Ghostly 4th Neutrino Most Likely Doesn’t Exist

An international team of researchers from the IceCube Neutrino Observatory just announced with 99% certainty that a proposed particle called a sterile neutrino doesn’t exist. Why is the fact that something doesn’t exist big news? This ghost particle may have helped explain several mysteries of the universe, such as the origin of dark matter and why matter exists at all.

We know that three types of neutrinos exist based on large, carefully designed experiments, but some experiments hinted at this mystery-solving fourth type. Eerie particles with practically zero mass that rarely interact with anything, neutrinos are often referred to as ghost particles. Trillions of neutrinos travel through your body every second without you even noticing. Although they are tiny and difficult to detect, neutrinos play an important role in the universe that we are still trying to understand.

To explore whether the hypothetical sterile neutrino exists, IceCube researchers did a comprehensive analysis of two years of data. Led by Francis Halzen from the University of Wisconsin-Madison, IceCube is a one-of-a-kind neutrino observatory located in the South Pole. Most of the detector is buried more than a mile deep in the ice. Neutrinos are extremely difficult to detect because they rarely interact with matter, so scientists have gotten creative.

The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers that collect raw data from the detector buried in the ice.
Image Credit: Christian Krueger, IceCube/NSF, April 2016.

IceCube works like this. Neutrinos are produced when cosmic rays, high energy particles traveling through space, hit particles in the Earth’s upper atmosphere. (They are produced in other ways too, but this study focused on atmospheric neutrinos.) Most of these neutrinos travel right through the Earth as if it didn’t even exist. However, once in a while a neutrino will interact with a molecule in the ice in a way that produces a flash of blue light. IceCube consists of 5,160 sensors that keep an eye out for these flashes. The sensors are frozen at various depths within a cubic kilometer of ice. From the direction and size of the flash, researchers can determine the type, energy, and origin of the neutrino.

For this analysis, the researchers looked only at flashes consistent with neutrinos traveling up through the detector after passing through the Earth. A neutrino signal can be easily overwhelmed by signals from the collisions of other particles, like electrons, with molecules in the ice. However, these particles can’t travel through the Earth like neutrinos. By looking only at flashes that travel upward through the detector, the team cut out most of the background noise.

IceCube detects about one atmospheric neutrino every six minutes, the energy of which ranges from about 1 tera electron Volt (TeV) to several hundred TeV. These neutrinos are a great tool for particle physics studies such as the search for sterile neutrinos—a theoretical type of neutrino that has no “flavor“. In this image, an incoming cosmic ray (red) interacts with a particle in the atmosphere and produces other particles including a neutrino (yellow). The neutrino travels through the Earth and up through the IceCube detector. νµ indicates the specific type of neutrino measured in this study.
Image Credit: IceCube Collaboration.

Sterile neutrinos are especially tricky to detect because the hypothetical particles don’t ever collide with anything! They are even more ghost-like than the other ghost particles. Luckily, there is an indirect way to test their existence. Experiments show that neutrinos change from one type into another in a predictable way as they fly through the universe. If the fourth type exists, theory predicts that it “mixes” with the other types, meaning that it also changes types. This would show up in a distinct way within the data collected by IceCube.

The thing is, it didn’t. In two comprehensive, independent studies of tens of thousands of events each, IceCube found no trace of sterile neutrinos. Their method and results were published today in the American Physical Society’s Physical Review Letters.

Some people may be disappointed by the lack of sterile neutrinos, but there is a flip side to this result. Although the sterile neutrino would solve some problems, it would create others. In particular, the Standard Model—a framework that describes our best understanding of the particles and forces that make up our universe—only allows for three types of neutrinos. According to Halzen, “If you throw in a fourth neutrino, it changes everything”. As exciting as it would be to verify the existence of a fourth neutrino (science loves a good discovery), there is something to be said for results that reinforce our understanding of the universe, even if it means some mysteries remain.

Kendra Redmond

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