What is a Neutrino…And Why Do They Matter?

BY Jenny Marder  January 25, 2011 at 4:15 PM EST

Neutrinos are teeny, tiny, nearly massless particles that travel at near lightspeeds. Born from violent astrophysical events like exploding stars and gamma ray bursts, they are fantastically abundant in the universe, and can move as easily through lead as we move through air. But they are notoriously difficult to pin down.

“Neutrinos are really pretty strange particles when you get down to it,” says John Conway, a professor of physics at University of California, Davis. “They’re almost nothing at all, because they have almost no mass and no electric charge…They’re just little whisps of almost nothing.” Ghost particles, they’re often called.

But they are one of the universe’s essential ingredients, and they’ve played a role in helping scientists understand some of the most fundamental questions in physics.

For example, if you hold your hand toward the sunlight for one second, about a billion neutrinos from the sun will pass through it, says Dan Hooper, a scientist at Fermi National Accelerator Laboratory and an associate professor of astronomy and astrophysics at the University of Chicago. This is because they’re shot out as a byproduct of nuclear fusion from the sun – that’s the same process that produces sunlight.

“They’re important to our understanding of the kind of processes that go on in the sun, and also an important building block for the blueprint of nature,” Hooper said.

Particle physicists originally believed that neutrinos were massless. But in the 1990s, a team of Japanese scientists discovered that they actually have a smidgen of mass. This tiny bit of mass may explain why the universe is made up of matter, not antimatter. Early in the process of the Big Bang, there were equal amounts of matter and antimatter, according to Conway. “But as the universe expanded and cooled, matter and antimatter were mostly annihilated. And a slight asymmetry favored matter over antimatter. We think neutrinos may have something to do with that process…. And it’s a puzzle, why we’re made out of matter and not antimatter.”

Studying neutrinos is difficult. They’re tough to detect since they interact so weakly with other particles. But the newly-completed IceCube Neutrino Observatory will study neutrinos inside a cubic kilometer block of ice in Antarctica. Here’s how: when the neutrinos interact with atoms inside the deep arctic ice detectors, they sometimes give off puffs of energy.

“As neutrinos pass through and interact, they produce charged particles, and the charged particles traveling through the ice give off light,” Conway said. “That’s how they’re detected. It’s like having a telescope for neutrinos underground.”

Fermilab National Laboratory has an experiment that hurls a beam of neutrinos 400 miles underground from Wisconsin to Northern Minnesota in about two milliseconds, and the lab is also planning a massive linear accelerator called Project X that will study the subatomic particles by sending them even farther.

“If 100 years ago, I told someone that the universe was filled with massless, chargeless particles with no energy, I wonder if they’d have believed you,” Conway said. “Who knows where we’ll be 100 years from now.”

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