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World record in high energy physics


In a new world record, the international KArlsruhe TRItium Neutrino Experiment (KATRIN) has constrained the mass of neutrinos with unprecedented precision, thus breaking a barrier in neutrino physics.

An international research team has succeeded in reducing the upper limit of the mass of the neutrino, the lightest particle of matter, to 0.8 electron volts, thus enabling more precise models of the universe, both the primitive one and the current one.

This is a real advance, since it breaks the “psychological barrier” that we had of not knowing if the neutrino weighs more than 1 eV. Importantly, we now know that the neutrino is at least 500,000 times lighter than the electron, notes physicist Björn Lehnert in PhysicsWorld.

Neutrinos are among the most abundant material particles in the universe. Tens of billions of neutrinos pass through every square centimeter of the earth’s surface every second without our noticing.

They are a significant remnant of the Big Bang, forming in large quantities in the cores of some stars and accompanying the decay of many radionuclides, such as tritium, the heaviest isotope of hydrogen.

Neutrinos play an important role in the arrangement of large galactic structures. In the quantum field, it remains a mystery why the neutrino mass is so small, and the answer to this question could lead to the expansion of physics beyond current theories.

important goal

We have known for some time that its mass is non-zero. In at least one of the three states in which neutrinos occur, it is 0.05 eV.

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Therefore, determining the rest mass of neutrinos is an important goal, both in particle physics and in astrophysics and cosmology.

A group of 150 scientists, technicians and students from six countries, led by Dr. Magnus Schlösser, from the Karlsruhe Institute of Technology (KIT), and Professor Susanne Mertens, from the Max Planck Institute for Physics and the Technical University of Munich, undertook the direct, ie model-independent, determination of the neutrino mass within the international KArlsruhe TRItium Neutrino experiment (KATRIN).

International cooperation

At KIT, the international team that has developed this research built a 70-meter long apparatus after many years of effort, whose main parts are the most intense source of tritium gas in the world, and a powerful electron spectrometer, with the that the energies of decaying electrons can be measured with unprecedented precision.

A series of control measurements has shown that the device makes it possible to study the energy spectra of the tritium decay electrons in the most perfect way so far. Scientific measurements began in 2018.

The result of the second series of measurements is unequivocal. The KATRIN team compared the measured spectrum with the theoretical prediction for different values ​​and obtained a new result, again the best in the world, with 90% statistical confidence.

Thus, after many years of efforts by experimental physicists, the symbolic value of 1 eV for the upper limit of the neutrino mass was significantly exceeded.

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This refinement allows scientists to create more reliable models of the processes that take place both in the current universe and in its beginning, shortly after the Big Bang. In this way, they can exclude those who no longer resist refined value.

unprecedented measurement

The researchers carried out the second series of measurements under better conditions: they managed to increase the intensity of the cryogenic tritium source to four times the previous value, thus fulfilling one of the technical challenges of the KATRIN project. Background noise was reduced by 25%.

“Of the 60 billion electrons emitted by the tritium source in every second of the 750-hour measurement, four million electrons were selected to study the neutrino mass with unprecedented sensitivity,” explains Magnus Schlösser.

The reliability of the result was enhanced by the fact that the selected electrons were analyzed by three independent groups, who compared their partial results and theoretical predictions.

Simultaneous to the measurement of the neutrino’s mass, the experts of the KATRIN experiment team recently published remarkable results, which further limit the possible existence of new types of light neutrinos.

So it’s a legitimate hope that a giant spectrometer could explore a number of key properties of this remarkable particle by the end of this decade, the researchers conclude.


Direct neutrino-mass measurement with sub-electronvolt sensitivity. The KATRIN Collaboration. Nature Physics volume 18, pages160–166 (2022).

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