Almost every particle has an antimatter counterpart: a particle with the same mass but opposite charge, among other qualities.

This seems to be true of neutrinos, tiny particles that are constantly streaming through us. Judging by the particles released when a neutrino interacts with other matter, scientists can tell when they’ve caught a neutrino versus an antineutrino.

But certain characteristics of neutrinos and antineutrinos make scientists wonder: Are they one and the same? Are neutrinos their own antiparticles?

This isn’t unheard of. Gluons and even Higgs bosons are thought to be their own antiparticles. But if scientists discover neutrinos are their own antiparticles, it could be a clue as to where they get their tiny masses—and whether they played a part in the existence of our matter-dominated universe.

Dirac versus Majorana

The idea of the antiparticle came about in 1928 when British physicist Paul Dirac developed what became known as the Dirac equation. His work sought to explain what happened when electrons moved at close to the speed of light. But his calculations resulted in a strange requirement: that electrons sometimes have negative energy.

“When Dirac wrote down his equation, that’s when he learned antiparticles exist,” says André de Gouvêa, a theoretical physicist and professor at Northwestern University. “Antiparticles are a consequence of his equation.”

Physicist Carl Anderson discovered the antimatter partner of the electron that Dirac foresaw in 1932. He called it the positron—a particle like an electron but with a positive charge.

Dirac predicted that, in addition to having opposite charges, antimatter partners should have another opposite feature called chirality, which represents one of the inherent quantum properties a particle has. A particle can have either a right-handed or left-handed chirality.

Dirac’s equation allowed for neutrinos and antineutrinos to be different particles, and, as a result, four types of neutrino were possible: neutrinos with left- and right-handed chirality and antineutrinos with left- and right-handed chirality.

But if the neutrinos had no mass, as scientists thought at the time, only left-handed neutrinos and right-handed antineutrinos needed to exist.

In 1937, Italian physicist Ettore Majorana debuted another theory: Neutrinos and antineutrinos are actually the same thing. The Majorana equation described neutrinos that, if they happened to have mass after all, could turn into antineutrinos and then back into neutrinos again.