Weighing Neutrinos – The Lightest Know Particle

KATRIN – a spectrometer weighed neutrinos for four weeks. And while that was a short weighing it managed to lower the upper limit of the mass of this tiny particle.

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The Karlsruhe Tritium Neutrino Experiment (KATRIN) is a spectrometer that measures the energies of electrons emitted when tritium decay. But the electron spectrum also carries information about the mass of neutrinos. During the beta decay of tritium three objects get created. The first being helium-3, second an electron, and the third is an anti-neutrino. These three split the energy that is created during the decay. The neutrino cannot be left with less energy than its resting mass. This partially affects the spectrum of the emitted electrons. And tritium is the perfect candidate for measuring mass because the energy released during its decay is one of the lowest – just roughly 19 keV. Just to give you an idea of how little that is – the resting mass of an electron, the second lightest known particle is 511 keV.

The spectrometer is built so it is capable of examining the decay of gaseous tritium. This excludes the effects of bonds between atoms of tritium in a solid form that was used previously. The whole spectrometer is roughly 70 kilometers long. From all the electrons emitted during the tritium decay, the ones with the highest energies are separated. These are then sent into the primary spectrometer where their energy is measured precisely.

An electrostatic spectrometer works by slowly increasing the voltage that then holds electrons with higher and higher energies back. This allows us to find out how many electrons have energy high enough to cross the barrier. The spectrum itself is obtained by deducting the measured amounts of electrons. But you need an insanely precise and stable spectrometer.

In the first half of the year, first measurements using gaseous tritium took place. These lasted for about four weeks and the analysis of the spectrum was done by three independent groups. Analyzing the electron spectrum is very difficult and there is a large need to exclude any chance of mistakes. But, luckily, all three groups reached the same results. The resting mass of a neutrino seems to be less than 1 eV. With about 90 % probability. This is an insanely small resting mass – half a million times smaller than the electrons.

Sources:

• https://arxiv.org/abs/1909.06048
• https://arxiv.org/abs/1909.06069

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