KATRIN experiment limits the mass of mysterious neutrino particles with unprecedented accuracy

View the main spectrometer of the KATRIN experiment to determine the neutrino mass on campus north of KIT. Author: Marcus Braig, KIT

New world record: neutrinos lighter than 0.8 electron volts.

Neutrinos are perhaps the most exciting elementary particles in our universe. In cosmology they play an important role in the formation of large-scale structures, while in particle physics their very small mass distinguishes them, pointing to new physical phenomena that go beyond our current theories. Without neutrino mass measurement our understanding of the universe will remain incomplete.

This is an international challenge KAрлсруэ THREEthis one NThe neutrino (KATRIN) experiment in KIT as the world’s most sensitive neutrino scale. Partners from six countries are involved. KATRIN uses the beta decay of tritium, an unstable isotope of hydrogen, to determine the mass of neutrinos through the energy distribution of electrons released during decay. This requires a lot of technological effort: the 70-meter-long experiment is the world’s most intense source of tritium, as well as a giant spectrometer to measure the decay energy of electrons with unprecedented accuracy. Since the start of scientific measurements in 2019, high quality data has been steadily improving over the past two years. “KATRIN as an experiment with the highest technological requirements now works like the perfect watch,” says Professor Guido Drexlin of KIT, project manager and one of the two co-secretaries of the experiment. Professor Christian Weinheimer,[{” attribute=””>University of Münster, the other co-spokesperson, adds: “Reduction of the background rate and increase in the signal rate were decisive for the new result.”

KATRIN Experiment Setup Schematic

Schematic representation of the setup of the KATRIN experiment. Credit: Leonard Köllenberger for the KATRIN collaboration

Detailed Data Analysis: First Push into the Range below 1 eV

In-depth analysis of these data represented a big challenge for the international team led by the two coordinators Dr. Magnus Schlösser, KIT, and Professor Susanne Mertens, Max Planck Institute for Physics and Technical University of Munich: Each and every effect on the neutrino mass, no matter how small, had to be investigated in detail. “This laborious and intricate work was the only way to exclude a systematic bias of our result due to distorting processes. We are particularly proud of our analysis team that accepted this huge challenge with great commitment and was successful, “ Schlösser and Mertens say. The experimental data from the first year of measurements and modeling based on a vanishingly small neutrino mass matched perfectly: It allowed them to determine a new upper limit of 0.8 eV for the neutrino mass, the scientists point out. This is the first time that a direct neutrino mass experiment has entered the sub-eV mass range that is of high relevance to cosmology and particle physics, as it is here where the fundamental mass scale of neutrinos is assumed to lie. “The particle physics community is excited that the 1-eV barrier has been broken by KATRIN,” comments neutrino expert John Wilkerson, University of North Carolina, who chairs the KATRIN Executive Board.

“The success was based decisively on the fact that the ramp-up of the source to nominal source intensities at the Karlsruhe Tritium Laboratory and the operation of the spectrometer, detector, and cryoinfrastructure went smoothly. This was due to the professional dedication of our highly motivated staff which supported this large-scale experiment,” Schlösser emphasizes.

Further Measurements to Improve Sensitivity

The researchers involved in the KATRIN project describe their next goals: “Further measurements of the neutrino mass will continue until the end of 2024. To exhaust the full potential of this unique experiment, we will steadily increase the statistics of signal events and continuously develop and install upgrades to further reduce the background rate.” The development of the new TRISTAN detector system will be of particular importance. From 2025, it will allow KATRIN to embark on a search for “sterile“ neutrinos with masses in the keV range. Such sterile neutrinos would be candidates for the mysterious dark matter that has already become manifest in many astrophysics and cosmological observations, but the physical nature of which is still unknown.

For more on this research, see New World Record: “Ghost Particle” Experiment Limits Neutrino Mass With Unprecedented Precision.

Reference: “Direct neutrino-mass measurement with sub-eV sensitivity” by The KATRIN Collaboration, 14 February 2022, Nature Physics.
DOI: 10.1038/s41567-021-01463-1 KATRIN experiment limits the mass of mysterious neutrino particles with unprecedented accuracy

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