After nearly three decades of experimentation, science finally caught wind of the elusive sterile neutrino for the second time. This mysterious particle is believed to be the culprit behind many unexplained phenomena, such as the existence of dark matter and baryogenesis, the hypothetical physical process responsible for the asymmetry between matter and antimatter.
The first-ever hint that the sterile neutrino might exist came in the 1990s when the Liquid Scintillator Neutrino Detector (LSND) experiment conducted at Los Alamos National Laboratory in New Mexico found clues of an enigmatic new particle in the universe.
Now, another major physics experiment came to the same conclusion, reports Live Science. The new experiment, called MiniBooNE, is a followup to the original LSND and has produced the most substantial evidence yet that sterile neutrinos might actually exist.
Also known as inert neutrinos, sterile neutrinos are essentially a non-charged, non-active type of neutrinos that can pass through matter without interacting at all.
These “sterile” particles are different from the active neutrinos known in the Standard Model of particle physics, which are charged under the universe’s weak force and therefore do interact, albeit only slightly, with matter.
As opposed to active neutrinos — which are known to exist in three “flavors,” electron, muon, and tau — sterile neutrinos only interact via gravity and avoid any of the fundamental interactions described in the Standard Model.
Just like the LSND experiment, MiniBooNE fired beams of neutrinos at a detector placed behind an insulator, whose purpose was to block out all other radiation. As reported by Live Science, MiniBooNE used a tank of oil to protect its neutrino detector from radiation, whereas LSND used water as an insulator.
The experiment took place at the Fermi National Accelerator Laboratory near Chicago and aimed to monitor how many muon neutrinos morph into electron neutrinos.
This switch between neutrino “flavors” occurs because these elementary particles periodically “oscillate” as they move through space, jumping from one “flavor” to another, Live Science explains.
The results showed that MiniBooNE detected several hundred more electron neutrinos than predicted, an excess which supports the findings of the previous LSND experiment.
According to Science News, the researchers found 2,437 interactions between electron neutrinos and their antimatter counterparts, antineutrinos — roughly 460 more than they were expecting to uncover.
This anomaly suggests that neutrinos are oscillating into a fourth, heavier “flavor,” whose greater mass could account for the higher number of electron neutrinos counted by the researchers.
“The MiniBooNE data are consistent in energy and magnitude with the excess of events reported by the Liquid Scintillator Neutrino Detector (LSND), and the significance of the combined LSND and MiniBooNE excesses is 6.1-sigma,” the team wrote in a paper available on the preprint server arXiv.
The 6.1-sigma result means that the finding has more than one-in-500 million odds of being a fluke, explains Live Science.
Although this discovery still needs to be corroborated by further research, it has stirred quite an excitement in the world of particle physics.
“That would be huge; that’s beyond the standard model; that would require new particles… and an all-new analytical framework,” said Kate Scholberg, a researcher from Duke University in North Carolina, who didn’t take part in the experiment.
Theoretical physicist Neal Weiner, from New York University, also chimed in on the MiniBooNE discovery.
“It’s clear there’s something to be understood, and I certainly hope it’s a fourth neutrino.”
However, as he told Quanta Magazine, “the threshold for the evidence is obviously very high,” considering that “this would be the first discovered particle beyond the standard model.”
The downside of these results is that, if it turns out that sterile neutrinos have indeed been spotted, this means that they are too lightweight to make up dark matter, for which science will have to come up with another explanation.
Nevertheless, cosmologist Kevork Abazajian of the University of California told Science News that, just because these sterile neutrinos are too light to account for dark matter, it doesn’t mean that there aren’t heavier ones out there waiting to be discovered.
“I’m very excited about this result, but I am not ready to say ‘Eureka!'” said study co-author Janet Conrad, a neutrino physicist at the Massachusetts Institute of Technology.
As reported by Live Science, previous particle physics experiments, such as the underground Oscillation Project with Emulsion-Tracking Apparatus experiment in Switzerland and the more recent IceCube Neutrino Observatory in Antarctica, have failed to yield any proof that sterile neutrinos are real.
“There are people who doubt the result,” Scholberg told the media outlet, “but there’s no reason to think there’s anything wrong [with the experiment itself].”