Particle Physics – US experiment invalidates sterile neutrino hypothesis

Neutrinos – as fast as light and extremely fleeting. (imago images / Science Photo Library)

It was in June 2018 when the US physicist Bill Louis caused the bang at a conference in Heidelberg: “There are more than three types of neutrinos. Such a fourth kind of neutrino would be a new elementary particle. It has to be sterile, it only reveals itself through its gravity. “

Neutrinos are the strangest of the elementary particles, fast like light and extremely volatile. They only react with the rest of the matter by gravity and by means of another natural force – the so-called weak force. But the textbook knowledge of particle physics, the Standard Model, only knows three types of neutrinos. If there were a fourth one that only reacts to gravity and not to the weak force, that would be a physical revolution.

Physics beyond the Standard Model?

For Bill Louis it was clear: “This gives us a clear indication of new physics beyond the standard model.” The MiniBooNE experiment in Chicago provided the information: An accelerator fires neutrinos at a spherical tank filled with mineral oil 500 meters away Bill Louis explained in 2018 that more neutrinos have turned into a different type on the fly than expected – for him a clear indication of a fourth neutrino.

The MiniBooNE experiment refuted measurement results about the mutability of neutrinos.

The MiniBooNE experiment with this accelerator in Chicago threw the physics world into turmoil in 2018. (

But other experts reacted cautiously, such as Manfred Lindner from Heidelberg: “Nature has many mechanisms to fake something that one would like to believe.” What was needed was a second experiment that confirmed or refuted the results. It was carried out also in Chicago, where the accelerator is called MicroBooNE instead of MiniBooNE.

New experiment with a different accelerator

“This is a completely new technology. The experiment is based on a huge tank, the size of a school bus, full of liquid argon, “explains Michele Weber from the University of Bern, a member of the 200-strong MicroBooNE team. As in the earlier experiment, an accelerator fires a certain neutrino Species on a tank. This tank, however, can not only detect a characteristic glow that occasionally occurs when the neutrinos strike, but much more.

Michele Weber, Professor for Experimental Particle Physics in Bern: “The traces of all the particles that are produced in these interactions can be measured there. We can really see for every neutrino interaction: What kind of particles were produced there? Is it really a neutrino or is it something else? The other experiment could only see these flashes of light and thus interpret something. And we were really able to analyze these events in 3D. “

A simple model of the fourth neutrino is unlikely

The experts have been collecting measurement data for years, and now they are presenting their first analyzes. According to this, no more neutrinos have turned into a different type than expected. The older experiment seems to be refuted.

“This simple model of a sterile neutrino has become really very, very improbable,” concludes Michele Weber. However, the particle physicist points out that at least theoretically there could still be a loophole for the sterile neutrino, which is solely the force of gravity feels: It could disintegrate so quickly that even the highly sensitive MicroBooNE sensors could lose it.

Dark matter captured by the Hubble Space Telescope and the Chandra X-Ray Observatory.  In the picture you can see stars and a kind of purple-pink nebula.

Are there still other particles? Possibly the ominous dark matter? (dpa / picture alliance / Landov)

Are there any other particles?

In order to test such speculations, the team now wants to upgrade its experiment and equip it with additional detectors. Because even if the hypothesis of the sterile neutrino is highly unlikely, the MicroBooNE measurement data show suspicious anomalies that do not really fit into the standard model for Michele Weber and her MicroBooNE team.

“It’s definitely something that comes from the beam and not from anywhere else. It may be that something else is also being produced there that we don’t know. ”It is possible that it is particles of ominous dark matter that are created there, or so-called axions – all of them particles that would be completely new in the particle zoo and that set the horizon of the Would expand physics significantly.

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