At the KATRIN experiment, researchers are studying the mass of elementary particles that interact with matter only extremely rarely: neutrinos. One type of these particles—which has remained hypothetical until now—is the sterile neutrino, which could potentially solve several problems in modern particle physics. Kathrin Valerius from the Karlsruhe Institute of Technology explains what the KATRIN experiment can reveal about this type of neutrino.
World of Physics: What is the KATRIN experiment?
Kathrin Valerius: The name is an abbreviation for “Karlsruhe Tritium Neutrino Experiment.” It is the world’s leading facility for directly investigating neutrino mass. The Standard Model of particle physics describes all elementary particles and their interactions, including neutrinos. However, unlike other elementary particles—such as electrons—the mass of neutrinos is still unknown. We only know that their mass is very small, but not zero. Using precision experiments like KATRIN, we have already succeeded in narrowing down the neutrino mass. But we still cannot specify it exactly.
How is neutrino mass studied at KATRIN?
We study what is known as the beta decay of tritium. This is a superheavy isotope of hydrogen that can decay into helium, releasing a neutrino—strictly speaking, an antineutrino, i.e., an antiparticle—along with an electron. Neutrinos also have antiparticles, just as positrons are the antiparticles of electrons. We don’t need to detect the released neutrinos at all—because they interact with other particles extremely rarely, they simply flow right through our experiment. Instead, we examine the energy spectrum of the released electrons very closely. This is because during beta decay, part of the total energy is transferred to the neutrino and the rest goes to the electron. By measuring the electron energies very precisely in many millions of tritium decays, we can now draw conclusions about the neutrinos, and in particular their mass.
But you’ve also investigated the possible mass range of another, hypothetical type of neutrino?
Yes, that’s an additional research result for which KATRIN wasn’t actually designed at all! But the data is so precise that we’ve also obtained new results for these so-called “sterile neutrinos.”
What kind of particles are these, and why are they called “sterile”?
There are three known types of neutrinos, and sterile neutrinos are particles that are still entirely hypothetical, for which there is no experimental evidence yet. But according to various theories, there could be one or even several additional types of neutrinos that differ from the three known ones in that they do not interact with normal matter at all—not even as weakly as normal neutrinos. Neutrinos are sometimes called “ghost particles” because they interact with matter only with very low probability and can thus pass through our planet undisturbed. Sterile neutrinos are even “invisibler”; they are, so to speak, particularly ghostly ghost particles.
But how could such particles be detected at all?
Direct detection via the weak interaction, as with ordinary neutrinos, is physically impossible. However, sterile neutrinos can manifest themselves indirectly through so-called neutrino oscillations. This is because the three known types of neutrinos—namely, electron, muon, and tau neutrinos—can transform into one another while in flight. Sterile neutrinos can participate in this oscillation process and serve as additional oscillation partners. After transforming into one of the three known types, they could then interact with ordinary matter after all. Incidentally, neutrino oscillation led to a major mystery several decades ago when neutrinos from the Sun were studied.
What was the mystery?
At the time, far fewer solar neutrinos were detected than expected. The solution lay in the fact that the electron neutrinos produced in the Sun partially transformed into other types of neutrinos on their way to Earth. The 2015 Nobel Prize in Physics was awarded for the detection of these neutrino oscillations. Then, a series of experiments measured unusual oscillation effects that did not quite fit the standard picture of the three known neutrinos. This led some researchers to postulate a new type of neutrino—namely, sterile neutrinos.
Dark matter in the universe also hardly interacts with ordinary matter. Could sterile neutrinos make up part of dark matter?
That, too, is a possible—though so far purely hypothetical—explanation. For sterile neutrinos to make a significant contribution to dark matter, they would need to be relatively massive and, at the same time, very weakly coupled to known neutrinos. Current experiments—including KATRIN—are primarily able to rule out lighter sterile neutrinos. In the future, with further improvements to the experiment and more data, KATRIN will be able to significantly expand the mass range under investigation. We are very eager to see if this will reveal any evidence of such particles. But even a negative result would be valuable, as it would provide important constraints for theoretical models and set the direction for future experiments.
