June 18, 2011 - Missing Neutrinos Found!

– 2001

When you're as teensy as an electron, it's hard to see you!

But electrons have a negative charge, so we can at least detect that charge. They interact with matter because they are charged—for example, the electrons streaming away from the Sun excite atoms in our atmosphere and cause colorful curtains of light called aurorae.

There are other particles streaming away from the Sun that are just as tiny as electrons but that have no charge. These neutral particles are called neutrinos. And boy, are they hard to detect!!!

Way back in 1930 theoretical physicist Wolfgang Pauli predicted that there should be such a thing as a neutrino. When physicists make such predictions, they are working with data and equations, and they sometimes find that something is missing in their data—the math doesn't come out right unless some other thing, still unknown, exists. Pauli was in that situation—he thought that there must be a particle as large as an electron, but with no charge. He named this not-yet-discovered particle a neutron.

A few years later, another physicist discovered a much larger neutral particle that he named neutron. So the as-yet-undiscovered particle got renamed neutrino.

In 1956, physicists were able to detect neutrinos (well, it turned out that they were closely-related particles called anti-neutrinos) created in nuclear reactors. And in the 1960s, scientists were able to detect and even count neutrinos coming from the Sun, using huge tanks of dry-cleaning liquid deep, deep underground. (Those few incoming neutrinos that hit a chlorine atom in the liquid changed the chlorine to argon. The amount of argon in the liquid was then measured.)

Once they were able to detect and measure neutrinos, scientists noticed that Earth was receiving only one third to one half of the expected solar neutrinos. Again, our equations were scrutinized—this time, the equations that modeled how nuclear fusion operates deep in the Sun's core. Could there be something wrong with our model? Could something be missing?

The Sudbury Neutrino Observatory reported on this day in 2001 that the missing neutrinos were there, all along, but had changed “flavors” in their long journey through the outer layers of the Sun, through space, and through our own atmosphere. The earlier neutrino detectors had only detected electron neutrinos, but the SNO used heavy-water detectors that also detected muon and tau neutrinos. With data on all three flavors of neutrinos, the expected number was found, and our model of how the Sun operates was confirmed.

Have you always wanted...

...a muon neutrino of your very own? Get it here. 

Older students might enjoy the Particle Adventure.