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Scientists slash the upper limit for the neutrino’s mysterious mass in half

Alan Boyle
The KATRIN neutrino experiment is located on the grounds of the Karlsruhe Institute of Technology in Germany. (KATRIN Photo)

Scientists from the University of Washington and other institutions around the world say they’ve reduced the upper limit for the mass of the neutrino by half.

Thanks to findings from the Karlsruhe Tritium Neutrino Experiment, or KATRIN, physicists now know to a 90% confidence level that the neutrino has a rest mass no greater than 1.1 electron volts, or 1.1 eV. The previous upper limit was 2 eV.

Nailing down the neutrino’s mass could solidify scientists’ grasp on the Standard Model, which describes the subatomic world in fine detail. It could also open a path to the mysterious realm beyond the Standard Model.

Neutrinos are among the ghostliest particles in the subatomic zoo. Billions of them pass through our bodies every second, but they’re virtually undetectable.

Decades ago, scientists suspected that neutrinos might be massless particles, like photons of light. But around the turn of the millennium, Nobel-winning experiments in Japan and Canada showed that they had to have at least a smattering of mass. The current lower limit is 0.02 eV.

KATRIN, which sports a spectrometer vessel that’s been compared in appearance to a 30-foot-wide blimp, arrives at its mass estimate by measuring the energy of electron-neutrino pairs emitted by radioactive tritium.

In most of the pairs, the two particles share 18,560 eV of energy equally. But in rare cases, the electron takes up nearly all the energy. Thanks to Albert Einstein’s equation on mass-energy equivalence, E=mc2, those high-energy electrons can help KATRIN’s physicists figure out how much “missing mass” to leave for the neutrino.

KATRIN’s latest results were announced last Friday at the  2019 Topics in Astroparticle and Underground Physics conference in Japan. Members of the KATRIN science team said the findings mark a significant step toward getting a definitive fix on neutrino mass.

“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,” Hamish Robertson, a KATRIN scientist and professor emeritus of physics at the University of Washington, said today in a news release.

For example, data from KATRIN could provide evidence for the existence of sterile neutrinos, which are among the theoretical solutions for the mystery of dark matter.

KATRIN is located on the grounds of the Karlsruhe Institute of Technology in Germany and involves researchers from 20 institutions around the globe.

“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,” co-spokespersons Guido Drexlin of the Karlsruhe Institute of Technology and Christian Weinheimer of the University of Münster said in a statement.

Scientists and engineers at UW built the spectrometer’s electron-detector system and helped create a suite of analytical software. Other U.S. institutions contributing to the effort include the University of North Carolina at Chapel Hill, the Massachusetts Institute of Technology, Lawrence Berkeley National Laboratory, Case Western Reserve University and Carnegie Mellon University.

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

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