If neutrinos had personalities, they’d be known best for their aloofness. These tiny particles, which can travel almost as fast as the speed of light, don’t have much regard for matter; they sail right on through walls and buildings and planets. If you sent a group of neutrinos through a light-year of lead shielding—six trillion miles thick—half of them would still cruise on through.

Physicists attempt to explain all fundamental particles, including neutrinos, with a set of rules known as the Standard Model. Mostly it works great, but it has a few flaws. One mystery that the theory fails to explain is the snobbish behavior of the tiny neutrino, which comes in three different flavors: muon, electron and—most elusive of all—the tau neutrino.

Now, particle physicists have experimental proof that muon neutrinos can change into tau neutrinos, a finding that confirms the existence of a big blind spot in the Standard Model. On Tuesday, Italy’s Gran Sasso Laboratory announced that the Oscillation Project with Emulsion-tRacking Apparatus, or OPERA experiment, had detected a muon-turned-tau neutrino—just the fifth time the transition had ever been observed.

How sure are they? Extremely sure; astoundingly sure—no one does statistics like a particle physicist. “There is a 1 in 10 million chance that we didn’t detect a tau neutrino in our muon neutrino beam,” says Giovanni De Lellis, a physicist at Gran Sasso and spokesperson for the team.

This group, made up of about 140 physicists from 26 research institutions in 11 countries, has spotted this suspected transformation four times before, but they weren’t confident enough in their observations to announce a discovery. With this fifth detection, the team is now sure that they saw what they saw.

Neutrinos have been spotted acting weird since the 1960s, when physicists began measuring electron neutrinos produced by the sun. They found far fewer neutrinos than were expected based on their understanding of Earth’s star, De Lellis says. Nothing in the Standard Model could explain why the number of detected electron neutrinos were so low.

So astroparticle researchers posited that neutrinos can swap from flavor to flavor, and that some of these electron-flavored neutrinos were morphing into tau neutrinos—a process called neutrino oscillation. Later measurements of cosmic rays, high-energy radiation coming from space, also hinted that muon neutrinos can turn into tau neutrinos.

To get a sense of how weird neutrino oscillation is, imagine a game of catch. “It’s like throwing an apple, and having some turn into grapefruits and some turn into oranges by the time they’re caught,” says Alan Poon, a neutrino physicist unaffiliated with OPERA at Lawrence Berkeley National Laboratory. A particle can be measured as one thing at the beginning of its path, and measured as something entirely different once it reaches a detector.

To detect this flavor change from muon to tau, De Lellis’s team produced a beam of muon neutrinos at CERN, Europe’s center for high energy physics research in Geneva, Switzerland, by smashing protons into a graphite target. When the protons collide with the graphite, they produce particles called pions and kaons that decay into muon neutrinos.

These then begin an epic journey more than 450 miles underground to a huge detector, heavier than 20 blue whales, at Gran Sasso. The long distance increases the likelihood the neutrinos will oscillate from their muon flavor to the tau variety. In Italy, the OPERA detector records any evidence of the neutrinos on layers—9 million in all—of photographic film.

“As far as I’m concerned, the chapter on neutrino oscillations on the change from muon to tau has closed,” Poon says. He thinks that now the big goals in experimental neutrino physics will be measuring the particle’s mass and finding out how a neutrino differs from1 its corresponding antiparticle, the antineutrino.

But for the time being, De Lellis is celebrating. Speaking on Tuesday evening, he was about to join his colleagues for dinner festivities. “We are in a very old monastery in the north of Rome, built in the fifth century after Christ,” De Lellis says. How fitting, to bask in their discovery on the foundations of old.

1 UPDATE 6:00 PM ET 06/17/15 This story has been updated to indicate that scientists will study neutrinos, not antineutrinos.