Since the discovery of neutrino oscillations, physicists know that neutrinos have non-zero mass. However, the absolute neutrino-mass scale remains unknown. Now, physicists from the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany report the upper limit on effective electron antineutrino mass — less than 0.8eV at a 90% confidence level.
“The neutrino masses are at least five orders of magnitude smaller than the mass of any other fermion of the Standard Model of particle physics, which may point to a different underlying mass-creation mechanism,” said KATRIN project leader Dr. Guido Drexlin and his colleagues.
“The determination of the neutrino mass would, thus, shed light on the fundamental open question of the origin of particle masses.”
“Despite the smallness of their masses, neutrinos play a crucial role in the evolution of large-scale structures of our cosmos due to their high abundance in the Universe,” they added.
“A direct measurement of the neutrino mass could, hence, provide a key input to cosmological structure formation models.”
Physicists have pursued direct mass measurements since they observed electron antineutrinos in 1956.
A direct method to probe the neutrino mass scale in the laboratory is provided by kinematic studies of weak-interaction processes such as beta-decay of tritium (3H), a rare and radioactive isotope of hydrogen.
KATRIN is now the most precise experiment of this kind. It combines a windowless gaseous molecular tritium source, pioneered by the Los Alamos experiment, with a spectrometer based on the principle of magnetic adiabatic collimation with electrostatic filtering.
These techniques allow the investigation of the endpoint region of tritium-decay with very high energy resolution, large statistics and small systematics.
KATRIN has been designed and built to refine this direct kinematic method to its ultimate precision level.
“KATRIN is an experiment with the highest technological requirements and is now running like perfect clockwork,” Dr. Drexlin said.
“The increase of the signal rate and the reduction of background rate were decisive for the new result,” said University of Münster’s Dr. Christian Weinheimer, co-spokesperson of the KATRIN Collaboration.
In the second neutrino-mass measurement campaign, the KATRIN experiment reached sub-electronvolt sensitivity (0.7 eV).
“The particle physics community is excited that the 1-eV-barrier has been broken by KATRIN,” said Professor John Wilkerson, a neutrino expert with the University of North Carolina.
Combined with the first campaign, the KATRIN team set an improved upper limit of < 0.8 eV.
“We, therefore, have narrowed down the allowed range of quasi-degenerate neutrino-mass models,” the physicists said.
“And we have provided model-independent information about the neutrino mass, which allows the testing of non-standard cosmological models.”
The results appear in the journal Nature Physics.
The KATRIN Collaboration. 2022. Direct neutrino-mass measurement with sub-electronvolt sensitivity. Nat. Phys 18, 160-166; doi: 10.1038/s41567-021-01463-1
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