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Neutrinos have a mass of less than 0.8 electronvolts


The worldwide KArlsruhe TRItium Neutrino Experiment (Katrin) at Karlsruhe Institute of Technology (KIT) has overcome a significant “barrier” in neutrino physics, which has implications for particle physics and cosmology. The mass of the neutrino has been given a new upper limit of 0.8 electronvolt (eV) based on fresh evidence. Katrin is able to restrict the mass of these lightweights of the cosmos with unparalleled accuracy thanks to this first push into the sub-eV mass scale of neutrinos via a model-independent laboratory approach.

Neutrinos are arguably our universe’s most fascinating elementary particle. They are vital in the production of large-scale structures in cosmology, and their minuscule but non-zero mass distinguishes them in particle physics, hinting to novel physics processes outside our present ideas. Our knowledge of the universe will be incomplete unless the mass scale of neutrinos can be measured.

This is the challenge that the international KATRIN experiment at Karlsruhe Institute of Technology (KIT) has taken on as the world’s most sensitive neutrino scale, with partners from six countries. It uses the energy distribution of electrons released during the beta decay of tritium, an unstable hydrogen isotope, to determine the mass of the neutrino. This needs a significant scientific effort: the 70-meter-long experiment holds the world’s most powerful tritium source as well as a massive spectrometer that can precisely detect the energy of decay electrons.

After beginning scientific measurements in 2019, the data quality has steadily improved over the last two years. “KATRIN is an experiment with the greatest technical criteria that is currently operating like a flawless clockwork,” says Guido Drexlin (KIT), the project leader and one of the experiment’s two co-spokespersons. “The rise of the signal rate and the lowering of the background rate were important for the new outcome,” says Christian Weinheimer (University of M√ľnster).

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Data analysis

The international analysis team lead by Susanne Mertens (Max Planck Institute for Physics and TU Munich) and Magnus Schl√∂sser (Max Planck Institute for Physics and TU Munich) demanded everything from the international analysis team (KIT). Every effect, no matter how minor, had to be thoroughly investigated. “We were able to exclude a systematic bias of our result due to distorting processes only through this laborious and intricate method, and we are especially proud of our analysis team, which successfully took on this huge challenge with great commitment,” the two analysis coordinators are pleased to report.

The first year of experimental data and modeling based on a vanishingly small neutrino mass match perfectly, allowing a new upper limit on the neutrino mass of 0.8 eV to be determined (Nature Physics, July 2021). This is the first time a direct neutrino mass experiment has reached the cosmologically and particle-physically significant sub-eV mass range, which is thought to be where neutrinos’ basic mass scale lies. “The particle physics community is ecstatic that KATRIN has broken the 1-eV barrier,” says John Wilkerson, a neutrino expert (University of North Carolina, Chair of the Executive Board).

“Our team at the MPP in Munich has developed a new analysis method for KATRIN that is specially optimized for the requirements of this high-precision measurement, which has been successfully used for past and current results,” Susanne Mertens explains. “My group is highly motivated: we will continue to meet the future challenges of KATRIN analysis with new creative ideas and meticulous accuracy.”

Sensitivity should be improved with more measurements

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“Further measurements of the neutrino mass will continue until the end of 2024,” say the co-spokespersons and analysis coordinators of KATRIN. “To realise the full potential of this unique experiment, we will not only steadily increase the statistics of signal events, but we are constantly developing and installing improvements to further lower the background rate.”

The development of a new detector system (TRISTAN) plays a key role in this, allowing KATRIN to begin searching for “sterile” neutrinos with masses in the kiloelectronvolt range as early as 2025, a candidate for the mysterious dark matter in the cosmos that has already been observed in a number of astrophysical and cosmological observations but whose particle-physical nature is still unknown.

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