Evrim Yazgin
Cosmos science journalist
Physicists in Germany have performed the most accurate measurement of the mass of the fundamental particle neutrinos. The finding deepens physicists’ attempts to uncover laws of nature beyond even the best current theories.
The standard model of particle physics – the best theory which explains the fundamental forces and particles of nature – posits that neutrinos are massless. Experiments, however, suggest that they do have mass.
Measuring the neutrino mass could explain the limitations of the standard model and offer insights into new areas of physics and beyond.
Neutrinos are the most abundant particles in the universe, but you wouldn’t know it. In fact, there are about 100 billion neutrinos passing through your body every second. They pass straight through us because they don’t interact with other particles very easily. They have no electric charge, are extremely tiny and travel at very close to the speed of light.
This also makes them very difficult to study.
A team at the Karlsruhe Tritium Neutrino Experiment (KATRIN) has published a paper in Science detailing their measurements of the neutrino mass.
Their upper limit for the mass shows that neutrinos are more than a million times smaller than electrons.
Physicists measure particle mass using a unit called an electron volt, eV. An eV is about 6 million billion billion billion times less than 1g. Electrons are about half a million eV. According to the KATRIN experiment, neutrinos are no more than 0.45 eV.
The mass measurement has a 90% confidence level.
KATRIN scientists measured the neutrino mass by analysing the decay of tritium – a rare and radioactive form of hydrogen which has 1 proton and 2 neutrons. During this decay, a neutron transforms into a proton, emitting both an electron and an electron antineutrino.
The electron antineutrino is the neutrino’s antiparticle. So, the neutrino’s mass can be gathered by analysing the distribution of total energy between the electron and the electron antineutrino.
About 36 million electron energies were gathered over 259 days between 2019 and 2021. The KATRIN collaboration’s dataset is 6 times larger than previous runs.
“The neutrino mass measuring campaign of the KATRIN experiment will end in 2025 after reaching 1000 days of data acquisition,” writes Loredana Gastaldo in a related Perspective in Science. Gastaldo was not an author on the paper and is a researcher at the Kirchhoff Institute for Physics in Germany.
“Analysis of the full data set gained from this grand project will allow for estimating the effective electron neutrino mass close to the projected value of 0.3 eV at 90% confidence level.”
