The Karlsruhe Tritium Neutrino Experiment, or KATRIN, is a massive detector based in the town of Karlsruhe, Germany, that has been designed to measure a neutrino’s mass with far greater precision than existing experiments.
KATRIN is addressing a very old question, one that was posed by [Enrico] Fermi in the 1930s, which is the question of how much mass a neutrino has. Decades ago, we thought the neutrino had to be massless. Then experiments in neutrino oscillations showed that no, that wasn’t actually true—neutrinos actually have a tiny mass. And now we know that different neutrinos have different masses from each other. What we don’t know is how much any one neutrino weighs—the absolute mass is still unknown.
E = mc2
KATRIN addresses this question by looking at energy conservation: E = mc2. We have a radioactive gas, in this case, tritium, that releases energy as it decays. Some of that energy goes to a neutrino, which flies off and we never see it again. Some of that energy goes to an electron, and if you measure the electron’s energy very precisely, it turns out that tells you how much the neutrino took away. And in particular, was that energy all kinetic, because the neutrino was going away at the speed of light? Or does it have a little bit of rest energy, or mass?
One of the most important open questions
Researchers say determining the mass of neutrinos is one of the most important open questions in particle physics and will help scientists better understand the history of the universe. Some 200 people from 20 institutions in seven countries are part of the project.
For other particles like the top quark and the electron, we might not understand why they have the masses they do, but we understand how they have mass. For the neutrino, that’s an open question. How the neutrino gets its mass is unknown. The hope is, by measuring the mass of the neutrino, you get a better sense of how a neutrino gets its mass. We have billions of neutrinos everywhere in the universe. If all of a sudden they have a mass, they will impact how the universe will evolve over time. For cosmologists, that information will be very useful.