Neutrinos -- once described as “the most tiny quantity of reality ever imagined by a human being” -- are perhaps the most exotic and least understood of all known subatomic particles.
These particles, which are produced by the decay of radioactive elements, have a unique trait -- they can change, or “oscillate,” between their three known types, or “flavors” -- the electron neutrino, the muon neutrino and the tau neutrino. And particle physicists believe that understanding this peculiar behavior is key to answering the longstanding cosmic quandary -- why is there something rather than nothing?
Scientists at the Fermi National Accelerator Laboratory's NOvA experiment -- a collaboration of 180 scientists from 28 institutions -- have now announced their first evidence of oscillating neutrinos, describing it as a “major leap” toward understanding these ghostly particles and their interactions.
“People are ecstatic to see our first observation of neutrino oscillations,” NOvA co-spokesperson Peter Shanahan said, in a statement released Friday. “For all the people who worked over the course of a decade on the designing, building, commissioning and operating this experiment, it’s beyond gratifying.”
Despite being one of the most abundant particles in the universe, neutrinos hardly interact with matter. While this characteristic allows neutrinos produced at Fermilab to travel 500 miles through the rocks below the Earth’s surface, it also means that only a very tiny fraction of the incoming particles interact with the NOvA detector -- a behemoth 50 feet tall, 50 feet wide and 200 feet long -- located in Ash River, Minnesota. Every second, Fermilab’s accelerator sends a high-powered beam containing trillions of neutrinos from its Illinois center to Minnesota, of which only a handful register at the detector.
When a neutrino bumps into an atom in the NOvA detector, it releases a signature trail of particles and light depending on its flavor. The beam originating at Fermilab is made almost entirely of muon neutrinos. In order to see if oscillation has occurred during the neutrinos’ 500-mile journey, the detector at Minnesota measures the relative fraction of mu and electron neutrinos.
“If oscillations did not occur, experimenters predicted they would see 201 muon neutrinos arrive at the NOvA far detector in the data collected; instead, they saw a mere 33, proof that the muon neutrinos were disappearing as they transformed into the two other flavors,” Fermilab said, in the statement. “Similarly, if oscillations did not occur, scientists expected to see only one electron neutrino appearance (due to background interactions). But the collaboration saw six such events, evidence that some of the missing muon neutrinos had turned into electron neutrinos.”
This is not the first time neutrinos have been spotted switching flavors. In June, physicists at Italy’s National Institute for Nuclear Physics found the first direct proof of oscillation between a tau and a muon neutrino in a beam produced at the European Organization for Nuclear Research (CERN).
Neutrinos oscillating between various flavors have an important implication -- they hint at the existence of physics beyond the Standard Model, which has so far failed to incorporate one of the four fundamental forces -- gravity. Additionally, if scientists are able to detect a neutrino switching to its anti-particle -- the antineutrino -- it might help them determine the reason behind current matter-antimatter imbalance in the cosmos, and, by extension, explain why the universe as we know it, exists.