Neutrino detectors, like the one used in the BOREXINO collaboration here, generally have an enormous tank that serves as the target for the experiment, where a neutrino interaction will produce fast-moving charged particles that can then be detected by the surrounding photomultiplier tubes at the ends. However, slow-moving neutrinos cannot produce a detectable signal in this fashion. (Credit: INFN/Borexino Collaboration)

If neutrinos have mass, where are all the slow ones?

If you’re a massless particle, you must always move at light speed. If you have mass, you must go slower. So why aren’t any neutrinos slow?

For many years, the neutrino was among the most puzzling and elusive of cosmic particles. It took more than two decades from when it was first predicted to when it was finally detected, and they came along with a bunch of surprises that make them unique among all the particles that we know of. They can “change flavor” from one type (electron, mu, tau) into another. All neutrinos always have a left-handed spin; all anti-neutrinos always have a right-handed spin. And every neutrino we’ve ever observed moves at speeds indistinguishable from the speed of light.

But must that be so? After all, if neutrinos can oscillate from one species into one another, that means they must have mass. If they have mass, then it’s forbidden for them to actually move at the speed of light; they must move slower. And after 13.8 billion years of cosmic evolution, surely some of the neutrinos that were produced long ago have slowed down to a reasonably accessible, non-relativistic speed. Yet, we’ve never seen one, causing us to wonder where are all the slow-moving neutrinos? As it turns out, they’re probably out there, just at…