensure that beta decay conserves energy and angular momentum. This experiment aims to measure the energies of the electrons emitted in beta decay and compare them to the total energy of the reaction. If the neutrino has mass, the ejected electron cannot have an energy equal to the total amount, since (according to Einstein’s famous equation, E=mc², relating energy and mass) some of this energy must have been used to make the neutrino.

The mass of the unseen neutrino can therefore be measured by carefully tallying the energy of the emitted electron to figure out how much energy is missing.

The current detector only has a resolution of about 15 electron volts, which is too imprecise to measure the neutrino’s mass. The 91Ě˝»¨team is now working to build a bigger apparatus and better precision for the energy measurements. Robertson believes the ultimate precision could be as low as 0.04 electron volts, much lower than any existing method. But he says it will take years to get there.

The researchers have named the new technique cyclotron radiation emission spectroscopy, or CRES.

“In addition to being a step on the path to measuring the neutrino mass, I’m excited that we’ve made a new tool for measuring radioisotope decay energies in general,” said , a 91Ě˝»¨research assistant professor of physics. “It is my hope that the CRES technique ends up having even broader applications. We’re exploring both ideas for testing fundamental physics and more practical applications.”

Many of the collaborators on this project met through the , a particle detector in a Canadian mine that collected some of the first measurements to show that the neutrino does have mass.

Rybka is also involved in a 91Ě˝»¨experiment using microwave radiation to find an , a hypothesized subatomic particle that would be a component of dark matter, a form of matter that cannot be detected but is thought to make up most of the mass of the universe.

Other 91Ě˝»¨scientists among the 27 authors who contributed to the paper are professor , research professor , research scientists and , doctoral students and , affiliate professor , and former staff member Natasha Woods.

The research was funded by the U.S. Department of Energy, the National Science Foundation and the Pacific Northwest National Laboratory.

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For more information, contact Robertson at rghr@uw.edu or 206-616-2745 and Rybka at grybka@uw.edu or 206-543-2540. To tour the lab, contact Fertl at mfertl@uw.edu or 206-543-9134.

Parts of this article were adapted from a on the American Physical Society website.