Undiscovered particles known as sterile neutrinos could explain some unusual experimental outcomes.

When physicists finally discovered the Higgs boson in 2012, they confirmed a 50-year-old theoretical prediction. However, not every particle that scientists are looking for has a past. Several experiments are looking for a particle that no theory predicted, but which could answer several unanswered puzzles in particle physics.

The particle, known as a sterile neutrino, is a sneakier counterpart of the ghostly neutrino. Neutrinos pass nearly totally unobserved through other matter; roughly 100 trillion pass through your body every second, albeit just a handful will interact in your body over your lifetime.

Neutrinos were previously assumed to have no mass, according to the Standard Model of particle physics. However, physicists discovered compelling evidence in 1998 that the three types of neutrinos—electron, muon, and tau—can oscillate, or alter, among themselves, which is only possible if the particles had mass. (They were awarded the Nobel Prize in Physics in 2015 for this finding.)

Another option emerged after the discovery of neutrino mass: a right-handed neutrino. In particle physics, handedness is a property that arises from the mass and spin of a particle. Neutrinos, as massless particles, would be unable to change their handedness; yet, with mass, they can.

Scientists have only seen left-handed neutrinos so far, but the right-handed counterpart could be lurking in the shadows. Right-handed neutrinos, on the other hand, interact simply through gravity, while left-handed neutrinos interact in two ways (through gravity and the weak force).

“Sterile neutrinos were always a possibility, but we didn’t have to be concerned because we didn’t realize neutrinos could have mass. So we just disregarded them,” recalls Richard Van de Water, a physicist at the Los Alamos National Laboratory of the US Department of Energy. “Neutrinos now have mass,” says the author. That suggests a right-handed, sterile neutrino might exist.”

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1 in a trillion chance

Two decades ago, scientists discovered the first evidence of sterile neutrinos. There are three types of oscillations that can be recorded if there are three neutrino states, as specified in the Standard Model. Scientists should be able to predict the third measurement based on the first two.

Physicists felt they had two figured out by the early 2000s. Experiments at Los Alamos’ Liquid Scintillator Neutrino Detector were supposed to validate the previous measurement. However, in 1995, LSND discovered excess neutrino oscillations where theory anticipated none.

“The fact that they observed something indicates that you can’t put all these different observations together in a coherent neutrino image with only three neutrinos,” says Matt Toups, a neutrino physicist at the Fermi National Accelerator Laboratory of the Department of Energy. “That’s why sterile neutrinos are now a thing.”

The findings of LSND were so unexpected that scientists developed a new detector at Fermilab to verify them. The MiniBooNE detector, which stands for Booster Neutrino Experiment, detected an excess of electron neutrinos in 2006, contradicting the conclusions of the LSND but indicating the potential of sterile neutrinos. (LSND was more consistent in later runs.)

Physicists need to look at interactions in numerous detectors situated at different distances from the neutrino source, such as the liquid-argon detectors in Fermilab’s Short-Baseline Neutrino Program, to better understand these excesses. The first neutrino interactions were observed in November by MicroBooNE, the first of three short-baseline detectors to be built.

Finding definitive evidence of the existence of sterile neutrinos will be difficult even with these modern detectors. The term “sterile” refers to their inability to interact with other matter through any of the Standard Model’s forces other than gravity. That implies they can only be observed through their oscillations into active neutrinos, which are extremely difficult to detect in and of themselves. Fewer than one in a trillion neutrinos will contact with another particle at LSND, leaving a trace that scientists can track.

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Van de Water says that just detecting neutrinos is tough. “As a result, it’s difficult to interpret the effects of sterile neutrinos on neutrino oscillations.”

Unexpected visitors

Because scientists had long predicted the existence of the Higgs boson, its absence would have completely changed the course of particle physics. This isn’t the case with sterile neutrinos, which would be a far more surprising find.

According to André de Gouvêa, a theoretical physicist at Northwestern University, “there are all these models out there that tell us we’re not meant to find any sterile neutrinos.” “The world does not change if you do not notice one.”

Despite this, the search for these strange particles is gaining traction, partly due to the numerous outstanding questions that sterile neutrinos could be tied to. It’s possible they have the answer to one of particle physics’ biggest mysteries: the source of neutrino mass. They could be essential in cosmology, providing new insights into the early universe’s development. Some physicists believe sterile neutrinos are the particles that make up dark matter, which, like sterile neutrinos, appears to be immune to all known forces save gravity.

Van de Water says, “We know there’s this huge 95% of the cosmos out there that we don’t understand.” “What better way to link to that than with a particle that barely interacts with the Standard Model itself?” says the author. What better way to conceal all of that physics than with a sterile neutrino? It might be a portal to the dark sector, and that’s what we’re beginning to discover.”

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Van de Water claims that their existence, whatever they are, would be solid evidence of physics outside the Standard Model. Finding them would require physicists to integrate them into existing ideas about the cosmos, and it would very certainly raise a slew of new concerns for them to address.

Toups speculates, “It could just be a portal onto a whole other collection of events.” “It would truly open up a whole avenue of research for the discipline that I believe would be productive for a number of years,” says the researcher.

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