An international research team, including scientists from the 天美影视传媒, 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 鈥� known as the or KATRIN 鈥� reports that the neutrino鈥檚 mass is below 0.8 electron volts, or 0.8 eV/c². Honing in on the elusive value of the neutrino鈥檚 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鈥檚 mass since the experiment began collecting data in 2018. The team鈥檚 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鈥檛 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.

鈥淭here are almost as many neutrinos in the universe as there are photons,鈥� said Doe. 鈥淪o, 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.鈥�

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麓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 UW is a founding member of the KATRIN collaboration, which was formed in 2001. Under the direction of co-author , a UW professor emeritus of physics, the UW was the lead U.S. institution for designing and acquiring KATRIN鈥檚 electron detection system. Led by co-author , 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鈥檚 mass at 0.02 eV/c².聽 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鈥檚 mass by making use of an atomic tritium source 鈥� rather than molecular tritium 鈥� 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鈥檚 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 鈥� or KATRIN 鈥� 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 天美影视传媒. “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 天美影视传媒, 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 UW 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 天美影视传媒 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 UW researchers and later moved to their current institutions. In addition to Robertson, other current UW scientists working on the KATRIN experiment are research professor of physics , research associate professor of physics and Menglei Sun, a postdoctoral researcher in the UW .

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 鈥� 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 鈥� leaving only a tiny amount for the neutrino. Those rare pairs are what KATRIN scientists are after because 鈥� thanks to E = mc² 鈥� 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 鈥� the “eye” of KATRIN 鈥� 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 M眉nster 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.