Neutrinos are light elementary particles that play a crucial role in particle physics and cosmology. However, their exact mass is not yet known. Physicists at the KATRIN experiment in Karlsruhe have now succeeded for the first time in limiting the neutrino mass to a maximum of 0.8 electron volts – less than a billionth of the mass of a proton. For the first time, measurements have entered the mass range of less than one electron volt, which is important for particle physics. In addition, the new value was determined independently of a cosmological model and thus also makes it possible to test hypotheses on exotic physics that do not follow the Standard Model.
Neutrinos are among the most common and at the same time most mysterious particles in our universe. Although billions of them flow through our body every second, they cannot be felt and are very difficult to measure. Because the elementary particles hardly interact with matter, have almost no mass and their three types can literally transform into one another on the fly. It is known that these “ghost particles” are produced during radioactive decay, in the sun and also during high-energy cosmic events. At the same time, they could play an important role in many still unexplained cosmic phenomena, from possible “new physics” to the nature of dark matter. In order to understand the possible contribution of neutrinos to this, however, one parameter is crucial: their mass. After neutrinos were long considered to be completely massless, experiments on neutrino oscillation show that these elementary particles must have a mass, albeit a tiny one. Previous measurements narrowed this value down to the range between 2 and 0.02 electron volts.
KATRIN experiment as a neutrino “balance”
In search of a more precise value for the neutrino mass, the international KATRIN experiment was launched at the Karlsruhe Institute of Technology (KIT) in 2017. This experiment is based on the beta decay of radioactive tritium gas. During this decay, an electron and an antineutrino are released. This antineutrino cannot be detected directly and its mass cannot be determined either – but that of the electron can. If the neutrino now has a mass, the energy and thus the mass of the electron would have to be exactly this proportion less than the total energy released during the decay of the tritium atom. The 70 meter long KATRIN experiment uses exactly this effect to limit the neutrino mass. To do this, tritium gas is pumped into a long tube where it decays. The electrons released in the process are guided into a spectrometer with the help of superconducting magnets. This can be adjusted in such a way that it only lets through electrons up to a certain energy – it acts like a filter. Behind the spectrometer is a detector that can measure the energy of these electrons with great precision.
In the first run of the KATRIN experiment in 2019, the physicists of the KATRIN collaboration managed to limit the mass of the antineutrino and thus also the neutrino to a maximum of one electron volt. Since then, the systems have been further optimized so that measurements with even greater precision were possible in the second runtime. “KATRIN as an experiment with the highest technological requirements now runs like clockwork,” reports project manager Guido Drexlin from KIT. The reduction of the interference signals and the increase of the signal rate were decisive. Progress has also been made in evaluating the enormous amounts of data generated by the detector. “It was only through this complex and meticulous work that we could really rule out a systematic influence on our result by other effects,” explain Magnus Schlösser from the KIT and Susanne Mertens from the Max Planck Institute for Physics.
New upper limit for the neutrino mass
The results of the new measurements are now available. The KATRIN team managed to limit the mass of the electron antineutrino to a maximum of 0.8 electron volts. This neutrino therefore has less than a billionth the mass of a proton. For the first time, a direct neutrino mass experiment advances into the cosmologically and particle-physically important mass range of less than one electron volt, in which the fundamental mass scale of neutrinos is assumed. “We now have information about the neutrino mass that is independent of cosmological models, which can now also be used to check non-standard models of cosmology,” write the physicists. They have also succeeded in reducing the statistical and systematic uncertainties by a factor of three and two, respectively.
“The particle physics community is excited that KATRIN has broken the 1 eV barrier,” comments neutrino expert John Wilkerson of the University of North Carolina. The aim now is to further increase the precision of KATRIN in further measurements. “Further measurements of the neutrino mass will continue until the end of 2024. In order to exploit the full potential of this unique experiment, we will not only continuously increase the statistics of the signal events, we will continuously develop and install improvements to further reduce the nuisance event rate,” the scientists explain. From 2025, KATRIN will be supplemented by the new TRISTAN detector system, which physicists will use to specifically search for “sterile” neutrinos – a hypothetical fourth type of neutrino. If it exists, it is considered one of the candidates for the dark matter particle.
Source: KATRIN Collaboration, Nature Physics, doi: 10.1038/s41567-021-01463-1