Matter and the universe

Apparently no fourth type of neutrino after all

So-called sterile neutrinos have been a promising explanation for anomalies observed in previous physics experiments for more than two decades. However, initial results from the international MicroBooNE Collaboration, in which the University of Bern is also involved, give no indication that the theoretical elementary particles actually exist. Thanks to this important zero result, the researchers can now investigate further hypotheses.

Along with photons, neutrinos are the most abundant elementary particles in the cosmos. Their role, for example, in the evolution of the universe is of great importance in physics. To date, there are three types of neutrinos. However, physicists suspected a previously undiscovered fourth type of neutrino – so-called sterile neutrinos – as a promising explanation for certain anomalies in earlier experiments.

However, in October 2021, first results of the MicroBooNE experiment at the particle physics research center Fermilab near Chicago (USA) dealt the theoretical elementary particles a blow: Four complementary studies of the international MicroBooNE Collaboration came up with no evidence for the actual existence of the sterile neutrinos. Instead, the results are consistent with the standard model of particle physics, the best physical theory to date of how the universe works. “We conducted very comprehensive studies of several types of neutrino interactions. The message is the same in each case: There is no evidence that sterile neutrinos exist,” says Michele Weber, scientific lead of the MicroBooNE experiment and Professor for Experimental Particle Physics at the University of Bern.

The hunt for the “ghost particles”

Neutrinos are produced by a variety of sources, including the sun, the atmosphere, nuclear reactors, and particle accelerators. However, since they rarely interact with other matter, they are difficult to detect and are therefore also called “ghost particles”. They can nevertheless still be made indirectly visible and investigated with particle detectors.

There are three types of neutrinos: the electron, myon and tau neutrino. You can switch between these types in a special way. This is referred to as “neutrino oscillation”. In the 1990s, during an experiment in the USA, more particle interactions were observed than theoretically expected to investigate this neutrino oscillation. The existence of a fourth type of neutrino, the sterile neutrinos, became a popular explanation of these strange happenings. This hypothetical particle would be even more difficult to grasp than its “colleagues” and would respond only to gravity. But with the detector technology available at the time, it would not even have been possible to prove the existence of such a neutrino. As a result, in 2007, the idea of MicroBooNE was born.

MicroBooNE: Particle detector with precision technology from Bern

MicroBooNE has been in operation since 2015. The particle detector, based on state-of-the-art technologies, is stored in a 12-meter long cylindrical chamber filled with 170 tons of liquid argon. Thanks to the detector, the almost 200 employees of the MicroBooNE Collaboration can take spectacularly precise 3D pictures of neutrino events and thus study the interactions in detail. “We helped develop this liquid argon technology here at the University of Bern and our group was also involved in the construction of MicroBooNE,” explains Igor Kreslo, Professor at the Laboratory for High Energy Physics (LHEP) at the University of Bern. Furthermore, a calibration system was developed and a detector component for demonstrating cosmic rays built at the Laboratory for High Energy Physics and at the Albert Einstein Center for Fundamental Physics (AEC) at the University of Bern. These are central to the precision of the results of MicroBooNE.

Important zero result opens lots of doors

The first three years of data from MicroBooNE have now been evaluated – and show no sign of sterile neutrinos. According to Michele Weber, this is a fascinating turning point in neutrino research: “Naturally, discoveries are more exciting than zero results – but these are all the more important. We can now more or less rule out the most likely explanation for the anomalies and explore other – more complex and perhaps more interesting – possibilities.” Half the MicroBooNE data still has to be analyzed, and the possibilities for explaining the anomalies are diverse: “These include fascinating things such as light that is generated by new processes in neutrino collisions as well as exotic things such as dark matter,” says Weber. The MicroBooNE particle detector allows researchers to study other types of particle interactions.

The future of neutrino research

MicroBooNE is one of a whole range of neutrino experiments looking for answers. The foundations created with MicroBooNE are indispensable for further experiments. What is crucial is that the liquid argon technology has proved itself as it is also used in the Deep Underground Neutrino Experiment DUNE. DUNE is an international flagship experiment at the Fermilab, which already involves more than 1,000 researchers from more than 30 countries. DUNE will investigate oscillations in a process involving the sending of neutrinos underground to detectors 1,300 kilometers away at the Sanford Lab in South Dakota (USA). The University of Bern is contributing the main component of what is called the DUNE “near detector” which is to demonstrate neutrinos as soon as they have been created. The “ArgonCube”, as this special detector is called, was designed and developed in entirety in Bern and has also already been built as a prototype.