When physicists finally detected the Higgs boson in 2012, they validated a theoretical prediction made some 50 years earlier. But not every particle that physicists are searching for has such a history. Several experiments are on the hunt for a particle that theory never demanded—but that could wind up answering several open questions in particle physics. Known as a sterile neutrino, the particle is an even sneakier version of the ghostly neutrino. Neutrinos stream through other matter almost completely unnoticed; about 100 trillion of them pass through your body every second, though only a few will interact in your body over your entire lifetime. According to the Standard Model of particle physics, neutrinos were originally thought to have no mass. But in 1998, physicists found clear evidence that the three known types of neutrinos—electron, muon and tau—can oscillate, or change, among each other, which is possible only if the particles have mass. (This discovery earned them the 2015 Nobel Prize for Physics.) The discovery of neutrino mass opened up another possibility: a right-handed neutrino. In particle physics, handedness is a quality that emerges from a particle’s mass and spin. As massless particles, neutrinos wouldn’t be able to change their handedness—but with mass, they can. Until now, scientists have only observed left-handed neutrinos, but the right-handed version might be lurking out of sight. And while left-handed neutrinos interact in two ways (through gravity and the weak force), right-handed neutrinos are even trickier, interacting perhaps only through gravity. “Sterile neutrinos were always out there as an idea, but we didn’t have to worry about it because we didn’t even know neutrinos had mass. So we just ignored them,” says Richard Van de Water, a physicist with the US Department of Energy’s Los Alamos National Laboratory. “We now say neutrinos have mass. That means there can be this right-handed, sterile neutrino.” Illustration by Sandbox Studio, Chicago with Ana Kova

One in a trillion Experimentalists found the first hint of the existence of sterile neutrinos two decades ago. If there are three neutrino states, as described in the Standard Model, then there are three different kinds of oscillations that can be measured. Any two measurements should allow scientists to predict the third. By the early 2000s, physicists thought they had two figured out. They had hoped to confirm the last measurement with experiments run at Los Alamos’ Liquid Scintillator Neutrino Detector. But in 1995, LSND had picked up excess neutrino oscillations where theory predicted there should be none. “The fact that they saw something says that you can’t put all these different measurements together in a coherent neutrino picture that has only three neutrinos,” says Matt Toups, a neutrino physicist at DOE’s Fermi National Accelerator Laboratory. “That’s why people now talk about sterile neutrinos.” LSND’s results were so surprising that physicists built a new detector at Fermilab to check their findings. Dubbed MiniBooNE—BooNE stands for Booster Neutrino Experiment—the detector picked up an excess of electron neutrinos in 2006 that conflicted with LSND’s results but still indicates the possibility of sterile neutrinos. (Later runs were more consistent with LSND.) To further probe these excesses, physicists need to look at interactions in multiple detectors set at different distances from the neutrino source, such as the liquid-argon detectors in Fermilab’s Short-Baseline Neutrino Program. MicroBooNE, the first of three short-baseline detectors to be installed, saw its first neutrino interactions in November. Even with these new detectors, finding definitive evidence of the existence of sterile neutrinos will be a challenge. The “sterile” in their name comes from their inability to interact with other matter through any of the forces in the Standard Model other than perhaps gravity. That means they can be observed only through their oscillations into active neutrinos, which are themselves incredibly difficult to detect. At LSND, fewer than one in 1 trillion neutrinos will interact with another particle, leaving a footprint for scientists to measure. “Just detecting neutrinos is difficult,” Van de Water says. “By extension, that makes interpreting the effects of sterile neutrinos on neutrino oscillations difficult.” Illustration by Sandbox Studio, Chicago with Ana Kova