Ghost Particle Explained As Sterile Neutrino Eludes Physicists in Groundbreaking Study

Physicists were left with more questions than answers this week after returning empty-handed following a painstaking search for an elusive theoretical particle called a sterile neutrino.

The results were released on Wednesday by scientists working with the MicroBooNE neutrino experiment based at Fermilab—a particle physics facility in Illinois.

Working with neutrinos is a difficult and painstaking process because they are incredibly small. In fact, for a long time scientists thought neutrinos didn't have any mass at all and they've even been dubbed "ghost particles."

What is a neutrino?

A neutrino is a fundamental particle of the universe, meaning that, as far as we know, it isn't made of anything else. These types of particles are also called elementary particles. An electron, for example, is also a fundamental particle.

But while an electron has a charge, neutrinos don't. Neutrinos are also very, very small. Their mass is thought to be less than one millionth that of an electron.

Scientists think that neutrinos might be able to help us answer some of the most fundamental mysteries of physics, including: why is there matter in the universe? Current predictions suggest that an equal amount of matter and anti-matter should have appeared and canceled each other out after the Big Bang, but clearly this didn't happen.

Helpfully, neutrinos are everywhere, including space. Of all particles with mass, they are the most abundant.

Unhelpfully, they're so small that they're very hard to detect, since they tend to pass straight through almost everything they encounter.

Neutrinos come in three types that we know of, called "flavors." These are called electron neutrinos, muon neutrinos, and tau neutrinos. Neutrinos tend to switch between these three flavors as they travel.

Around two decades ago, scientists began investigating the possibility of a fourth flavor—a sterile neutrino—after they observed more neutrino collisions in an experiment than they had predicted. One way to explain this was if a fourth, unknown neutrino was responsible.

Enter MicroBooNE

Scientists needed to look at this problem more accurately to see if a fourth neutrino really did exist, so they built MicroBooNE.

MicroBooNE is a huge particle detector that takes the form of a 40-foot-long hollow tube container. When it's in use, scientists fill it up with tons of pure liquid argon, a dense, transparent fluid.

Using a complicated array of sensors, the machine can then capture the trails left behind when neutrinos pass through the machine and occasionally bump into the dense argon filling. These collisions release additional particles that can then be recorded.

Using this method the MicroBooNE scientists set out to see if they could finally catch a glimpse of a sterile neutrino—but it was not to be. The experiments showed no more than the standard three neutrino flavors we know and love.

"They all tell us the same thing," said Bonnie Fleming, physics professor at Yale University and co-spokesperson for MicroBooNE, in a Fermilab press release, "and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino."

But the question remains: why did the 2002 experiments produce such odd results?

According to Fermilab scientist Sam Zeller, the earlier experiments don't lie. "There's something really interesting happening that we still need to explain," he said in the press release. "The data is steering us away from the likely explanations and pointing toward something more complex and interesting, which is really exciting."

A stock image shows an abstract illustration of an atom. Neutrinos are a type of elemental particle much smaller than an atom that make up the universe. aleksandarnakovski/Getty