For the second time in history, scientists spot signs of a ghost particle that is a viable candidate for dark matter.
The MiniBooNE experiment at the Fermi National Accelerator Laboratory (Fermilab) in Batavia was developed to follow up on the results of a well-known experiment in the 1990s.
After 15 years since its conception, MiniBooNE confirms the findings of the Liquid Scintillator Neutrino Detector based at the Los Alamos National Laboratory, which gave the world's first evidence of the existence of sterile neutrinos, despite other experiments suggesting otherwise.
The Standard Model Of Particle Physics
Since the 1970s, the theory of the Standard Model has dominated the realm of particle physics. This theory acknowledges the existence of neutrinos, high-energy particles rushing through the universe barely interacting with matter. Every day, billions of neutrinos stream from the sun but have very little effect on human bodies.
The Standard Model dictates that neutrinos come in three types, or "flavors": electron, muon, and tau. As they race through space, these neutrinos oscillate from one flavor to another.
Physicists have proposed the existence a fourth flavor decades ago. Sterile neutrinos can pass through matter without interacting with it, except through gravity. If they exist, they would completely overthrow the Standard Model of particle physics as well as the prevailing notions that govern cosmology.
"There are other potential cracks in the standard picture," Carnegie Mellon physicist Scott Dodelson says. "The neutrino paradox could point our way to a new, better model."
The MiniBooNE Experiment
MiniBooNE was set up to check on the results of LSND. In both experiments, beams of neutrinos were fired at a detector sitting behind an oil tank insulator that blocks out radiation. In LSND, researchers used a water tank insulator instead.
The goal was to count the number of neutrinos striking the detector. As the neutrinos were fired, some of the muon neutrinos would become electron neutrinos. This was detected as flashes of radiation when the electron neutrinos interacted with oil.
Two decades after LSND, MiniBooNE saw the same anomalous result in the number of electron neutrinos. The detector recorded 2,437 electron neutrinos, nearly 500 more than the expected outcome.
The anomalous results, the researchers believe, could be a sign of sterile neutrinos that could have caused more muon neutrons oscillating into electron neutrinos.
"The new data from MiniBooNE confirms that this tension in the data is real," says Sabine Hossenfelder, theoretical physicist at the Frankfurt Institute for Advanced Studies. "This data can (to the best of my knowledge) NOT be fitted with the standard framework. It requires either new particles (sterile neutrinos) or some kind of symmetry violation."
The results of the experiment are currently published in the preprint server ArXiv.org.
Sterile Neutrinos And Dark Matter
Sterile neutrinos have long been considered one of the best candidates for dark matter, the unobservable 25 percent of the rest of the universe that can only be detected through gravity.
However, the MiniBooNE results would mean sterile neutrinos are fairly lightweight. For them to make up dark matter, physicists believe sterile neutrinos would have to be a little heavier. The results do not completely negate the connection between sterile neutrinos and dark matter. Cosmologist Kevork Abazajian at the University of California, Irvine, believes sterile neutrinos may come in light and heavy forms.
"Sometimes people say they're like cockroaches," Abazajian explains. "If you have one [type of] sterile neutrino, you have many."