What is a “sterile” neutrino? Why an unexpected result opens the door to new physics

Particle physicists have gone to great lengths — including rowing onto the dark waters of a swimming pool-sized water tank and setting up a base in the Antarctic — to understand one of the universe’s smallest particles: the neutrino.

These minuscule subatomic particles are shot off from stars, nuclear reactors, and accelerated particles beams — and billions of them are flying through us at every moment. But while the indetectable tickle of a neutrino may be inconsequential to our daily life on Earth, scientists say they could help unravel some of the universe’s biggest mysteries, including dark matter.

However, according to new results out of a Fermilab experiment, answering these questions may be less straightforward than scientists initially hoped.

Bryce Littlejohn is an associate professor of physics at the Illinois Institute of Technology and a collaborator on MicroBooNE, the Fermilab neutrino experiment that released new results in October. With this experiment, Littlejohn and collaborators were hoping to find a new kind of neutrino in their data called a “sterile neutrino” that could unlock a number of previously unsolved mysteries in physics. Instead, the data examined so far has turned up null.

But that isn’t necessarily a bad thing, Littlejohn tells Inverse.

“I did get kind of disappointed because we could have clutched this brand new, beyond the standard model physics, and we didn’t with this particular result,” Littlejohn says. “[But] now I am actually back to being excited... [because] there are a lot of other explanations for what could be going on, and most of them involve new physics.”

What exactly is a neutrino?

But let’s take a step back first to understand neutrinos as science already knows it.

Literally meaning “little neutron,” as named by Italian physicist Wolfgang Pauli when he first predicted them in 1930, these particles are incredibly light (at least six million times lighter than an electron,) chargeless, fast, and have minimal interaction with matter.

For this reason, they are commonly referred to as ghost particles because they can pass through matter unnoticed and are rendered invisible through their indifference to photons.

While this does conjure charming images of teeny neutrino ghosts flitting through us like friendly spirits, these traits have been a headache for physicists wishing to study neutrinos because it makes them incredibly hard to observe.

In the 1950s, however, physicists cracked the code on how to spy on what cannot be seen: watch where it lands.

Like how ghosts in scary stories can make doors creek or rustle through leaves, scientists can spot neutrinos by studying the particles they break into when colliding with an atomic nucleus in specialized detection chambers.

What flavors of neutrinos are there?

Before scientists went looking for a fourth flavor of neutrino, they already knew that at least three types, or flavors, of neutrinos existed:

• Electron neutrinos
• Muon neutrinos
• Tau neutrinos

The big difference between these types of neutrinos is the fermion particles they break into when colliding with an atomic nucleus. As their names suggest, each type of neutrino will produce — among a smattering of other particles — either an electron, a muon, or a tau particle upon its demise.

It’s these fermions that scientists are on the lookout for when detecting neutrinos.

What is a sterile neutrino?

If regular neutrinos are a needle in a haystack, then sterile neutrinos are a dust mite in a haystack. Unlike the other known neutrinos, a sterile neutrino doesn’t break into a new fermion or particle, Littlejohn explains, because it doesn’t interact with matter at all to create a collision. Instead, physicists searching for this particle believe that it could play a kind of assistant role for the other three neutrinos to help them quick-change from one type to another.

Throughout the decades, scientists have observed strange happenings in their particle detectors that have suggested the existence of such a neutrino, including MicroBooNE’s predecessor MiniBooNE in the early 2000s. What makes this latest experiment special, Littlejohn says, is the millimeter-level precision at which MicroBooNE’s camera can capture a neutrinos’ collision.

The results — Here’s what they did: beams of muon neutrinos were sent through a school bus-sized chamber full of argon where they collided with detectors and broke into pieces. If sterile neutrinos were present in the chamber, Littlejohn explains that they would’ve observed electrons as well as muons — meaning the muon neutrinos had changed form via help from sterile neutrinos.

However, this isn’t what the team observed. Instead, the muon neutrinos remained the same without any hint of a fourth, invisible neutrino in their midst. While this finding was initially disappointing, Littlejohn says it’s still only just the beginning for both sterile neutrinos and MicroBooNE.

“Maybe it’s not a sterile neutrino, but it could be some other crazy thing,” Littlejohn says. “[Something axion-like] or beyond the standard model Higgs particles. All sorts of other crazy physics that theorists have been thinking about.

Sterile neutrinos and dark matter

One exciting possibility that may still be on the horizon for sterile neutrinos is what they might tell scientists about the existence of dark matter in the universe. While the kind of neutrinos Littlejohn and colleagues searched for aren’t likely to be the key to unlocking this mystery, Littlejohn says that its theoretical cousin, the “heavy” sterile neutrino, may just be.

“One thing theorists have been interested in lately is the coupling of neutrinos to dark sectors of new physics,” Littlejohn says, who himself is an experimentalist. “Dark matter is one example. If dark matter is existing out there in the universe as some sort of heavy [bosons] or something that might cause you to ask, ‘how would a dark [boson] interact with a neutrino?’ or ‘how will the dark [boson] decay into a neutrino?’”

These kinds of questions will help bring scientists a step closer to understanding the existence of this inky, cosmic mystery.

What’s next — As of now, scientists have only scratched the surface of data collected from MicroBooNE. Over the next six months, Littlejohn says there could still be exciting news yet to come from the experiment.

And MicroBooNE’s data is just the beginning of what will be an exciting rest of the decade for neutrino physics, Littlejohn says. An experiment called DUNE is set to start collecting data in 2029 and will provide scientists with more neutrino data than ever before.

“I feel like people have been kind of hypnotized by sterile neutrinos for a while,” Littlejohn continues. “The more recent 'renaissance' has been using our bulky neutrino detectors to look for new physics outside the neutrino realm — it's like our 'night job,' while neutrino hunting is our 'day job'.”

In the world of ghost-hunting particle physicists, things are only getting more interesting.